TEEJAY ALEXANDER O REAR B.S. (University of California, Davis) 2007 THESIS MASTER OF SCIENCE. Ecology. In the OFFICE OF GRADUATE STUDIES.

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1 Diet of an Introduced Estuarine Population of White Catfish in California By TEEJAY ALEXANDER O REAR B.S. (University of California, Davis) 2007 THESIS Submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE in Ecology In the OFFICE OF GRADUATE STUDIES Of the UNIVERSITY OF CALIFORNIA DAVIS i

2 Approved: Dr. Peter B. Moyle, Chair Dr. Sharon P. Lawler Dr. Eliska Rejmankova Committee in Charge 2012 ii

3 Abstract Unlike other trophic groups, invasive top predators are often able to establish new populations, with serious consequences for the invaded ecosystem. For example, introduced populations of large catfishes have harmed populations of native fishes through predation. One large catfish species that has been widely introduced outside its native range that has not been assessed for predation effects on native fishes is the white catfish (Ameiurus catus). Of particular concern is the increasingly abundant white catfish population in Suisun Marsh, a brackish-water network of tidal sloughs in the San Francisco Estuary that is vital habitat for declining native fishes, as well as species that support fisheries. To address this issue, I examined the diet of large juvenile and adult white catfish over a year. I found that they mainly ate abundant amphipods and either introduced fishes or native fishes that are widespread and abundant. The summer diet was mainly comprised of species that are produced within the marsh's sloughs or in downstream bays. However, much of the autumn and spring diet was related to management of Suisun Marsh's large acreage of managed wetlands. These managed wetlands both (1) produced abundant invertebrates and fishes that were discharged into the tidal sloughs inhabited by white catfish, and (2) contributed to hypoxic conditions within the sloughs that allowed white catfish to feed on fishes and shrimp killed or rendered immobile by the low oxygen levels. The diets revealed that white catfish present little threat to at-risk fishes. However, their heavy use of food items produced by or affected by managed wetlands may make them dangerous for human consumption iii

4 since the managed wetlands contribute to the methylation of mercury. This potential risk to fishermen should be promptly assessed. iv

5 ACKNOWLEDGEMENTS A very special thanks to Peter Moyle, my major professor, who stuck with me through thick and thin. Without your prodding, insight, trust, and guidance, this project would not have been possible. I am indebted to Alpa Wintzer, who so effectively and efficiently taught me how to manage the Suisun Marsh Fish Study, from which this project was undertaken. Thanks to the following folks who hung in there with me for countless hours under the uncaring sun of the marsh while I processed (read: barfed) white catfish: Natalie Craig, Kin Lam, Matt Young, Priya Shukla, Amy Chandos, Karin Petrites, Nick Buckmaster, Anna Steel, and Mike Wigginton. Big-time thanks to Dr. Robert Schroeter, who has so often opened my eyes to marshy things I never suspected. I owe the same gratitude to Steve Chappell and Orlando Rocha of the Suisun Resource Conservation District for enlightening me on so many things that happen on the landward side of the marsh's dikes. Thanks also to Ted Sommer, Chris Enright, Kate Le, Bill Burkhard, Randall Brown, Laura Bermudez, Terri Fong, and all others at the California Department of Water Resources who have continued to champion and fund the Suisun Marsh Fish Study, without which this study would have been nearly impossible. And, finally, none of this would have been within reach without the understanding and selfless support of my former girlfriend Amber Manfree - thank you. v

6 1 INTRODUCTION Invasions by non-native species, by actions of humans, has become a serious problem affecting all types of ecosystems worldwide. Aquatic ecosystems have been especially susceptible due to myriad modes of introduction: ballast-water discharges, legal and illegal introductions for sport or commercial fishing, fish-farm escapees, release of aquatic pets into the wild, escape and survival of animals used for bait, and interdrainage water exchanges (Gido and Brown 1999, Moyle 2002). Nevertheless, many organisms introduced to aquatic ecosystems fail to become permanent members of the invaded system with the exception of organisms that are either omnivores or top predators (Moyle and Light 1996). These two types of organisms are generally able to establish populations because of the often unlimited or naive food available in the invaded ecosystem (Moyle and Light 1996). In the case of top predators, their establishment and integration into the new ecosystem can result in declines or extinctions of native organisms (Moyle 2002). Large predatory catfishes of the genera Pylodictus, Ictalurus, and Ameiurus have been introduced widely outside their native range, especially into the western United States (Page and Burr 1992, Dill and Cordone 1997). Most of these introductions have been intentional for sport fishing, commercial fishing, or other purposes; many of these populations are now large, self-sustaining, and support important fisheries (e.g., Schaffter 1997). A negative effect of these introductions has been reduction of native fish populations through predation. For instance, illegally introduced flathead catfish (Pylodictus olivaris) have decimated redbreast sunfish (Lepomis auritus) and native bullhead (Ameiurus spp.) populations in the Altamaha River, Georgia (Thomas 1993,

7 2 Weller and Robbins 1999). Declines in green sunfish (Lepomis cyanellus) and smallersized ameiurid catfishes in the Cape Fear River, North Carolina, occurred concurrent with introduction and establishment of flathead catfish (Guier et al. 1981) due to predation (Quinn 1987, Weller and Robbins 1999). Introduced populations of channel catfish (Ictalurus punctatus) and flathead catfish are a major impediment to restoration of razorback suckers (Xyrauchen texanus) in the Gila River, Arizona, through predation on reintroduced razorback sucker juveniles (Marsh and Brooks 1989). Small channel catfish have also been found to prey on larval suckers (Carpenter and Mueller 2008). Channel catfish have additionally been documented to consume large amounts of outmigrating salmonid (Oncorhynchus spp.) smolts in the tailraces of dams in the Columbia River (Poe et al. 1991, Vigg et al. 1991). A large catfish species whose predation effects are poorly understood is the white catfish (Ameiurus catus), even though it is abundant in the San Francisco Estuary and other habitats in California where native fishes are in decline (Moyle 2002). In 1874, either 56 or 74 white catfish from the Raritan River in New Jersey were introduced to the San Joaquin River near Stockton (Dill and Cordone 1997). From this stock, white catfish spread throughout the Sacramento-San Joaquin watershed, inhabiting rivers, sloughs, reservoirs, and brackish-water estuaries (Moyle 2002). White catfish have also colonized reservoirs and waterways in southern California via the state's intricate water-supply conveyance system, in addition to being introduced to a number of other waterways (e.g., the Russian River and its tributaries, Ruth Reservoir on the Mad River in northern California, the Eel River, and Clear Lake). In Clear Lake, Lake County, the proliferation of white catfish occurred concomitant with declines in native fishes such as hitch

