TEST OF SALT MARSH AS A SITE OF PRODUCTION AND EXPORT OF FISH BIOMASS WITH IMPLICATIONS FOR IMPOUNDMENT MANAGEMENT AND RESTORATION PHILIP W.

Transcription

1 TEST OF SALT MARSH AS A SITE OF PRODUCTION AND EXPORT OF FISH BIOMASS WITH IMPLICATIONS FOR IMPOUNDMENT MANAGEMENT AND RESTORATION By PHILIP W. STEVENS A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2002

2 ACKNOWLEDGMENTS Dr. Clay Montague, my advisor and mentor, offered his valuable time, advice, and unique ecological perspective that made this dissertation a very enjoyable and rewarding experience. Dr. Kenneth Sulak, my co-chair and friend, gave me the encouragement, employment, and freedom to pursue this project. I also appreciate his confidence in my abilities to lead projects and author scientific papers during my residence with the US Geological Survey. My committee members, Dr. Thomas Crisman and Dr. Franklin Percival, provided valuable input and helpful critique. Dr. George Dennis provided knowledge of fish identification, statistical analyses, and project organization. Andy Quaid maintained datasondes in the study area, which rendered the much needed water level data that drive the ecology of this nontidal environment. The refuge managers and staff at Merritt Island National Wildlife Refuge and Canaveral National Seashore provided access to field sites, knowledge and history of impoundment ecology, and protection from the ever-present NASA security. Florida Sea Grant and the Aylesworth Foundation provided academic financial assistance, and the US Geological Survey Coastal Restoration Initiative provided logistics and employment. All of my field work could not have been done alone, and the following people provided enthusiastic field assistance despite the hardships of impoundment research: Cliff Bennett, James Berg, Nick Flavin, Mike Randall, Eric Rolla, James Russell, Pam Schofield, Mat Schreiner, Scott Stahl, George Stevens, and George Yeargin. ii

3 Throughout my endeavors, my parents, George and Yvonne Stevens, and my sister, Carla Stevens, have provided support and encouragement. My wife, Jackie Stevens, has always encouraged me to pursue my goals and aspirations, and has made many sacrifices on my account. I would like to thank them for their continued support and especially their patience. iii

4 TABLE OF CONTENTS ACKNOWLEDGMENTS... ii LIST OF TABLES... vii LIST OF FIGURES... x ABSTRACT... xiv CHAPTER 1 INTRODUCTION... 1 The Role of Nekton in Saltmarsh/Estuarine Interactions... 2 Production and Export of Nekton from Salt Marshes... 5 Saltmarsh Impoundments in East-Central Florida... 6 Hydrology of Saltmarsh Impoundments in East-Central Florida... 9 Consumption and Migration of Fish Biomass from the Saltmarsh Management and Restoration of Impoundments Hypotheses Resident Migrations Hypotheses Trophic Relay by Transients FIELD METHODS Study Area Water Conditions Fish Standing Stock Fish Ingress/Egress Piscivores Statistical Analysis FIELD RESULTS Water Conditions Pattern of Fish Use and Piscivore Abundance within the Impoundment Fish Standing Stock Impoundment Marsh Surface Impoundment Ditch, Creek, and Estuary Shoreline Size Distribution of Fishes within the Impoundment iv

5 Fish Ingress/Egress Piscivorous Fishes Piscivorous Birds Notable Observations FIELD DISCUSSION Water Conditions Resident Fish Hypotheses Juvenile Transient Fish Hypotheses Large Piscivorous Fish Hypotheses Other Piscivorous Animals Fish Use of Marsh Habitats Conclusions BIOMASS BUDGET DERIVATION Production Estimate from Ricker Equations Production Estimate from Biomass Budget Estimates of Biomass Budget Parameters Sensitivity Analysis of Biomass Budget BIOMASS BUDGET RESULTS BIOMASS BUDGET DISCUSSION OVERALL DISCUSSION Impoundment Management Strategies Implications of Impoundment Management Strategies Conflicts between Bird and Fish Management Impoundment Restoration Strategies Implications of Impoundment Restoration Strategies Conclusions APPENDIX A LENGTH-WEIGHT RELATIONSHIPS FOR FISHES B CORRELATIONS AMONG FISH CATCHES AND WATER CONDITIONS C ESTIMATED BIOMASS OF FISH CAPTURED DURING STUDY LIST OF REFERENCES v

6 BIOGRAPHICAL SKETCH vi

7 LIST OF TABLES Table page 1. Species cited in study Terms used to describe various places, habitats, and species Summary of monthly gear deployments Fish captured by cast net on the marsh surface Fish density on the marsh surface Fish captured by cast net within ditch and creek in Impoundment C20C Fish captured by cast net along Banana Creek shoreline Comparison of fish density among saltmarsh habitats and estuary shoreline Comparison of fish catch between reduced flow and unmodified culvert traps Number of small fish moving out of Impoundment C20C captured by culvert trap Number of small fish moving into Impoundment C20C captured by culvert trap Net egress of resident fish from Impoundment C20C over the study period Comparison of net fish ingress among culvert locations Number of large fish moving in and out of Impoundment C20C captured by culvert trap Fish captured by gill net within Impoundment C20C Fish captured by gill net along Banana Creek shoreline vii

8 17. Comparison of piscivorous and nonpiscivorous fish catch among saltmarsh habitats and estuary shoreline Stomach contents of piscivorous fish caught in gill nets Birds present within Impoundment C20C during monthly counts Birds present along Banana Creek shoreline during monthly counts Comparison of bird abundance among saltmarsh habitats and estuary shoreline Estimates of monthly fish standing stock within Impoundment C20C Estimates of monthly net fish ingress into Impoundment C20C Estimates of Florida gar abundance within the impoundment Conversion from catch per unit effort to impoundment gar population Daily prey consumption per predator body weight for selected estuarine fishes Estimates of monthly fish consumption by piscivorous fishes within Impoundment C20C Daily fish consumption by piscivorous birds Estimates of monthly fish consumption by birds within Impoundment C20C Fish production within Impoundment C20C estimated from Ricker equations Fish production within Impoundment C20C estimated from fish biomass budget Sensitivity analysis of impoundment fish biomass budget Estimated fish production incorporating net fish ingress from marsh surface and other disappearance Estimates of fish production among estuarine communities Length-weight relationships of fishes collected within Merritt Island NWR Monthly water conditions and culvert water flow in Impoundment C20C during the study period viii

9 37. Pearson s correlation among average monthly animal catches and average monthly water conditions Spearman s correlation among average monthly animal catches and average monthly water conditions Biomass of fish captured by cast net on the marsh surface Biomass of fish captured by cast net within ditch and creek in Impoundment C20C Biomass of fish captured by cast net along Banana Creek shoreline Biomass of small fish moving out of Impoundment C20C captured by culvert trap Biomass of small fish moving into Impoundment C20C captured by culvert trap Biomass of large fish moving in and out of Impoundment C20C captured by culvert trap Biomass of fish captured by gill net within Impoundment C20C Biomass of fish captured by gill net along Banana Creek shoreline ix

10 LIST OF FIGURES Figure page 1. Energy flow diagram illustrating impoundment trophic interactions within a saltmarsh impoundment and exchanges with the adjacent estuary Nekton biomass budget in a saltmarsh impoundment Illustration of Resident Hypothesis Illustration of Resident Hypothesis Illustration of Juvenile Transient Hypothesis Illustration of Juvenile Transient Hypothesis Illustration of Juvenile Transient Hypothesis Location of study site (Banana Creek) within the northern Indian River Lagoon system Map of Impoundment C20C within Kennedy Space Center Map of Impoundment C20C showing vegetation types, culvert locations, and datasonde locations Gill net deployment locations within Impoundment C20C Water level during 21 months, including study period, from datasondes that continuously measured water conditions hourly in Impoundment C20C and Banana Creek Water temperature during 21 months, including study period, from datasondes that continuously measured water conditions hourly in Impoundment C20C and Banana Creek Diurnal changes in temperature during a typical 10-day period x

11 15. Salinty during 21 months, including study period, from datasondes that continuously measured water conditions hourly in Impoundment C20C and Banana Creek Dissolved oxygen during 21 months, including study period, from datasondes that continuously measured water conditions hourly in Impoundment C20C and Banana Creek Redox potential during 21 months, including study period, from datasondes that continuously measured water conditions hourly in Impoundment C20C and Banana Creek Diurnal changes in dissolved oxygen during a typical 10-day period Diurnal changes in redox potential during a typical 10-day period ph during 21 months, including study period, from datasondes that continuously measured water conditions hourly in Impoundment C20C and Banana Creek Diurnal changes in ph during a typical 10-day period Average monthly water flow through Impoundment C20C culverts during study period Average water level, and numbers involved in net fish migration, fish standing stock, piscivorous fish catch per unit effort, and piscivorous bird abundance Average water level, and biomass involved in net fish migration, fish standing stock, piscivorous fish catch per unit effort, and piscivorous bird abundance Number and biomass of the standing stock of resident fishes by habitat Number and biomass of the standing stock of transient fishes by habitat Size frequency of Cyprinodon variegatus caught by cast net on the marsh surface at Impoundment C20C Size frequency of Poecilia latipinna caught by cast net on the marsh surface at Impoundment C20C Size frequency of Poecilia latipinna caught by cast net in ditch and creek within Impoundment C20C xi

12 30. Size frequency of Gambusia holbrooki caught by cast net in ditch and creek within Impoundment C20C Size frequency of Cyprinodon variegatus caught by cast net in ditch creek within Impoundment C20C Size frequency of Menidia peninsulae caught by cast net in ditch and creek within Impoundment C20C Size frequency of Mugil cephalus caught by cast net in ditch and creek within Impoundment C20C Size frequency of Lucania parva caught by cast net in ditch and creek within Impoundment C20C Catch rate of resident fish in culvert traps during study period Net resident fish ingress by culvert location Catch rate of transient fish in culvert traps during study period Net transient fish ingress by culvert location Piscivorous fish catch per unit effort in gill nets by habitat Size frequency of Sciaenops ocellatus captured by culvert trap and gill net during study period in Impoundment C20C Size frequency of Elops saurus captured by cast net, culvert trap, and gill net during study period in Impoundment C20C Size frequency of Leiostomus xanthurus captured by cast net, culvert trap, and gill net during study period in Impoundment C20C Size frequency of Cynoscion nebulosus captured by culvert trap and gill net during study period in Impoundment C20C Number and estimated biomass of piscivorous birds by habitat Derivation of fish production equation for biomass budget Estimated fish production, water level, and marsh elevation in Impoundment C20C during study period xii

13 47. Consumption and migration of fish production from Impoundment C20C Management strategies of coastal wetland impoundments in the Indian River Lagoon, Florida Restoration strategies of coastal wetland impoundments in the Indian River Lagoon, Florida xiii

14 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy TEST OF SALT MARSH AS A SITE OF PRODUCTION AND EXPORT OF FISH BIOMASS WITH IMPLICATIONS TO IMPOUNDMENT MANAGEMENT AND RESTORATION By Philip Stevens August 2002 Chairman: Clay L. Montague Major Department: Environmental Engineering Sciences Salt marshes are among the most productive ecosystems in the world, and although they are thought to enhance the productivity of open estuarine waters, the mechanism by which energy transfer occurs has been debated for decades. One possible mechanism is the transfer of saltmarsh production to estuarine waters by vagile fishes and invertebrates. Saltmarsh impoundments in the Indian River Lagoon, Florida, that have been reconnected to the estuary by culverts provide unique opportunities for studying marsh systems with respect to aquatic communities. The boundaries between salt marshes and the estuary are clearly defined by a system of dikes that confine fishes into a known area, and the exchange of aquatic organisms are restricted to culverts where they may be easily sampled. A multi-gear approach was used monthly to estimate fish standing stock, fish ingress/egress, and predation. Changes in saltmarsh fish abundance, and exchange with the estuary reflected the seasonal pattern of marsh flooding in the xiv

15 northern Indian River Lagoon system. During a six month period of marsh flooding, saltmarsh fishes had continuous access to marsh food resources. Piscivorous fishes regularly entered the marsh via creeks and ditches to prey upon marsh fishes, and piscivorous birds aggregated following major fish migrations to the marsh surface or to deep habitats. As water levels receded in winter, saltmarsh fishes concentrated into deep habitats and migration to the estuary ensued. The monthly estimates of fish standing stock, net fish ingress, and predation were used to develop a biomass budget to estimate annual production of fishes and the relative yield to predatory fish, birds, and direct migration to the estuary. Annual production of saltmarsh fishes was estimated to be 17.7 g m -2 salt marsh, which falls within the range of previously reported values for estuarine fish communities. The relative yields were at least 21% to piscivorous fishes, 14% to piscivorous birds, and 32% to export. Annual export of fish biomass was 5.6 g fish m -2 salt marsh, representing about 2% of saltmarsh primary production. Saltmarsh fishes convert marsh production to high quality vagile biomass (fishes concentrate energy, protein, and nutrients as body mass) and move this readily useable production to the estuary, providing an efficient link between salt marshes and estuarine predators. xv

16 CHAPTER 1 INTRODUCTION Salt marshes are among the most productive ecosystems in the world (Day et al. 1989; Montague and Wiegert 1990; Mitsch and Gosselink 1993; Montague and Odum 1997), and although they are thought to enhance the productivity of open estuarine waters, the mechanism by which energy transfer occurs has been debated for decades (Haines 1979; Nixon 1980; Dame 1994). Historically, salt marshes were thought to contribute to estuarine productivity by exporting large quantities of detritus, which then form the base of the estuarine food web (Teal 1962; Odum and de la Cruz 1967; Wiegert et al. 1975). However, detrital and dissolved organic export from salt marshes is variable among locations and is often only a minor contribution to estuarine productivity (Heinle and Flemer 1976; Marinucci 1982; Montague et al. 1987; Dame et al. 1991; Williams et al. 1992; Borey et al. 1993; Taylor and Allanson 1995). Salt marshes were also thought to supply nutrients to estuarine waters, thereby enhancing estuarine primary production (i.e., phytoplankton production). However, the majority of nutrients contained within saltmarsh plant tissues are recycled within the marsh and little is directly exchanged with the estuary relative to demand in estuarine waters (Haines et al. 1977; Valiela and Teal 1979; Nixon 1980; Hopkinson and Schubauer 1984). Marsh sediments and their associated microbial communities can be sources or sinks for nutrients depending on the nutrient supply from other sources such as river discharge, upland runoff, and sewage effluent (Haines et al. 1977; Valiela and Teal 1979; 1

17 2 Kaplan et al. 1979). For example, marshes with high input of nitrogen (the limiting nutrient in many estuaries) are likely to be sites of net denitrification, whereas marshes with low input of nitrogen are likely to be sites of net nitrogen fixation (Haines et al. 1977; Valiela and Teal 1979; Kaplan et al. 1979; Montague et al. 1987). Although the historical paradigm of salt marshes supplying detritus and nutrients directly to open estuarine waters remains equivocal, an alternative paradigm has been emerging that emphasizes the role of salt marshes in providing food and refuge to young estuarine nekton and the role of nekton in transferring saltmarsh production to estuarine waters (Werme 1981; Haines 1979; Haines and Montague 1979; Boesch and Turner 1984; Montague et al. 1981; Montague et al. 1987; Knieb 1997). The Role of Nekton in Saltmarsh/Estuarine Interactions Nekton studies in coastal wetlands recognize two components of aquatic communities: resident (those species that complete their life history within marshes), and transient (those species that use marshes periodically for food and refuge). Many transient species (e.g., drums, mullets, swimming crabs) use marshes as nurseries during early stages in their life history (Weinstein 1979; Boesch and Turner 1984; Pattillo et al. 1997). These species often arrive in saltmarsh creeks during postlarval and juvenile stages after being transported there by wind and currents directly, or by actively positioning themselves within the water column to take advantage of the movement of water masses favorable for reaching nursery areas (Weinstein et al. 1980; Pietrafesa et al. 1986). The nursery areas often include salt marshes, and also seagrasses, because these habitats offer greater food availability to many species of nekton relative to unvegetated areas (Rozas and Minello 1998), and also provide substantial refuge from predation from