8 3 (Lavinia exilicauda; Dill and Cordone 1997), on which white catfish had been found to feed (Miller 1966). Recently, an increase in white catfish numbers in Suisun Marsh, the central part of the San Francisco Estuary, has happened concurrent with decreases in native fishes such as longfin smelt (Spirinchus thaleichthys) and delta smelt (Hypomesus transpacificus; O'Rear and Moyle 2009, 2010), which have also been recorded as prey for white catfish (Turner 1966). However, it is unknown if white catfish predation has contributed to these changes. Suisun Marsh represents some of the best habitat remaining to declining, commercially important species such as striped bass (Morone saxatilis), as well as to native fishes that are of special concern such as Sacramento splittail (Pogonichthyes macrolepidotus) and fishes listed under the federal and/or California Endangered Species Act such as longfin smelt and delta smelt (Lund et al. 2007). Nevertheless, the value of Suisun Marsh as habitat for important fishes could be compromised if they are preyed upon heavily by a large white catfish population. Past studies of white catfish diets in California, which focused on freshwater populations, found white catfish to commonly be benthic generalist carnivores that prey or scavenge on the most available animal taxa, with larger white catfish [i.e., those longer than 20 cm total length (TL)] being more piscivorous than smaller individuals. Miller (1966) reported that white catfish from Clear Lake fed on a wide variety of fishes [hitch (Lavinia exilicauda), sculpins (Cottus spp.), bluegill (Lepomis macrochirus), tule perch (Hysterocarpis traski), black crappie (Pomoxis nigromaculatus), and common carp (Cyprinus carpio)], in addition to frogs, aquatic insects, and clams. However, Mississippi silversides became the dominant food item of Clear Lake white catfish after

9 4 the silversides were introduced for gnat control (Moyle 2002). In the Delta, Miller (1966) found that white catfish fed primarily on invertebrates (amphipods, mysid shrimp, and chironomids) and that fishes made up only a minor portion of the diet. Ganssle (1966) had similar results for white catfish caught in Suisun Bay: amphipods and mysids dominated the diet. White catfish captured in the 1950s from the southern Delta, most of which were adults, fed heavily on Americorophium amphipods (Borgeson and McCammon 1967). However, adults taken in 1954 from the Sacramento River near Verona concurrent with the spawning run of American shad (Alosa sapidissima) contained substantial amounts of American shad parts (Borgeson and McCammon 1967). Young-of-year white catfish captured in 1963 and 1964 from the southern Delta had diets dominated by Americorophium spp. (Turner 1966). Minor food items differed depending on where the white catfish were caught. For example, the opposum shrimp Neomysis mercedis was a relatively important food in the western Delta during summer and fall, while chironomid larvae and pupae were fed on in the southern Delta in the same seasons (Turner 1966). While invertebrates (e.g., Americorophium) continued to be an important food for one-year-old and older white catfish, fish became increasingly important for white catfish larger than 20 cm fork length (Turner 1966). The fishes eaten were threadfin shad (Dorosoma petenense), American shad, striped bass, delta smelt, bluegill (Lepomis macrochirus), river lamprey (Lampetra ayresi), and Pacific herring (Clupea harengus); herring are commonly used as fish bait and consequently were scavenged (Turner 1966). Fishes were eaten throughout the year, although they were most frequently found in diets during summer (Turner 1966), coinciding with the population peaks for the preyed-upon fishes. Similarly, white catfish longer than 20 cm TL ate

10 5 almost exclusively threadfin shad in a California water-supply reservoir during summer, while immature chironomid midges were important during the cooler seasons (Goodson, Jr. 1965). The diets of white catfish in the estuary have not been examined extensively since the 1960s, and since that time declines of many native species have occurred with increases in introduced species such as largemouth bass (Micropterus salmoides; Lund et al. 2010). Although trends in white catfish populations throughout the San Francisco Estuary are not fully known, they have continued to support a substantial fishery in the Delta and Suisun Marsh and have shown a long-term increase in Suisun Marsh from the early 1990s (Matern et al. 2002, O'Rear and Moyle 2010). Consequently, elucidating the diet of white catfish in Suisun Marsh is an important first step towards determining their potential threat to native fishes. To that end, this study addressed the following interrelated questions: Do the diets of white catfish in Suisun Marsh differ substantially from those of the Delta in the 1960s? Does the diet of white catfish track shifting prey abundances related to changes in abiotic factors (e.g., salinity, temperature)? Does the diet of white catfish in Suisun Marsh follow ontogenetic patterns found in previous studies (i.e., are fish a more important prey for larger white catfish than for smaller individuals)? Are white catfish important as predators on native or declining fish species?

11 6 Although the exact species of likely prey for white catfish in Suisun Marsh differs from that of the Delta, representatives of each of the important food categories found in the Delta - fishes, amphipods, and mysids - are present in Suisun Marsh and occupy the same ecological roles as those in the Delta (R.E. Schroeter, pers. comm., Carlson and Matern 2000, Peterson and Vayssieres 2010). Because most fishes that rear in the marsh spawn during spring (e.g., striped bass, yellowfin goby, delta smelt, prickly sculpin; Meng and Matern 2001), small fishes that could serve as forage (e.g., young-of-year striped bass) for white catfish are most abundant during summer (Moyle et al. 1986, Meng et al. 1994, Meng and Matern 2001, Matern et al. 2002, O'Rear and Moyle 2011). Conversely, mysids are generally most abundant during late winter and spring (Moyle et al. 1986, O'Rear and Moyle 2011) while amphipods - mainly gammaroids and corophiids - are usually available throughout much of the year (Batzer and Resh 1992). Thus, based on the above, I tested several hypotheses: White catfish diets in Suisun Marsh are similar to those in the Delta and have not changed much over the last 50 years, with mysids and amphipods important in the cooler seasons and fish most important during summer. The major prey species of white catfish in Suisun Marsh change with environmental conditions, as was the case in the Delta with fishes and mysids (Turner 1966). As white catfish grow, the importance of fish in the diet increases, with the proportion of the diet comprised of fish rising linearly with length of white catfish after the onset of piscivory. This is a common pattern found in a wide range of other fishes (e.g., ictalurids: Minckley and Deacon 1959