18 3 larger estuarine species (Rozas and Odum 1988; McIvor and Odum 1988). Refuge from predation is also found in shallow water (whether vegetated or not), a common feature of saltmarsh systems (Ruiz et al. 1993; Miltner et al. 1995). Transient juvenile nekton use salt marshes as nurseries for a few weeks to a few months (Weinstein et al. 1980; Gilmore et al. 1982). Transient juvenile species are most commonly found in tidal creeks, and although some venture onto the marsh along the edge of tidal creeks, most are never found on the interior marsh surface (Minello et al. 1994; Peterson and Turner 1994). Few transients on the marsh surface have also been reported for seasonally flooded marshes in the Indian River Lagoon system, even during periods when the marshes were continuously flooded (Klassen 1998). Resident nekton (e.g., killifishes, livebearers) complete their entire life history within the marshes and may comprise a major portion of the forage base for larger transient nekton and avian wildlife (Kushlan 1980). The marsh surface offers a wide variety of food resources for resident nekton such as detritus, benthic microalgae, cyanobacteria, plankton, and benthic invertebrates (Harrington and Harrington 1961; Kneib and Wagner 1994). It also provides refugia for resident fishes (e.g., killifishes, livebearers) (McIvor and Odum 1988; Knieb and Wagner 1994; Rozas 1995), although predation by birds within marshes may be quite high in some places (Kushlan 1980). Resident fishes remain on the marsh surface during high water and retreat to pools, depressions, and creeks during low water periods to escape desiccation (Knieb and Wagner 1994; Rozas 1995). Trophic interactions among resident and transient nekton that occur along interaction hot-spots (e.g., creek banks) transport marsh production across the saltmarsh landscape to the open estuary (Kneib 1997).

19 4 Kneib (1997) has described the movement of energy across the marsh landscape to the estuary via predator-prey relationships as the trophic relay and has developed a detailed conceptual model of these interactions. Resident nekton convert saltmarsh production into vagile biomass, which then may move across the marsh landscape as smaller residents are consumed by larger residents. The larger residents are more likely to move to subtidal creeks or seagrass beds during low water to escape desiccation or thermal stress on the marsh surface, which makes them susceptible to predation by young transient nekton. The latter may move to deeper estuarine waters as they mature, or may be eaten by larger adult transients that occasionally foray into marsh creeks seeking prey. Non-predatory transient nekton, (e.g., herrings, silversides, and mullets) also incorporate marsh foods into vagile biomass when present in the marsh as juveniles. They may be consumed by larger predatory transients during their residence within the saltmarsh, but they eventually emigrate from salt marshes to the open estuary as they reach maturity, moving the incorporated saltmarsh resources with them (Deegan 1993; Kneib 1997). According to Kneib (1997), the migrations involved in the trophic relay occur at different spatial and temporal scales. For example, daily intertidal migrations occur as resident nekton move between marsh surface and creek habitats, or transient nekton move in and out of marsh creeks. Also, seasonal migrations occur as larval and early juvenile stages move into marshes and later emigrate to the open estuary, and in some cases, the open ocean. The interspecific interactions between resident marsh fishes and transient predators that occur within the marsh may be important mechanisms for moving the marsh resources that have been incorporated into resident nekton to the adjacent estuary.

20 5 Production and Export of Nekton from Salt Marshes Export of fishes from salt marshes has been addressed by Deegan (1993) and Herke et al. (1992). Deegan (1993) used wing nets to sample the migration of Brevoortia patronus from a Louisiana estuary and found that B. patronus transported substantial quantities of carbon, nitrogen, and phosphorus from the estuary to the nearshore Gulf of Mexico. Estimated export of menhaden from the Louisiana estuary was 38 g fish m - 2 yr -1, which represented 5 10% of estuarine primary production (Deegan 1993). Herke et al. (1992) used a combination of weirs and fish traps to sample all nekton that left two saltmarsh impoundments in Louisiana and estimated average emigration to be 21.7 g wet weight m -2 y -1. Net export from the Louisiana marshes was not known, however, because immigration was not measured (Herke et al. 1992). Several investigators have estimated the production of a single species within salt marshes (Welsh 1975; Valiela et al. 1977; Meredith and Lotrich 1979; Weinstein 1983), but only a few have estimated the production of an entire saltmarsh fish community (Schooley 1980). Estimates of immigration and emigration were not necessary in these studies because the species under investigation were marsh residents with limited home range (Welsh 1975; Valiela et al. 1977; Meredith and Lotrich 1979), marsh residents isolated from the estuary within saltmarsh impoundments (Schooley 1980), or transient nekton with marked periods of recruitment so that production could be measured after the recruitment phase (Weinstein 1983). Despite the potential importance of nekton in energy transfer from the marsh to the estuary, few studies have estimated both estuarine nekton biomass production and export. The boundaries of salt marsh systems are open and ill defined, and the conduits

21 6 of transport (i.e., creeks) are broad and complex. Thus, estimating the boundaries within which production has occurred, and measuring the exchange of nekton between salt marshes and the estuary, create difficult challenges with respect to sampling. Open saltmarsh impoundments in East-Central Florida, however, provide unique opportunities for study of marsh systems with respect to aquatic communities. The boundaries between salt marshes and the estuary are clearly defined by a system of dikes, thereby confining nekton into a known area, and the exchange of aquatic organisms is restricted to culverts. Open culverts allow sampling nekton as they move between salt marshes and the adjacent estuary. Although saltmarsh impoundments differ morphologically from natural systems, understanding the function of impounded marsh systems should provide a general model of saltmarsh ecology and linkages to the adjacent estuary by movement of aquatic organisms. Saltmarsh Impoundments in East-Central Florida In many areas of the world, salt marshes have been impounded to control water levels within the marsh for a variety of objectives including mosquito control, wildlife enhancement, vegetation mitigation, prevention of saltwater intrusion, reduction of erosion, and prevention of extreme water level fluctuations (Rogers et al. 1994). In East- Central Florida, the majority of salt marshes were impounded by diking around the perimeter of the salt marsh and installing water control structures (i.e., flapgated culverts and riserboards) to manage water levels for mosquito control, wildlife, and vegetation (Montague et al. 1985; Montague et al. 1987; Smith and Breininger 1995; Brockmeyer 1997).

22 7 An energy flow diagram that illustrates trophic interactions within saltmarsh impoundments and exchanges with the adjacent estuary is shown in Fig. 1. The sources (circles), storages (tanks), producers (bullets), and consumers (hexagons) within the impoundment and their interactions are similar for both natural and impounded marshes. Salt marshes contain a network of creeks, flooded marshes, and open ponds (top left of Fig. 1, inside the heavy storage tank) that offer habitat for resident nekton. Lines are drawn from the habitat components (ponds, flooded marsh, creeks) to resident nekton to represent the effect of habitat. The resident nekton, in turn, consume and incorporate saltmarsh products (detritus, algae, phytoplankton, benthic invertebrates, and mosquito larvae) into fish biomass. Lines are drawn from the various foods (detritus, algae, phytoplankton, benthic invertebrates, and mosquito larvae) to represent the conversion of these foods to resident nekton. Saltmarsh creeks and ditches (top left of Fig. 1, inside the storage tank) are adequately deep to be used by adult transient nekton that may exploit both resident and juvenile transient nekton present within the marsh. Other wildlife such as wading birds and reptiles also exploit resident and juvenile transient nekton. The trophic relay is shown in the center of the diagram. Lines are drawn from prey to predators where appropriate. The trophic relay begins with detritus, phytoplankton, and algae, and moves progressively across the diagram left to right towards larger species such as adult transient nekton, wading birds, and reptiles (e.g., alligators, terrapins, and snakes). As larger animals consume smaller animals, the embodied saltmarsh production moves through higher trophic levels, and across the marsh landscape elsewhere. Transient fishes move saltmarsh secondary production to the estuary as they eventually emigrate from the marsh.

23 8 Reptiles may move saltmarsh secondary production across land boundaries as they access adjacent marshes or the open estuary. Piscivorous birds may move secondary production from the marsh system at even greater scales (regionally, or globally) as they migrate along the Atlantic coast and cross continental boundaries. Although the fundamental trophic interactions within impoundments are similar to natural systems, the distinguishing features of impounded marshes are the system of dikes and borrow ditches that surround the perimeter of the marsh (Fig. 1, outer outline represents dikes), and the culverts that provide aquatic exchange to the adjacent estuary (shown at right in Fig. 1). Perimeter ditches within impounded salt marshes, although artificial, are similar to creeks in other marsh systems (lines are drawn from creeks and ditches to nekton to illustrate that both provide nekton habitat). They may provide similar refuge from predation, a conduit for transport of materials from the marsh to the estuary, and access to predators seeking prey when water control structures are open to the estuary (Gilmore et al. 1982; Rey et al. 1990b). Exchange of water, detritus, nutrients, and aquatic organisms between the marsh system and the adjacent estuary is confined to culverts (dashed box at right in Fig. 1), where water control structures may be opened or closed depending on management objectives (e.g., resource management, mosquito control). A line drawn from management at top right in Fig. 1 to culverts represents the influence of management in opening or closing the culverts (culverts are indicated as a switch in Fig. 1). A primary concern over saltmarsh impoundments has been the reduction of access to salt marshes by estuarine nekton, especially for transient species that dependent on marshes during early stages of their life history (Montague et al. 1985; Rogers et al. 1994; Brockmeyer et al.

24 9 1997). Where water level management is not a priority, impoundments can be left open year round to optimize access for wetland-dependent fishes and invertebrates (Brockmeyer 1997). Hydrology of Saltmarsh Impoundments in East-Central Florida When impoundments are open year round, they are directly subject to the hydrology of the adjacent lagoon. Salt marshes in the northern Indian River Lagoon system lie at the extremities of three such lagoons (Indian River Lagoon, Banana River, and Mosquito Lagoon). The waters of these lagoons are isolated from the ocean and have little or no diurnal tidal range (< 5 cm) (Smith 1986). Short-term changes in water levels and circulation are primarily driven by wind. Long-term changes in water levels are dramatically influenced by seasonal variations in sea level. The water levels are typically low in spring and summer, but about 27 cm higher in fall due to thermal expansion and seasonal rainfall (Smith 1986). Due to this seasonal variation in water level, many Indian River Lagoon marshes are dry during much of summer, but are almost continuously inundated during fall and early winter (Montague et al. 1985). The hydrology of the northern Indian River Lagoon System may be similar to other seasonally flooded and wind-driven estuaries (e.g., Pamilco Sound, North Carolina; Laguna Madre, Texas; Camargue, southern France), but differ greatly from more typical tidal marshes such as those in Georgia where daily tides are predictable and tidal range masks seasonal changes in water level. The unique hydrology of the Indian River Lagoon influences saltmarsh use and exchange rates of nekton with the estuary. Use of natural marshes and open saltmarsh impoundments by large juvenile and adult transient species is greatest during high water levels of fall, and use of marshes by young juveniles is

25 10 greatest during spring (Gilmore et al. 1982; McLaughlin 1982; Rey et al. 1990a, 1990b; Karlen 1991; Weiher 1995; Poulakis 1996; Lin and Beal 1995; Klassen 1998; Taylor et al. 1998; Faunce and Paperno 1999). Use of saltmarsh impoundments and their ditches by fishes is lowest during summer when water level is low and water temperature and salinities are high. Dissolved oxygen is low under these conditions. Hence, summer water conditions may be suboptimal for many transient fishes (Gilmore et al. 1982; Rey et al. 1990b; Lin and Beal 1995; Klassen 1998). In contrast to marshes affected by daily tides, the cycle of resident marsh fish migrations to and from the marsh surface in seasonally flooded marshes is expanded over many months, rather than hours. Thus, the movement of saltmarsh resources via the trophic relay would also occur on similar time scales (weeks to months). As in tidal marshes, the trophic relay would also include seasonal movements of nekton from marshes to the estuary as a result of changing habitat preferences as they grow. Direct migration of resident marsh fishes to the estuary may be especially important in the Indian River Lagoon system where hydrology is seasonal and the relatively narrow marshes are in close proximity to open estuarine waters. Consumption and Migration of Fish Biomass from the Salt Marsh The artificial boundaries, creeks, and connections to the estuary, together with the seasonal hydrology associated with open saltmarsh impoundments in the Indian River Lagoon, permit a biomass budget of saltmarsh fishes to be constructed (Fig. 2). Outputs from the standing stock of nekton within the impoundment are the result of emigration and predation/death. Inputs to the standing stock of nekton within the impoundment are the result of immigration and production. Monthly estimates of standing stock,

26 11 immigration, emigration, and predation can be used to estimate annual production of fishes in the saltmarsh (production equals change in biomass divided by change in time minus immigration plus emigration plus consumption by predators). This estimate of saltmarsh fish production can be compared to those of other locations and other estuarine habitats such as seagrass beds and open estuarine waters. More importantly, the yield to predatory fish, birds, and emigration can be quantified. Management and Restoration of Impoundments The patterns of fish use and predator abundance within seasonally flooded salt marshes, and the consumption and migration of fish biomass from the marsh are relevant to management and restoration of saltmarsh impoundments in the Indian River Lagoon, Florida. Almost all salt marshes in the Indian River Lagoon system have been impounded, and over 65% of saltmarsh impoundments (by area) are located within Merritt Island National Wildlife Refuge (MINWR) (Brockmeyer et al. 1997). Thus, management and restoration activities within the refuge potentially influence the entire Indian River Lagoon system. The impoundment situation in the Indian River Lagoon system, and MINWR in particular, was an appropriate study area to meet the objectives of the U.S. Geological Survey Coastal Restoration Initiative. The goal of the Coastal Restoration Initiative was to develop fundamental knowledge of community ecology to guide restoration strategies for engineered salt marshes to maintain species biodiversity and ecosystem integrity. To interpret the consequences to fish production by various impoundment restoration alternatives, a comparison of fish use among perimeter ditches, marsh surface, marsh creeks, and estuarine shorelines was needed, because each will be affected by restoration

27 12 efforts. Thus, scientific knowledge developed regarding saltmarsh trophic interactions, fish migrations, and differential fish use among marsh habitats have direct applications to contemporary resource management objectives. Hypotheses Resident Migrations During the seasonal high water period (July November), resident fishes are dispersed on the marsh surface where they make use of abundant food and spawning sites. As water levels seasonally recede (December February), resident fishes leave the marsh surface, entering marsh creeks and ditches to escape desiccation. Resident Fish Hypothesis 1 (Fig. 3): A portion of the resident fish population emigrates from the impoundment contributing marsh production to the estuarine food chain. Resident marsh fishes such as Cyprinodon variegatus, Gambusia holbrooki, and Poecilia latipinna are concentrated into ditches and creeks as water levels recede. If crowding occurs, some of the resident fishes may leave the impoundment in search of additional forage. During strong winds and rain that drive water exchange through culverts, some resident fishes may be forced from the impoundment to the estuary. These resident fishes add to the forage of larger estuarine fishes. Resident Fish Hypothesis 2 (Fig. 4): Resident fishes remain within the impoundment and few individuals move into the estuary. The creeks and perimeter ditches within the marsh offer adequate food and the relatively deep waters and the high edge/open water ratio provide refuge from predation by wading birds and aquatic predators. The resident fishes do not leave the refuge of these impoundment creeks and ditches and can avoid export through culverts during periods of high current velocity. Previous studies conducted in tidal marshes have shown that resident fish densities are