12 7 Busbee 1968, Keast 1985, Poe et al. 1991, Hill et al. 1995, Weller and Robbins 1999, Edds et al. 2002; moronids: Rulifson and McKenna 1987, Nobriga and Feyrer 2008; salmonids: Frantz and Cordone 1970, Nowak et al. 2004). Given the abundance of native and commercially important fishes in Suisun Marsh and the documented predation by white catfish on these species (Miller 1966, Turner 1966), native and commercial fishes, especially young striped bass that can be extremely abundant in Suisun Marsh, are commonly preyed upon by white catfish in the marsh. METHODS Study Area Suisun Marsh is a tidal brackish-water marsh covering about 34,000 hectares (California Department of Water Resources 2001). Roughly two-thirds of the marsh area is diked wetlands managed for waterfowl; the remainder consists of sloughs that separate and deliver water to the wetland areas and undiked marsh plains (California Department of Water Resources 2001). The marsh is contiguous with the northern boundary of Suisun, Grizzly, and Honker bays and is central to the San Francisco Estuary (Figure 1). There are two major tidal channels in the marsh: Montezuma and Suisun sloughs (Figure 1). Montezuma Slough generally arcs northwest from the confluence of the Sacramento and San Joaquin rivers, then curves southwest and terminates at Grizzly Bay (the major embayment of Suisun Bay). Major tributary sloughs to Montezuma are Denverton and Nurse; Cutoff Slough and Hunter Cut connect Suisun and Montezuma sloughs (Figure 1). Suisun Slough begins near Suisun City and meanders south until

13 8 emptying into Grizzly Bay southwest of the mouth of Montezuma Slough. Major tributaries to Suisun Slough, from north to south, are Peytonia, Boynton, Cutoff, Wells, Cordelia, and Goodyear sloughs (Figure 1). First and Second Mallard sloughs are tributary to Cutoff Slough and are part of Solano Land Trust's Rush Ranch Open Space preserve. Suisun and Montezuma sloughs are generally meters (m) wide, 3-7 m deep, and partially riprapped (Meng et al. 1994). Tributary sloughs are usually m wide, 2-4 m deep, and fringed with common reed (Phragmites australis) and tules (Schoenoplectus spp.). Substrates in all sloughs are generally fine organics, although a few sloughs also have bottoms partially comprised of coarser materials (e.g., Denverton Slough), and the larger, deeper sloughs (e.g., Montezuma Slough) can have sandy channel beds. The amount of fresh water flowing into Suisun Marsh is the major determinant of its salinity. Fresh water enters the marsh primarily from the Sacramento River through Montezuma Slough, although small creeks, particularly on the northwest and west-central sides of the marsh, also contribute fresh water. As a result, salinities are generally lower in the eastern, northwestern, and west-central portions of the marsh. Freshwater inflows are highest in winter and spring due to rainfall runoff and snowmelt in the Sacramento and San Joaquin hydrologic regions; consequently, marsh salinities are lowest in these seasons. Salt water enters the marsh through lower Suisun and lower Montezuma sloughs from Grizzly Bay via tidal action. In general, minimum whole-marsh salinities average about 1.8 ppt and maximum whole-marsh salinities average about 5.5 ppt, with some sloughs, particularly on the northwest and eastern sides of the marsh, being wholly

14 9 fresh throughout winter and spring (O'Rear and Moyle 2011). During extreme tides, water depths can change as much as 1.7 m over a tidal cycle, often dewatering more than 50% of the smaller sloughs at lower low tide and overtopping dikes during high spring tides. Water currents are generally moderate in the sloughs, with speeds approaching 2 feet per second during strong ebb tides. A number of water management facilities and their operation influence hydrology and water quality of the marsh. State Water Project and Central Valley Project water export facilities in the southern Delta affect the timing and magnitude of freshwater inflow into Suisun Marsh. The Suisun Marsh Salinity Control Gates, located in Montezuma Slough just downstream of the confluence of the Sacramento and San Joaquin rivers, are operated to inhibit saltwater intrusion into the marsh during flood tides, which provides fresher water for managed wetlands (California Department of Water Resources 2001; Figure 1). Goodyear Slough is now connected to Suisun Bay by a channel that was built to depress salinities in the slough for water diverters in the southwestern portion of the marsh. Numerous water control structures, most of which are unscreened for fish, are located in dikes protecting managed wetlands throughout the marsh. They are opened in early autumn for flooding the wetlands to attract wintering waterfowl and again in late and winter and spring to leach salts from wetland soils and to set seeds of desired plants (Figure 2). Managed wetlands are usually flooded to at least some extent from autumn to spring and are dry during summer to allow for maintenance. Managed-wetland drainage water is discharged directly into adjacent sloughs. At the onset of flood-up and for about the following three weeks, dissolved oxygen (DO) is either very low or absent in some managed-wetlands drainage water; consequently,

15 10 sloughs receiving that water can have hypoxic or anoxic conditions (Schroeter and Moyle 2004, O'Rear et al. 2009, O'Rear and Moyle 2011, Siegel et al. 2011). Mild DO depressions also occur during springtime, although levels rarely fall below that required for most marsh fishes (i.e., 5 mg/l; Siegel et al. 2011). Ponded areas of the managed wetlands are shallow, have abundant aquatic vegetation, and are lentic. Most sloughs in the marsh are diked and thus separated from marsh plains to at least some extent. Diking has been most extensive in the western marsh, with concurrent water management and its effects (e.g., hypoxia or anoxia) especially intense in the smaller sloughs (e.g., Boynton Slough; Siegel et al. 2011). The northeastern marsh's sloughs are bordered by a mosaic of diked and undiked wetlands and consequently appear less affected by water-management activities than sloughs of the western marsh (O'Rear and Moyle 2011). Sloughs that enter the north side of Cutoff Slough (e.g., First Mallard Slough) are entirely undiked and are generally unaffected by water management in other regions (e.g., the northwest marsh) during most conditions. Both Montezuma and Suisun sloughs are diked throughout almost their entire lengths, except for small marshes along the eastern part of Montezuma Slough, along the river-left bank of upper Suisun Slough, and along the mouths of both sloughs where they empty into Grizzly Bay. Most diking was completed in the late 1800s and early 1900s, with the current geographic configuration of the marsh in place by about the 1930s. Sampling Field Sampling White catfish were captured by otter trawling as part of the standard monthly sampling of the Suisun Marsh Fish Study (O'Rear and Moyle 2011); those from June