28 13 high along creek edges, especially during low tide (Valiela et al. 1977; Meredith and Lotrich 1979; Peterson and Turner 1994). In such marshes, however, fish will again have access to the marsh surface in a matter of hours when the tide rises. This situation differs from the Indian River Lagoon where the marsh surface may remain dry up to several months. Hypotheses Trophic Relay by Transient Fishes Juvenile Transient Fish Hypothesis 1 (Fig. 5): Juvenile transient fish enter the impoundment, grow, and eventually emigrate to the estuary in greater biomass (although fewer individuals) than when they entered. Large Piscivorous Fish Hypothesis 1 (not figured): Large predatory transient fish enter the impoundment, prey upon saltmarsh fish (residents and small transients), then leave. Young transient fish enter saltmarsh impoundments, either actively as juveniles, or passively as larvae, during periods of high current velocity through culverts. Young transient fish not only enter the impoundments, but also some survive and eventually emigrate from the impoundment to the estuary. Exported fish biomass may be greater than that imported. Juvenile fish abundance (no. m -2 ) within the impoundment equals or exceeds that outside the impoundment, if juveniles seek saltmarsh habitat. Large predatory fish enter the impoundment to feed upon resident and small transient saltmarsh fish, then leave. Juvenile Transient Fish Hypothesis 2 (Fig. 6): Juvenile transient fish are abundant along estuary shorelines, but absent from impounded marshes. Large Piscivorous Fish Hypothesis 2 (not figured): Large predatory transients are abundant along estuary shorelines, but absent from impounded marshes. Transient fish seldom enter the impoundment, but are more often found along the estuary shoreline among

29 14 saltmarsh vegetation. Estuary shorelines may be the preferred habitat for transient fishes. They may avoid impoundments because water quality is too poor, or because food and cover are greater along the estuary shoreline. Transient fish densities (no. m -2, g m -2 ) are greater along the shoreline than within the impoundment ditches and creeks. Although studies report the occurrence of transient fishes in impoundments when they are open to the estuary (Gilmore et al. 1982; McLaughlin 1982; Rey et al. 1990a, 1990b, Karlen 1991; Weiher 1995; Poulakis 1996; Lin and Beal 1995; Klassen 1998; Taylor et al. 1998; Faunce and Paperno 1999), few compare fish densities in impoundments with those along the estuary shoreline and seagrass beds (Klassen 1998). Klassen (1998) found that an estuary shoreline in Mosquito Lagoon, Florida, provided a different type of habitat for transient fishes than within an impoundment. For example, Leiostomus xanthurus was more abundant along the shoreline adjacent to an impoundment than inside the impoundment. Also, several species Cynoscion nebulosus, Lagodon rhomboides, Strongylura marina, and Symphurus plagiusa) collected along the shoreline were absent from collections within the impoundment (Klassen 1998). Juvenile Transient Fish Hypothesis 3 (Fig. 7): Saltmarsh impoundments act as a sink for juvenile fish. Juvenile transients enter the impoundment but most do not leave. Large Piscivorous Fish Hypothesis 3 (not figured): Large predatory fish do not survive well in saltmarsh impoundments. Fish are trapped within the wetland and die from either poor water quality or predation by birds and other wildlife such as alligators. Biomass of juvenile fish is not exported to the estuary greater than that of the earlier immigrating juvenile fish. The artificial accessibility to marshes provided by perimeter ditches may expose transient fish to poor water quality associated with seasonally flooded marshes

30 15 (Poizat et al. 1997). The openness of natural creeks, embayments, and estuarine shorelines may be necessary for juvenile survival and predatory fish access. Such openness may offer greater opportunity for transient species to escape the marsh during poor water quality conditions. Culverts may restrict access because of smaller size of their opening relative to complete dike removal. These three hypotheses regarding transient fish use of Indian River Lagoon marshes are not mutually exclusive. Although transient fishes have often been reported to use salt marshes as nursery habitat, considerable debate exists (e.g., Miller et al. 1984). Also, transient fish occurrence within salt marshes varies among species. Some species use the Indian River Lagoon saltmarsh creeks and ditches (e.g., Mugil cephalus; Klassen 1998), while others may occur only along the estuary shoreline (e.g., L. xanthurus; Klassen 1998). Which hypothesis is supported by analyses may ultimately depend on the available suite of transient species present in the estuary adjacent to the study marshes. The proposed transient hypotheses may depend upon which resident hypothesis transpires. For example, large transient fishes that prey on residents may benefit from concentrated food if residents remain within the impoundment for several months after water levels recede during late winter. Young transients entering the impoundment in spring, however, would face severe competition with resident marsh fishes, resulting in less available food and space. If, however, resident fishes leave the impoundment after water levels recede from the marsh surface during late winter, then competition between residents and young transients in the ditches and creeks during spring may be much lower, possibly resulting in greater transient survival and growth.

31 Fig. 1. Energy flow diagram illustrating trophic interactions within a saltmarsh impoundment and exchanges with the adjacent estuary. Outer outline represents the dikes that define the impoundment border. Bold outline contains within it some fundamental trophic interactions found in all flooded marshes. Bold dashed box at right contains aquatic sources that are exchanged with the impoundment. Ditches, culverts, and perimeter dikes (impoundment boundary) are unique to impounded salt marshes. 16

32 Immigration Emigration Production Standing Stock Predation 17 Fig. 2. Nekton biomass budget in a saltmarsh impoundment: Production = standing stock / time immigration + emigration + predation

33 Resident Hypothesis 1 a Dike Culvert Marsh Surface Estuary Ditch Impoundment b Dike 18 Culvert Marsh Surface Estuary Ditch Impoundment Fig. 3. Illustration of Resident Hypothesis 1: As seasonal water level recedes (b), resident fishes are concentrated into subtidal habitats within the marsh (creeks and ditches). A portion of the resident fish population leaves the impoundment, contributing to the estuary prey base. a) Seasonal High Water (July February) b) Seasonal Low Water (February July). Arrows indicate net direction of travel by fishes.

34 Resident Hypothesis 2 Dike a Culvert Marsh Surface Estuary Ditch Impoundment Dike b 19 Culvert Marsh Surface Estuary Ditch Impoundment Fig. 4. Illustration of Resident Hypothesis 2: As water level recedes (b), resident fishes are concentrated into subtidal habitats within the marsh (creeks and ditches) and few leave the impoundment. a) Seasonal High Water (July February) b) Seasonal Low Water (February July).

35 Juvenile Transient Hypothesis 1 Dike a Culvert Marsh Surface Estuary Ditch Impoundment Dike b 20 Culvert Marsh Surface Estuary Ditch Impoundment Fig. 5. Illustration of Juvenile Transient Hypothesis 1: Juvenile transient fishes (small fish shown in a ) enter the impoundment, grow, (large fish shown in b ) and some eventually emigrate to the estuary. More biomass may leave than entered (although fewer individuals). a) Seasonal High Water (July February) b) Seasonal Low Water (February July). Arrows indicate net direction of travel by fishes.

36 Juvenile Transient Hypothesis 2 Dike a Culvert Marsh Surface Estuary Ditch Impoundment Dike b 21 Culvert Marsh Surface Estuary Ditch Impoundment Fig. 6. Illustration of Juvenile Transient Hypothesis 2: Juvenile transient fishes are abundant along estuary shorelines, but absent from impounded marshes. a) Seasonal High Water (July February) b) Seasonal Low Water (February July). Small fish in a indicate young fish. Large fish in b indicate fish that have grown.

37 Juvenile Transient Hypothesis 3 a Dike Culvert Marsh Surface Estuary Ditch Impoundment Dike b 22 Culvert Marsh Surface Estuary Ditch Impoundment Fig. 7. Illustration of Juvenile Transient Hypothesis 3: Impounded marshes are a sink for juvenile transient fishes. Juvenile transients enter the impoundment, but most do not leave. a) Seasonal High Water (July February) b) Seasonal Low Water (February July). Arrows indicate net direction of travel by fishes. Small fish in a indicate young fish. Large fish in b indicate fish that have grown.

38 CHAPTER 2 FIELD METHODS Study Area Merritt Island National Wildlife Refuge (Fig. 8), lies along the transition between temperate and subtropical climate in Florida where mangroves and salt marsh alternate in response to periodic freezes that reset mangrove development (Kangas and Lugo 1990; Stevens 1999). Saltmarsh plant species covered the marshes after consecutive freezes in the 1980s killed existing mangrove forests consisting of some Rhizophora mangle, and many Avicennia germinans and Laguncularia racemosa. However, dead mangrove wood and fringing live L. racemosa remain conspicuous features of the estuarine shorelines. The vegetation typical of seasonally flooded marshes at Merritt Island consists of Distichlis spicata, Paspalum vaginatum, Batis maritima, and Spartina bakerii (Montague et al. 1985). Spartina alterniflora may fringe the estuary shoreline, but is seldom found on the marsh surface. Merritt Island s transitional location enhances the diversity of nekton communities, as both temperate and subtropical fish species co-exist (Snelson 1983). The selection criteria for the study impoundment included the following: the impoundment was located within Banana Creek where joint University of Florida and U.S. Geological Survey fish community research was ongoing, the impoundment was covered by saltmarsh vegetation indicative of Merritt Island (as opposed to exotic marsh plants or predominately upland species), culverts were open year round to allow the 23

39 24 exchange of aquatic organisms between the impoundment and the estuary, the culverts had been open for several years so the ecological community was not undergoing dramatic successional changes during the study period, and adequate access to the impoundment by project personnel was available throughout the course of the study. The impoundment selected for study was C20C (134 ha) located in Banana Creek across from the Kennedy Space Center shuttle landing facility and adjacent to the KSC Vehicle Assembly Building (Fig. 9). The impoundment saltmarsh vegetation is dominated by D. spicata and P. vaginatum, which are typical saltmarsh species found in the vicinity of Merritt Island (Fig. 10). A perimeter ditch (approximately 10 m wide and 1 m deep) separates the dike from the interior of the impoundment and a major creek, Drainout Creek, is located within the impoundment (Fig. 10). L. racemosa fringes the estuarine shoreline, Drainout Creek, and the perimeter ditch. In addition to live L. racemosa, the estuary shoreline is fringed by decaying wood from dead mangroves and a narrow band (1 3 m) of S. alterniflora often occurs seaward of live white mangroves. Sediments are soft, especially within the perimeter ditch and Drainout Creek. Sediments along the estuary shoreline consist of firm mud within embayments and a mixture of mud and sand in areas directly adjacent to Banana Creek. Impoundment C20C is connected to the Banana Creek estuary by four sets of 0.91 m diameter culverts (Fig. 10). Each set consists of one culvert with riser boards, and one culvert with a flapgate. These water control structures enable resource managers to manipulate water levels within the impoundment when desired, but no manipulation was undertaken over the course of this study. The flapgate allows water to enter the impoundment when lagoon water levels exceed those in the impoundment. Riser boards

40 25 (each about 15 cm high) can be added or removed as needed to maintain a desired water level. If the impoundment water level exceeds the riser boards, then water will spill over the boards into the estuary. There are eight culverts connecting the impoundment directly to Banana Creek (Fig. 10). Two additional culverts are located within the impoundment; one connects C20C to an adjacent creek, and the other connects C20C to a small ditch at the southern end of the impoundment (Fig. 10). These two culverts were closed during the study, but the remaining eight were open (flapgate open and riserboards removed), providing estuarine communication with the impoundment. Lights used to guide the space shuttle to the landing facility are located along a boardwalk that extends across the southwestern portion of the impoundment. Hence, adequate access to the impoundment was available by project personnel because the US National Aeronautics and Space Administration (NASA) must maintain the dike roads surrounding the impoundment for maintenance of the lights. A list of common names of all animal and plant species cited in this study is shown in Table 1. For clarity, the following definitions regarding habitat and fish classifications are provided (Table 2). Salt marsh (natural or impounded) refers to the dominant marsh area that is located behind the estuary/marsh berm, or impoundment dikes, indicative of the Indian River Lagoon marshes. Estuary/marsh berm is a natural levee located at the outer mean high water mark (Provost 1973), and is the location where dikes were constructed during marsh impoundment (Montague et al. 1985). Salt marsh as defined here includes the vegetated marsh surface, tidal creeks that penetrate the salt marsh plane, marsh islands, and perimeter ditches if the marsh is impounded. Indian

41 26 River Lagoon estuary refers to the Indian River Lagoon, Banana River, Banana Creek, and Mosquito Lagoon (see Fig. 8). The estuary typically includes open water areas and seagrass flats between the barrier island and mainland, intracoastal waterways, and the shoreline up to the estuary/marsh berm (or impoundment dike). Saltmarsh fishes refer to the entire suite of resident, transient, and incidental fishes present within the salt marsh. Incidental fishes are those species resident in the estuary and occasionally occur in salt marshes by accident, or as a negligible extension to their regular habitat (usually seagrass-associated fishes). These species do not use the salt marshes as nursery habitat for young or as regular feeding areas for adults, but more typically use estuary habitats. Examples include anchovies, gobies, and pipefish. Water Conditions To determine short-term and seasonal variation in water conditions within the impoundment relative to the adjacent estuary, water level (m), temperature (ºC), salinity ( ), dissolved oxygen (mg L -1 ), turbidity (NTU), and oxygen redox potential (Eh) were monitored hourly with two continuously deployed datasondes (Hydrolab Datasonde 3); one within the impoundment and one in Banana Creek adjacent to the impoundment (Fig. 10). The datasondes were calibrated prior to deployment and twice during the study period in accordance with the manufacturer s recommended procedures. Impoundment water level was recorded to within the nearest 0.3 cm from a permanent staff gauge within the impoundment. Water level measurements taken by the datasondes were corrected to correspond to water level readings on the permanent staff gauge. This was accomplished by taking the difference between datasonde water-level readings and observed readings on the impoundment staff gauge during three periods of little wind,

42 27 water flow, rainfall, and water level changes. These differences were averaged to determine a correction factor. The impoundment staff gauge was later surveyed by NASA personnel, which provided a correction with respect to the 1988 National Geodetic Vertical Datum (NGVD). Fish Standing Stock Standing stock, immigration, emigration, and predator abundance (by large fishes and also piscivorous birds) were sampled monthly for a period of one year beginning July 2000 and ending July 2001 (Table 3). The monthly sample usually required one week to complete. Small fishes in the impoundment and along the adjacent Banana Creek shoreline were quantified with a 1.15 m (4 ft) radius, 6 mm (1/4 inch) bar mesh, monofilament cast net. Cast nets have been used by commercial fisherman along the Indian River Lagoon to target fast-moving species such as mullet, and are often used to capture bait for recreational fishing (Schoor et al. 1995). Cast nets can be quickly deployed and cleared of fish, an advantage over other gear types such as drop and throw traps requiring long processing times to clear fish from the gear. Thus, more deployments can be made and more area covered with cast nets than with drop and throw traps, which improves the chances of finding relatively rare species such as juvenile transients. Also, cast nets can be easily deployed by casting from the shoreline or from a small boat, thereby allowing the perimeter ditches and marsh creeks to be sampled effectively without fixed boardwalks that are necessary when sampling impoundments with seines (Gilmore et al. 1982). The diameter and mesh size of the cast net were chosen because the small diameter is manageable to throw, thereby reducing variation in deployment