16 through May 2010 were used for diet analysis. Trawling was conducted monthly in nine sloughs (Figure 1) throughout the marsh using a four-seam otter trawl with a 1.5 m X 4.3 m opening, a length of 5.3 m, and mesh sizes of 35-mm stretch in the body and 6- mm stretch in the cod end. The otter trawl was towed at 4 km/hr for 10 minutes in large sloughs (to compensate for small catches) and 5 minutes in small sloughs. For each site, temperature (degrees Celsius, C), salinity (parts per thousand, ppt), and specific conductance (microsiemens, µs) were recorded with a Yellow Springs Instruments (YSI) 85 meter. Dissolved oxygen parameters (milligrams per liter, mg/l, and % saturation) were also measured with the YSI 85. Water transparency (Secchi depth, cm), tidal stage (ebb, flood, high, low), and water depths (m) were recorded as well. White catfish were immediately placed into large containers of water after being removed from the trawl. Each fish was measured for standard length and weighed on an Acculab EC-411 scale to the nearest gram. Gastric lavage was then performed on fish larger than 160-mm standard length (SL) with the use of a deck hose, a series of three valves (for fine-tuned control of water pressure), and a 6.4-mm-diameter flexible copper tube measuring 20-cm long and capped with 30 mm of surgical tubing to protect the catfish's epithelial tissue. Gastric lavage has been used previously with success for other species of large catfish [e.g., flathead catfish: Haas et al. 2001, Pine, III, et al. 2005; blue catfish (Ictalurus furcatus) and flathead catfish: Waters et al. 2004]. For each catfish larger than 160-mm SL, the copper tube was gently inserted into the stomach, the water turned on, and the contents washed out and onto a filter. Each fish was lavaged until no other gut contents were expelled, at which point forceps were used to remove from the throat any item that became lodged during lavage. The fish was then tagged with a

17 12 numbered or colored spaghetti tag inserted just below the dorsal fin on the left side of the body, with the needle first sterilized in a hydrogen-peroxide solution. The catfish was then released. The gut contents were then washed from the filter through a funnel into a glass jar, were preserved in a 10% formalin solution, and were stained with rose Bengal to aid in identification. To verify the complete removal of diet items from catfish stomachs by gastric lavage, a subsample of gastric-lavaged fish was killed; their stomachs were removed, labeled, and preserved in 10% formalin; and their contents were identified in the laboratory. All such stomachs contained no diet items (n = 6). Fish smaller than 160-mm SL were immediately killed with a sharp blow to the base of the skull, and their digestive tracts were excised, wrapped in cheesecloth, labeled, and preserved in 10 % formalin for later laboratory analysis. Laboratory Procedures Once in the laboratory, each gut-content sample was poured through a 250-µm mesh filter to remove the formalin solution and then soaked in water for 24 hours to leach any remaining formalin out of the gut contents. The water was then decanted, and the sample put through another water soak for one hour. Thereafter, the contents were removed from the filter and placed into a Petri dish. Animals except bryozoans were sorted and identified to the lowest possible taxonomic grouping, counted, blotted dry, and weighed to the nearest g with an AT201 Mettler balance. If only parts from an animal were present, they were not used to estimate the mass of the entire animal since such an approach would have to assume that white catfish were only eating live prey, whereas previous studies have documented scavenging by this species (Turner 1966, Borgeson and McCammon 1967). Masses of fishes and crustaceans not identifiable to

18 13 species were partitioned into species categories according to the proportions of the species present in either the rest of the gut contents, in white catfish captured in the same trawl as the sample fish, or in white catfish captured from nearby sloughs that exhibit similar water-quality characteristics and biotic communities (e.g., Boynton and Peytonia sloughs; Matern et al. 2002, O'Rear and Moyle 2009). Algae and bryozoans were isolated, blotted dry, and then weighed, but no counts were possible. Rocks, sand, and plant debris (e.g., fragments of rhizomes from tules) were sorted, blotted dry, weighed, and were combined as "debris." Debris values were not included in any of the analyses based on the assumption that such material contributes no food value to white catfish. Invertebrates were identified via Carlton (2007), Merritt et al. (2008), and Pennak (1978), while vertebrates were identified following Wang (1986) and Moyle (2002). Data Analysis To address the hypothesis that amphipods and mysids were the most important diet items of white catfish during autumn, winter, and spring, while fish dominated the diet in summer, standardized masses for fish, amphipods, mysids, the green alga Enteromorpha spp., and "other" (all other diet items except debris) were summed for each season (December - February = "winter," March - May = "spring," June - August = "summer," and September - November = "autumn"; Turner 1966) and graphed. Standardized mass is the mass of each diet item divided by the mass of the white catfish that had eaten it. To compare diets between white catfish in the freshwater Delta and those in the brackish marsh, two methods were used. First, the proportion of the total standardized diet mass for each of the categories except Enteromorpha was calculated for the entire Suisun Marsh sample; these values were then graphed with the proportions of

19 14 the volumes for the same categories for Delta white catfish (Turner 1966) and compared. While several monitoring programs have tracked changes in the fish and invertebrate assemblages of the San Francisco Estuary since at least the 1970s [e.g., the California Department of Fish and Game's Fall Midwater Trawl Survey (Feyrer et al. 2007), the California Department of Water Resources's benthic surveys (Peterson and Vayssieres 2010)], and diet studies have been performed within the period of those monitoring programs for estuary fishes [e.g., Suisun Marsh fishes (Feyrer et al. 2003); striped bass, largemouth bass, and Sacramento pikeminnow (Ptychocheilus grandis; Nobriga and Feyrer 2007)], none of these programs has been complemented with diet studies of white catfish, making comparisons between my study and studies of estuary white catfish after the 1960s impossible. Consequently, the Turner study was used because it was the most comprehensive, detailed, and recent diet study of white catfish in the San Francisco Estuary. Although mass was measured in my study while volume was measured in Turner's study, both measurements assess the bulk contribution of a diet item and are therefore generally comparable (Hyslop 1980). Second, frequency-of-occurrence values for mysids, fishes, and amphipods were calculated via the equation Frequency of Occurrence = number of fish containing diet item x number of fish with gut contents for each season in Suisun Marsh and then graphed with seasonal frequency-of-occurrence values for the same groups taken from white catfish sampled in the Delta in 1963 and 1964 (Turner 1966). Seasonal proportions of volume or mass between the Delta and

20 15 Suisun Marsh were not calculated because seasonal volume or mass values for the Delta white catfish were not given (Turner 1966). To describe factors influencing the diet of white catfish through the year, a suite of abiotic variables was related to variation in the standardized diet-item masses: salinity, temperature, dissolved oxygen (DO), turbidity (as measured by Secchi depth), mean depth of slough, and number of managed-wetlands diversions per slough-kilometer (d/skm). The number of d/skm is a metric roughly describing the extent and type of connectivity the sloughs have to the marsh plain. Those sloughs with higher d/skm have lower connectivity to the adjacent marsh plains because connections occur only during activities associated with managed-wetlands maintenance. Sloughs with low or no diversions are connected during high tides that flood the marsh plain. Because of their different hydroperiod, lentic conditions, and large amount of vegetative cover, managedwetlands ponds tend to have a distinctive fauna (Batzer and Resh 1992, de Szalay and Resh 1996) that could subsidize the diet of white catfish that reside in sloughs. Standardized masses of diet items were used instead of indices that combine frequency of occurrence, mass or volume, and numerical abundance because of the inability to effectively determine the unit of the individual for some groups of important diet items (i.e., algae and bryozoans; Bowen 1996). The standardized-diet and abiotic data were ordinated with nonmetric multidimensional scaling (NMDS). This method was chosen over other ordination techniques (e.g., principal components analysis or canonical correspondence analysis) because (1) the diet data contained many zeros, (2) some of the relationships among the variables were not linear, and (3) distributions of most variables were not Gaussian

21 16 (McCune and Grace 2002, Zuur et al. 2007). NMDS has been frequently used successfully in other fish diet studies (e.g., Nelson et al. 2003, McHugh et al. 2008). NMDS runs were performed in PC-ORD Version 5 using a random configuration and Sorenson distance. Fifty runs were conducted each on the real data and randomized data. Once dimensionality was selected, the data were re-run with a maximum number of 200 iterations to acquire the ordination that best expressed the structure in the original data matrices. Only diet items that were in more than 1% of the white catfish sampled or were among the most abundant diet items contributing at least 95% of the standardized dietitem masses for at least one month were included in the ordination. To address the hypothesis that fish become more important in the diet as white catfish increase with size, two approaches were used. First, a two-way analysis of variance (ANOVA), with season and whether white catfish had eaten other fish or not as the two factors, and standard length as the response variable, was performed to see if white catfish that had eaten fish were significantly longer than those that had not eaten fish. Using season as a factor was necessary because the average size of white catfish varied with time. Two catfish were excluded from the analysis - these were the smallest juveniles and were likely from younger year-classes than the other fish. They were also not used in any other analyses since their guts were empty. Standard length was Box- Cox transformed prior to analysis in order to normalize residuals. After transformation, validity of model assumptions was assessed by examining various scatterplots of the residuals for linearity and heteroscedasticity, testing for autocorrelation via the Durbin- Watson test, and testing for normality with a goodness-of-fit test; no violations of model assumptions were found. Second, multiple linear regression was used, with standard

22 17 length and those abiotic variables from the NMDS analysis that were correlated with the axis to which fishes in the diet was correlated as the predictor variables, and with the proportion of the diet consisting of fish for white catfish that had eaten fish (excluding white catfish that had eaten no fish was necessary since the high number of zeroes would have violated the assumptions of such a parametric approach) as the response variable. Assessment of model assumptions was undertaken as for the previous analysis; no deviations from the assumptions were found. Both analyses considered all possible interactions since they appeared plausible and were carried out using JMP Version 8.0. To assess the influence of the abundance of prey fishes in the environment on the presence of prey fishes in the white-catfish gut contents, otter-trawl catch-per-unit-effort (CPUE) values were calculated for each season and for each major prey fish and graphed with the number of each prey fish per each white catfish sampled for those seasons. Additionally, seasonal CPUE values for the prey fishes were compared to the number of prey fishes per white catfish sampled in each respective season with Pearson's productmoment correlation coefficients; this analysis was also carried out in JMP Version 8.0. RESULTS A total of 304 white catfish were sampled from June 2009 through May Sample sizes were greater than 28 fish for all seasons. The majority of white catfish were captured in sloughs of the northwest and eastern marsh, reflecting the species's association with fresher water (O'Rear and Moyle 2011). Most of the white catfish (85%) were adults (i.e., fish longer than 182 mm SL). No young-of-year white catfish (SL < 75 mm caught after June) and probably only one yearling (SL < 110 mm ) and one two-year-

23 18 old fish were captured (Table 1). Consequently, these results and the following discussion are only applicable to larger juveniles and adult white catfish. Of the 304 white catfish sampled, 10% (n = 30) had empty stomachs. Of the 274 fish with material in their stomachs, 96% (n = 263) had eaten material other than debris. Sixty-one white catfish (20%) had consumed other fish; none of the fishes eaten were sport, commercial, or at-risk species. A total of 60 different diet items were consumed by white catfish, with 24 being abundant enough to be included in the NMDS ordinations (Table 2). A total of 259 white catfish were used in the NMDS ordination; the remaining four did not contain any of the 24 major diet items. Contrary to the hypothesis that amphipods and mysids would dominate the diet in winter, spring, and autumn while fish would be most important in summer, amphipods comprised a substantial portion of the diet in all seasons except autumn, mysids were never an important component, and fishes were eaten mostly in spring and, especially, autumn (Figure 3). The species of amphipods changed in the diet through the seasons, with E. confervicolus dominant in winter and spring, A. spinicorne most important in spring and summer, and Gammarus daiberi and talitrids most abundant during summer (Table 3). Fishes were the dominant food in autumn, especially in October when DO levels were very low in sloughs of the northwest marsh where many of the white catfish that month were captured (Figure 3). Fishes also comprised a considerable proportion of the diet during springtime; two of the species threespine stickleback (Gasterosteus aculeatus) and prickly sculpin (Cottus asper) breed within the managed wetlands and are discharged into the sloughs during leaching cycles and water-recirculating periods (Batzer and Resh 1996, California Department of Fish and Game 1996, 1997, 1998;