43 28 area, and the mesh size is effective for capturing juvenile fishes (Woodward 1989). Only areas of low vegetation density can be sampled effectively without fouling. The cast net was deployed among the primary habitats associated with both transient juvenile fishes and resident nekton. The areas sampled monthly with the cast net were the impoundment ditch (ca. 1 m depth), and creek (ca. 1 m depth). The estuary shoreline (< 1 m depth) was also sampled for comparison with the impoundment ditch and creek. The estuary shoreline was sampled along the dike within at least 1 m of the emergent shoreline vegetation and often within low-density smooth cordgrass. The seasonally flooded marsh surface (< 0.5 m depth) was sampled during July 2000 (the onset of marsh flooding), and October 2000 (the time of seasonally high water). The marsh surface consisted of consolidated mud substrate, emergent vegetation, open water ponds, and small potholes (2 3 m diameter). The marsh surface was sampled in open water areas near emergent vegetation and within small potholes. Deeper pools within the marsh were also sampled during January 2001 (after water had receded from the marsh) to measure the density of any fish remaining in ponded water. A pilot study conducted during March 2000 determined that a minimum of 14 cast net deployments were needed to detect a statistical difference between the mean numbers of fish from each habitat (α = 0.05, β = 0.90; Sokal and Rohlf 1995). Each habitat type (Drainout Creek, perimeter ditch, marsh surface, and estuary shoreline) was mapped using aerial photography (USGS 1995). The 14 cast-net deployment stations along the estuary shoreline were selected by randomly choosing among 136 sampling points spaced 16 m apart along the perimeter dike. The 14 stations in the perimeter ditch were selected by randomly choosing from among 98 sampling points (a 615 m segment of the perimeter

44 29 ditch was excluded due to inaccessibility) spaced 16 m apart along the centerline of the perimeter ditch. For the Drainout Creek sampling, 14 points from among 55 potential sampling points (30 m apart) were selected for cast net deployment. For the marsh surface sampling, 14 points were chosen in areas of low vegetation density and depth of at least 10 cm (knotgrass marsh/shallow ponds; Fig. 10). A small jon boat equipped with an electric trolling motor was used to access the perimeter ditch, creek, and estuary shoreline for cast net deployment. For the estuary shoreline sampling, the cast net was deployed as close to the shoreline as possible without fouling the net in wood debris or overhanging trees. For perimeter ditch sampling, a coin was tossed prior to each cast net deployment to determine whether to cast the net along the edge or near the center of the ditch. Collected fish were identified to species, counted, measured (total length), and released. Weight was later determined from length-weight relationships, which are given in Appendix A. Cast nets occasionally capture large fish (> 150 mm total length). Fish larger than 150 mm total length were excluded from the cast net analysis because they were better represented in gill net catches (see piscivorous fish section). Cast-net deployment area was estimated by throwing the net 14 times from the bow of a jon boat onto a grass surface (on land). The shape of the net after landing approximated an ellipse. The deployment area was estimated from the area formula for an ellipse (π ½ major axis length ½ minor axis length). The 14 calculated areas were averaged. Cast net sampling data were reported as fish m -2.

45 30 Fish Ingress/Egress Culverts isolated the ingress and egress of fishes to specific pathways, which provided an opportunity to quantify fish movements between coastal wetlands and the adjacent estuary using culvert traps. Culvert traps are oversized minnow traps with a dividing screen placed at the center so that the direction of movement of captured fish can be determined (Gilmore et al. 1982). Although culvert nets have been used in previous impoundment studies (e.g., Weiher 1995), such nets only capture fish moving with current. Culvert traps have the added advantage of capturing fish moving against the current. A modified version of the stainless steel culvert trap used by Gilmore et al. (1982), Rey et al. (1990a), and Taylor et al. (1998) was constructed using typical fish trap material (plastic coated wire and 0.25 cm bar mesh Vexar). The traps were 80 cm in diameter, which was slightly smaller than the diameter of the culverts (91 cm diameter) to account for fouling organism growth inside the culverts. The opening of the culvert trap was 42 cm high by 8 cm wide, and the typical gap between the culvert trap and the sides of the fouled culvert walls was 2 cm. Four culvert traps were available to document fish movements. Culvert trap sampling was conducted for four days and four nights continuously each month for one year (Table 3). One culvert trap was placed at each culvert location (culvert locations 1 4; see Fig. 10). Two culverts are present at each culvert location, but only one was sampled with the trap, the other remained unobstructed. Captured fish within the culvert traps were removed twice per day (early morning, and late afternoon) except if detrital fouling necessitated more frequent processing to keep the traps clear. Water flow through the culverts was measured with a flow meter during each nekton

46 31 collection before and after the culvert trap was installed inside the culvert. Collected fish were identified to species, and their direction of travel recorded. Fish were counted, measured (mm total length), and released in the direction they were traveling. Weights were determined from length-weight relationships. Numerical and biomass culvert trap catches were reported as fish culvert -1 hr -1. A separate study was conducted during July 2001 to determine if fished and adjacent unfished culverts differed in fish passage rates. There is one flapgate and one riserboard culvert at each culvert location (see Fig. 10), but only one is fished with a culvert trap. When a culvert trap was added to a culvert, the current through the culvert was reduced by about 50%, while current in the adjacent culvert (unfished) remained the same. To simulate this condition, both culverts at a location were fished simultaneously. One trap was fitted with an additional dividing screen to reduce the flow by 50% relative to the adjacent culvert. After fishing the culverts for two days and two nights, the trap with the added dividing screen and the unmodified trap were switched, and then fished for an additional two days and two nights. This test was repeated at each of two culvert locations (culvert location 1 and 4; see Fig. 10). A study testing the retention efficiency of the culvert traps was also conducted during July Fish were caught by cast net, fin-clipped, and added to culvert traps. The number of fin-clipped fish remaining in the traps at the end of the sets relative to the number that were added represented the retention efficiency of the traps. Approximately 20 fin-clipped fish (species common at time of collection) were added to each of two culvert traps (one trap at culvert location 1, and one trap at culvert location 4; see Fig. 10) and left for hrs. At the end of the set, the number of fin-clipped fish remaining in

47 32 the trap was counted. This procedure was repeated nine times in one culvert trap and 11 in the second trap (n = 20 replicates total). Piscivores Large transient fish within impoundments and the adjacent lagoon were sampled with gill nets to estimate the abundance of potential predators of resident and juvenile transient nekton (Table 3). Gill nets, 38 mm (1.5 inch) monofilament bar mesh, were deployed at permanent stations where the nets could be easily set, observed, and retrieved (typically near culvert locations; Fig. 11). The nets were 1.2 m high, and the depth of the water in which they were fished ranged from m. Gill nets were set in the impoundment ditch (four 10 m net sections), the impoundment creek (one 30 m net), and the Banana Creek shoreline (two 30 m nets). The nets deployed in the ditch were stretched across the ditch, those along the estuary shoreline were oriented perpendicular to the shoreline, and the net in the creek was run from culvert location 1 along the centerline of the creek perpendicular to the creek shoreline. Each net was deployed one day each month (for one year) for two to three hours between Eastern Standard Time, and checked for fish every 30 minutes during deployment. The perimeter ditch and creek gill nets were deployed, checked for fish, and retrieved using a small jon boat to minimize disturbance to the soft sediments. The shoreline gill nets were deployed, checked for fish, and retrieved by wading along the gill net. Weights of piscivorous fish were determined from length-weight relationships. Large fish (greater than about 800 mm total length) such Megalops atlanticus, Pogonias cromis, and Sciaenops ocellatus broke through the 38 mm bar mesh monofilament nets during the estuary shoreline sets and were not assessed by this method.