24 19 O Rear and Moyle 2011). Additionally, shimofuri gobies (Tridentiger bifasciatus) were also eaten; they have been entrained into the managed wetlands (California Department of Fish and Game 1996, 1997, 1998). Other diet items were important at various times of the year. Several taxa associated with wetlands for example, Daphnia magna and corixids contributed to the diet in spring (Table 3). Daphnia magna was also important in autumn, as was Enteromorpha spp.; nevertheless, nearly 60% of the total standardized mass in autumn was comprised of fish and the large introduced shrimp Siberian prawn (Exopalaemon modestus). Enteromorpha spp. was also a substantial component of the diet in summer (Figure 3), as was the isopod Gnorimosphaeroma insularae, although amphipods were still a major food, especially the species A. spinicorne and G. daiberi (Table 3). Diets of white catfish from Suisun Marsh and those from the Delta were similar in some respects and different in others, which is not surprising given the differences between the two regions. The frequency at which both populations fed upon amphipods and mysids through the seasons was remarkably consistent, with the exception of mysids being preyed upon less frequently by white catfish in the marsh during autumn (Figure 4). However, fishes were more commonly eaten by white catfish in the marsh than in the Delta, especially in spring and autumn (Figure 4). Conversely, fishes contributed nearly two times the amount by bulk to the diet of white catfish in the Delta compared to white catfish in the marsh (Figure 5). The lower contribution of fishes by mass in the diet of marsh white catfish was nearly totally compensated for by a concomitant increase in amphipod biomass (Figure 5). When the frequency-of-occurrence values and volumes/masses of the two populations are considered together, it appears that white

25 20 catfish in Suisun Marsh had eaten smaller fish and larger amphipods than those eaten by white catfish in the Delta. Diet items contributing to the other category between the two regions were disparate. Signal crayfish (Pacifastacus leniusculus) and Asian clams (Corbicula fluminea) were substantial in the diet of Delta white catfish, while Enteromorpha spp., D. magna, and earthworms were eaten in considerable amounts in Suisun Marsh. A three-dimensional ordination was determined to best represent the structure in the original datasets for the following reasons: (1) the probability of a run with randomized data resulting in lower stress than a run with the real data increased substantially with more than three axes, (2) Monte Carlo runs with randomized data resulted in stress lower than that for the real data less than 2% of the time, and (3) there was a considerable reduction in stress going from two to three axes. The final threedimensional solution had a rather high stress of 20.3, suggesting weak relationships within the data, although stability was satisfactory ( ). Axes one, two, and three explained 16%, 17%, and 14% of the variation found in the original dataset, respectively. Each axis had one abiotic variable to which it was relatively strongly correlated: the first axis represented most strongly a gradient in DO, the second represented temperature, and the third represented salinity (Table 4). Mean slough depth was also somewhat important to the first two axes, as was salinity to the first axis. Secchi depth and d/rkm were only weakly correlated, if at all, with the axes. In sum, the ordination distilled the important abiotic variables affecting diet to temperature, DO, salinity, and, to a lesser extent, slough depth.

26 21 Axes one and two together appeared to describe a change in the diet from food items supplied by managed wetlands (e.g., Eogammarus confervicolus) during the relatively cool autumn and spring seasons, when water temperatures and DO levels were both comparatively low, to a diet dominated by slough-produced or bay-produced food items (e.g., California bay shrimp, overbite clam) during summer,when water temperatures and DO levels were higher (Figure 6). Additionally, two fishes eaten by the white catfish shimofuri goby and threespine stickleback were more commonly eaten in abundance when DO and water temperature were both relatively low. These two species and prickly sculpin scored rather low on the third axis (Figure 7), reflecting their occurrence in the diet after both a large storm in autumn and higher Delta outflows in spring freshened areas of the marsh where the predation occurred. Notably, Siberian prawn was only eaten in substantial amounts in autumn when DO was at its minimum (Figure 6). Diet-item types appeared to be fairly randomly distributed with respect to the third axis (Figure 7), although the axis seemed to somewhat separate taxa most prevalent in winter when salinities are low (e.g., E. confervicolus) from those abundant in late summer when salinities are highest (e.g., Talitridae, Enteromorpha spp.). Overall, white catfish appeared to have a threshold size at which they became piscivorous, and the degree of piscivory was affected by water quality. Based on the NMDS, temperature and dissolved oxygen concentration were included in the model with standard length to assess the effect of white catfish size on the proportion of the diet consisting of fish (salinity was deemed unimportant because of the broad tolerances of the three major prey fishes shimofuri goby, prickly sculpin, and threespine stickleback to the salinities measured over the course of the sampling; Moyle 2002). Of all three

27 22 variables and their interactions, only one was associated with the proportion of fish in the diet DO (Table 5) in which lower DO values were associated with a higher proportion of fish in the diet of white catfish. On average, white catfish that had eaten other fish were larger than those that had not when season was taken into account (Table 6). Size of white catfish that had eaten other fish was not affected by the combination of a certain size range of catfish occurring during a certain season (Table 6). Sport, commercial, or at-risk fishes were never found in white-catfish gut contents. Graphs comparing CPUE values for prey fishes, particularly threespine stickleback and prickly sculpin, showed that the number of fish eaten by white catfish tracked the abundance of those fishes in the environment well in all seasons except autumn, when all three prey fishes were eaten in considerable amounts despite being in relatively low abundance in the environment (Figure 8). The correlation matrix reflected these patterns, with strong positive correlations for threespine stickleback and prickly sculpin that, because of the high numbers of sticklebacks and prickly sculpins in the diet but low numbers in the environment during autumn, were not significant (Table 7). DISCUSSION Although white catfish have become exceedingly abundant in Suisun Marsh while native and commercial fishes have been in decline (O'Rear and Moyle 2010), the concurrence of these events appears to be due more to abiotic factors and changes in the food web (Kimmerer 2004) rather than any negative effects of white catfish predation on the declining species (Moyle and Light 1996). This is probably partially due to the omnivory of white catfish, especially during summer when certain species such as young-of-the-year striped bass are extremely abundant, diverting the foraging of white

28 23 catfish away from potential fish prey to foods located closer to the base of the food web (e.g., Enteromorpha spp.). Also, while white catfish in other systems have behaved as top-level predators [e.g., white catfish in Pine Flat Reservoir (Goodson, Jr. 1965), white catfish in Clear Lake (Moyle 2002)], they only assumed this role in Suisun Marsh when predation on fish was facilitated by water operations of managed wetlands, which appeared due to (1) low DO levels immobilizing forage fish, making them easy to capture by white catfish; and (2) high production of forage fish and their subsequent introduction into the sloughs from the managed wetlands. In other words, the flexible feeding habits of white catfish may have not only contributed to their successful invasion and integration into the fish assemblage of Suisun Marsh, but may have also buffered any potential predatory effects of white catfish on declining fishes. Overall, white catfish in Suisun Marsh ate various types of food that were seasonally very abundant and apparently very easy to capture. Much of the white catfish diet was comprised of corophiid and gammaroid amphipods, the abundance and population growth rates for which are likely high enough so that this food source is not limiting (Spilseth and Simenstad 2011). Further, while many fishes in the marsh switched from feeding predominantly on mysids to gammarid amphipods after the invasion of the overbite clam (Feyrer et al. 2003), gammarids namely, G. daiberi were not nearly as prominent in the diet of white catfish as the native A. spinicorne or the introduced anisogammarid E. confervicolus (Table 3). White catfish never ate sport, commerical, threatened, or endangered fishes, despite commonly co-occurring with some of them (e.g., striped bass, Sacramento splittail; O Rear and Moyle 2010, 2011). This is not a surprising result for longfin and delta smelt due to their low population sizes over

29 24 the study period (O'Rear and Moyle 2010), although the absence of small striped bass, which were very abundant in Suisun Marsh over the study period, from the diet was unexpected, particularly given previously documented predation (Turner 1966). Although digestion rates were no doubt very high in summer when potential forage fish were most abundant, it is highly unlikely that fish, including small striped bass, would have gone unnoticed in the gut contents: bones were often the only remnants of fish that had been eaten, and those bones (e.g., pelvic girdles, jawbones, cleithra) were always easily identifiable to at least the genus level. Consequently, factors other than abundance resulted in the absence of striped bass in the diet of white catfish. Additionally, white catfish appeared to only eat fish when water management made it feasible. The preyedupon species, including both native (threespine stickleback and prickly sculpin) and introduced [shimofuri goby, yellowfin goby, western mosquitofish (Gambusia affinis), rainwater killifish (Lucania parva)] fishes, are abundant and widespread. High production of fish and invertebrates by managed wetlands could contribute to a larger white-catfish population, which could, in turn, result in a white-catfish population so large that they could be an important source of mortality for at-risk fishes just through incidental predation. As a result, should white catfish continue to increase in the marsh, replication of this study should be undertaken to assess any risk to listed fishes. Nevertheless, it is unlikely that white catfish would be important predators of longfin or delta smelt in the future, given the ecological similarities between the two smelts and small striped bass (e.g., pelagic planktivores; Moyle 2002, Feyrer et al. 2007) and the current lack of striped bass in the diet of white catfish.

30 25 Although amphipods were the most commonly consumed food item and made up the greatest proportion of the standardized diet mass over the entire study period (Table 3), several other foods were seasonally important, especially the chlorophyte Enteromorpha spp. Enteromorpha was eaten in all seasons, especially during summer and autumn (Table 3). Although tubes of corophiid species (mainly A. spinicorne) were frequently found within the Enteromorpha, the sheer amount of the chlorophyte in the diet suggests that white catfish were specifically targeting it rather than consuming it while foraging for other prey. Ingestion of plant or algal material has been observed in white catfish from both a deep, clear-water water-supply reservoir (Goodson, Jr. 1965) and a very large, shallow, Atlantic-drainage reservoir (Stevens 1959). Similarly, channel catfish in a South Dakota reservoir fed substantially on both macrophytes and algae, although the abundance of vegetation in the diet declined with an increase in the abundance of fish in the diet (Dagel et al. 2010). Consequently, this suggests that white catfish may be a true omnivore, capable of subsisting at least in part on plant or algal material. However, the importance of vegetation to the diet of white catfish during summer could be biased because white catfish are a warm-water species (Moyle 2002), and digestion rates are both higher for animal rather than vegetative material and at higher water temperatures, then the predominance of Enteromorpha in the diet could be due in part to high digestion rates of animals such as amphipods. The differences of diets between white catfish of Suisun Marsh and those of the Delta appear due in part to both salinities and the influence of wetlands. Asian clams and signal crayfish, which formed a considerable portion of the diet of white catfish in the Delta, are generally most abundant in fresh water (Hazel and Kelley 1966, Fields and

31 26 Messer 1999). Conversely, D. magna has exhibited maximum population growth rates at a salinity around 4 ppt (Arner and Koivisto 1993, Schuytema et al. 1997), a salinity that is commonly measured in Suisun Marsh. As the NMDS ordination and multiple linear regression model both showed, predation of white catfish on other fishes in Suisun Marsh was facilitated by water manipulations in managed wetlands. Threespine stickleback and prickly sculpin are abundant in the managed wetlands and are flushed into the sloughs during late winter and spring. In the Delta, where such wetlands are a negligible proportion of the acreage both today (Lund et al. 2007) and in the 1960s during Turner's study (Kelley 1966) compared to Suisun Marsh, the catfish diet was dominated by pelagic fishes that spawn in lotic, freshwater habitats: threadfin shad, American shad, striped bass, and delta smelt (Turner 1966). These species can attain much larger sizes than prickly sculpins, threespine sticklebacks, and shimofuri gobies, which explains why fishes contributed more to the bulk of the diet of white catfish in the Delta even though they were eaten much less frequently than in the marsh (Figure 5). A similar situation probably accounts for the fact that amphipods made up nearly double the bulk of the diet in Suisun Marsh compared to the diet in the Delta, despite both populations feeding on amphipods at basically the same frequencies: E. confervicolus, the amphipod having the highest mass in the Suisun Marsh diet, attains a much larger size than A. spinicorne or A. stimpsoni, the major amphipods eaten by Delta white catfish (Carlton 2007). Although the stress of the NMDS ordination was rather high, there was a clear linear gradient in the composition of the white catfish s diet with respect to water temperature and DO concentration (Figure 6). Many of the taxa on the lower portion of the line, where DO and temperature are both rather low, are associated with still-water