48 33 Attempts to use larger 50 mm bar mesh nets were thwarted by the occurrence of large alligators, which broke through the 38 mm bar mesh, but would entangle in the larger 50 mm bar mesh. All uninjured fish captured by gill net were fin-clipped, measured, and released. Fish injured during gill net capture were frozen whole and returned to the laboratory for stomach content analysis. Number and biomass data for gill net samples were reported as catch per unit effort (fish 10 m net -1 hr -1 ). Catch per unit effort was standardized to a 10 m net length by dividing the fish catch by the number of 10 m sections fished (10 m of net deployed at each ditch location, and 30 m (i.e., three 10 meter sections) of net deployed at the creek and each estuary shoreline location). Other potential predators of resident and juvenile transient nekton were piscivorous birds such as wading birds and diving birds. Piscivorous bird abundance was determined by visual surveys. Piscivorous birds within the impoundment were identified and counted by visual survey taken from the dike road between Eastern Standard Time twice during each month. Birds along the estuary shoreline adjacent to the impoundment (within 20 m of the dike) were counted separately. Bird biomass was determined from estimates of adult birds given for each species (Terres 1980). Number and biomass of piscivorous birds were reported as birds impoundment -1 d -1. Statistical Analysis Pearson s correlation was used to test for linear relationships among monthly mean animal catches and water conditions. Water conditions included in the correlation were temperature, salinity, dissolved oxygen, turbidity, and culvert water flow. Animal catches included in the correlation were fish standing stock (monthly averages of all

49 34 species combined from cast net catches), net fish ingress (monthly averages of all species combined from culvert trap catches), piscivorous fishes (monthly averages of all piscivores in gill net catches), and piscivorous birds (monthly averages of all piscivorous birds in bird counts). Also included were the monthly averages of the four most common small species (caught in cast nets and culvert traps) and the four most common species caught in gill nets. In addition to Pearson s correlation, Spearman s rank correlation was used to test for nonlinear relationships among the same variables. Fish standing stock, net fish ingress, piscivorous fish, and piscivorous bird results were plotted, and differences among months were analyzed graphically. Standing stock (cast net) data were log transformed and months pooled. Comparisons of fish density among the various habitats sampled (perimeter ditch, creek, and estuary shoreline) were then performed using analysis of variance. Significant differences were analyzed further by Tukey s pairwise comparisons. These statistical analyses were performed for resident and transient fishes separately (number and biomass). Length frequency plots were constructed for species where more than 100 individuals were collected. For net fish ingress, piscivorous fish, and piscivorous bird results, differences among habitats (pooling months) were compared with Kruskal-Wallis One-Way Analysis of Variance. Significant differences were analyzed further with Wilcoxon signed-rank tests adjusting p-values to avoid an inflated overall level of significance (p-value number of pairwise comparisons; Sokal and Rohlf 1995). For net fish migration, these analyses were performed for both resident and transient fish separately (numerical and biomass). All statistical analyses were performed with Systat 8.0 (SPSS) software.

50 Table 1. Species cited in this study. Species Common Name Species Common Name Fishes and Invertebrates* Alpheus heterochaelis snapping shrimp Hippocampus erectus lined seahorse Anchoa mitchilli bay anchovy Hippocampus reidi longsnout seahorse Arius felis hardhead catfish Jordanella floridae flagfish Bairdiella chrysoura silver perch Lagodon rhomboides pinfish Brevoortia smithi yellowfin menhaden Leiostomus xanthurus spot Brevoortia patronis gulf menhaden Lepisosteus platyrhincus Florida gar Callinectes sapidus blue crab Lepomis macrochirus bluegill Centropomis undecimalis common snook Lucania parva rainwater killifish Chasmodes bosquianus striped blenny Megalops atlanticus tarpon Chilomycterus schoepfi striped burrfish Menidia peninsulae tidewater silverside Caranx hippos crevalle jack Microgobius gulosus clown goby Cynoscion nebulosus spotted seatrout Micropogonias undulatus Atlantic croaker Cyprinidon variegatus sheepshead minnow Mugil cephalus striped mullet Dasyatis sabina Atlantic stingray Palaemonetes pugio grass shrimp Diapterus auratus Irish pompano Penaeus aztecus brown shrimp Elops saurus ladyfish Pogonias chromis black drum Eucinostomus argenteus spotfin mojarra Poecilia latipinna sailfin molly Floridichthys carpio goldspotted killifish Sciaenops ocellatus red drum Fundulus confluentus marsh killifish Sphoeroides nephelus southern puffer Fundulus grandis gulf killifish Strongylura marina Atlantic needlefish Fundulus heteroclitus mummichog Strongylura notata redfin needlefish Gambusia holbrooki eastern mosquitofish Symphurus plagiusa blackcheek tonguefish Gobiosoma bosc naked goby Syngnathus scovelli gulf pipefish Gobiosoma robustum code goby Trinectes maculatus hogchoker 35

51 Table 1. Continued Birds Ajaia ajaja roseate spoonbill Hydranassa tricolor Louisiana heron Anhinga anhinga American anhinga Megaceryle alcyon belted kingfisher Ardea herodias great blue heron Mycteria americana wood stork Butorides striatus green heron Nycticorax nycticorax black-crowned night heron Casmerodius albus great egret Pandion haliaetus osprey Dichromanassa rufescens reddish egret Pelecanus erythrorhynchos white pelican Egretta thula snowy egret Pelecanus occidentalis brown pelican Eudocimus albus white ibis Phalacrocorax auritus double-crested cormorant Florida caerulea little blue heron Plegadis falcinellus glossy ibis Himantopus mexicanus black-necked stilt Reptiles Alligator mississipiensis American alligator Malaclemys terrapin diamondback terrapin 36 Mammals Dasypus novemcinctus armadillo Sus scorfa feral hogs Lynx rufus floridanus bobcat Trichechus manatus manatee Procyon lotor raccoon Vegetation Avicennia germinans Black mangrove Paspalum vaginatum knotgrass Batis maritima saltwort Rhizophora mangle Red mangrove Distichlis spicata saltgrass Spartina alterniflora smooth cordgrass Laguncularia racemosa White mangrove Spartina bakerii bunch cordgrass *American Fisheries Society official common names

52 Table 2. Terms used to describe various places, habitats, and species. Term Definition Salt marsh/impoundment Indian River Lagoon estuary Resident species Dominant area located behind the estuary/marsh berm, or impoundment dikes. Includes the vegetated marsh surface, creeks that penetrate the marsh plane, marsh islands, and perimeter ditches if the marsh is impounded. Indian River Lagoon, Banana River, Banana Creek, and Mosquito Lagoon Includes open water areas, seagrass flats, intracoastal waterway, and shoreline up to the estuary/marsh berm Species that complete their life cycle within the salt marsh. Transient species Species that use salt marshes during some portion of their life cycle either as nursery habitat for young or as regular feeding areas as adults. 37 Incidental species Saltmarsh fishes Species resident to the estuary and occasionally occur in salt marshes by accident, or as a negligable extension of their regular habitat (usually seagrassassociated species). Collection of resident, transient, and incidental fishes within the salt marsh.

53 Table 3. Summary of monthly gear deployments. Number of Deployments ( ) Biomass Budget Gear Type and Habitat Parameter Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Total Standing Cast net - drainout creek Stock Cast net - perimeter ditch Cast net - estuary shoreline Cast net - marsh surface Imigration/ Culvert trap - days * * Emigration Culvert trap - nights * Fish Predation Gill net - drainout creek Gill net - estuary shoreline Gill net - perimeter ditch Bird Predation Morning bird counts *Limited by security concerns within NASA (i.e., shuttle launches)

54 Fig. 8. Location of study site (Banana Creek) within the northern Indian River Lagoon system 39

55 40 Vehicle Assembly Building Shuttle Landing Facility Estuary shoreline Banana Creek Drainout Creek Impoundment C20C Impoundment C20A Fig. 9. Map of Impoundment C20C within Kennedy Space Center.

56 41 Banana Creek Highway 3 Dike Road Drainout Creek Boardwalk to NASA lights F R F R 1 F R 3 2 Impoundment C20C F R 4 R Creek Impoundment C20A R Legend Dike Road Creek/Ditch Knotgrass Marsh/Shallow Ponds White Mangrove/Brazilian Pepper Uplands/High Wetlands Cattail Marsh R F R Flapgated Culvert Riserboard Culvert n Culvert Location Datasonde Location Fig. 10. Map of Impoundment C20C showing vegetation types, culvert locations, and datasonde locations.

57 42 Estuary Drainout Creek Ditch Ditch Estuary Ditch Fig. 11. Gill net deployment locations within Impoundment C20C. Lines indicate orientation of gill nets.