32 27 conditions found in the managed wetlands. Eogammarus confervicolus, larval dytiscid beetles, and threespine stickleback are known to reproduce in the marsh s managed wetlands during the flooded period (Batzer and Resh 1992), as well as other taxa that appeared in lesser numbers in the diet but at the same time and place [corixids, chironomid larvae, hydrophilid adults, red swamp crayfish (Procambarus clarkii); Batzer and Resh 1992, California Department of Fish and Game 1996, 1997, 1998; Table 3], namely, springtime and the northwest marsh s managed wetlands. Additionally, the cladoceran D. magna is generally most abundant in ponds and temporary waters where it exhibits two population peaks due to hatching of ephippial eggs and parthenogenic reproduction of the resultant females: one in autumn and one in spring (Pennak 1978). It, too, was most abundant in the diet during spring and, to a lesser extent, autumn in sloughs of the northwest marsh. On the upper part of the line are several taxa that are either produced outside, but ride tidal currents into, the marsh (e.g., California bay shrimp, overbite clam; Hatfield 1985, Schroeter 2010; Figure 6) or complete their lifecycle within the estuary s bays and sloughs (mysids; Kimmerer 2004). These taxa were also most frequently eaten in the larger, deeper sloughs (i.e., Montezuma, Suisun, and Nurse), which are subject to a greater tidal excursion, and therefore the influence of the bays, than the smaller, more-interior sloughs. Consequently, it appears that the diet of white catfish is subsidized by managed wetlands during autumn and springtime and shifts more to foods produced within the sloughs and bays during the summer. It should be noted that while the white-catfish diet in autumn and springtime was comprised of taxa known to occur and reproduce in the managed wetlands, this does not preclude a food subsidy provided by undiked marsh plains that are inundated regularly by

33 28 high tides to adjoining sloughs. Only one white catfish during springtime was sampled from First Mallard Slough, which is not diked on either bank; this fish had fed on large amounts of species that are common in the managed wetlands (e.g., E. confervicolus, threespine stickleback). Thus, large food subsidies may come from both managed wetlands and undiked marsh plains into adjacent sloughs; however, other features of the undiked sloughs not related to food may make them less-than-ideal habitat for white catfish, as seems to be the case with First Mallard Slough. Given the above, d/skm should also be correlated with the first axis of the NMDS ordination; however, it was basically unrelated to any patterns in the dataset. This result is probably due to the fact that though there may be a very high density of water control structures in a given slough, only a few of those structures were opened during the sampling period. For instance, managed wetlands between Boynton and Peytonia sloughs have the ability to draw water from either slough, in addition to the ability to discharge into either slough or upper Suisun Slough (Siegel et al. 2011). These wetlands have been known to draw water exclusively from Boynton Slough and a waste-water treatment plant while discharging only into Peytonia Slough (S. Siegel, pers. comm.). Consequently, the impact of discharge water on the sloughs would be more a function of the routing of water rather than just the number of water control structures. It is also possible that the negligible importance of d/skm was due to the small range of values ( d/skm); however, this is unlikely given that both salinity and DO had ranges that were not much larger ( ppt and mg/l, respectively) yet were still relatively strongly correlated to the NMDS axes.

34 29 While production of certain taxa and their introduction into the sloughs by the managed wetlands appears to be one mechanism through which white catfish gain food (Figure 8), lowered DO levels due to managed-wetland water operations appear to facilitate feeding of white catfish on fish and large invertebrates. This is further supported by the fact that the numbers of fish eaten by white catfish during autumn occurred concurrently with those species being eaten at or close to their annual population minimums (Figure 8). Consequently, the occurrence of fish in the diet of white catfish during autumn was not due to high numbers in the environment, as was likely the case for the springtime predation (Figure 8). The family Ictaluridae is wellknown for having hemoglobin with a very high affinity for oxygen that gives them the ability to persist in hypoxic waters (Moore 1942, Basu 1959, Becker 1983, Smale and Rabeni 1995, Ludsin et al. 2001, Torrans 2008). This insensitivity to low DO levels likely gives white catfish an ability to feed in areas that exclude other piscivorous fishes (e.g., adult striped bass) where potential prey is dead, dying, or stunned. Although there are no studies that have assessed the tolerance of Siberian prawn to low oxygen levels, comatose and dead Siberian prawns and California bay shrimp have been captured in areas of the marsh affected by low DO in autumn (personal observation, Schroeter and Moyle 2004). Thus, it is likely that predation on this large shrimp by white catfish was also due to a lowered scope of activity for the prawn because of low DO, which would explain the substantial weight of this invertebrate in the autumn diet. Not surprisingly, white catfish that had eaten fish were larger than those that had not. However, once they reached a size large enough to start preying on other fishes, additional size did not equate to more fish eaten. This may be due to the fact that once

35 30 white catfish reach a certain size, they are no longer limited by gape for feeding on fish. In fact, one of the key morphological characteristics of white catfish is the disproportionately large head relative to the size of the body as the catfish grows (Stevens 1959). As a result, other factors, such as DO or density of fish, appear more important in affecting piscivory than size once white catfish reach approximately 220-mm SL. Although no surveys were undertaken to evaluate the selectivity for food of white catfish, the wide range of items eaten (Table 3), the large amounts of disparate food types consumed [e.g., Enteromorpha spp., E. confervicolus, terrestrial earthworms, threespine stickleback, a large zooplankter (D. magna)], the various sources of these food items (e.g., downstream bays, managed wetlands), and the differences between diets of white catfish from Suisun Marsh and the Delta suggest that white catfish are highly opportunistic in their feeding habits. In sum, white catfish in Suisun Marsh are opportunistic omnivores that take advantage of a range of locally abundant food items. Managed wetlands subsidize the diet of white catfish by introducing large amounts of food in spring and, to a lesser extent, autumn; they also contribute to the diet by rendering larger food items fish and shrimp more vulnerable to capture by lowering slough DO levels. However, white catfish also derive a notable proportion of their diet from taxa produced within the sloughs and from downstream bays. Despite the piscivory of larger white catfish, they have not eaten fishes targeted for conservation, and much of the food they eat is either not utilized by at-risk or commercially important fishes or is unlikely to be limiting. Consequently, white catfish appear to be relatively harmless to populations of other fishes in Suisun Marsh.

36 31 A potential concern, however, is the effect of eating white catfish from Suisun Marsh on humans. Undoubtedly, white catfish are the third-most sought-after sport fish in the marsh behind striped bass and white sturgeon (Acipenser transmontanus; personal observation). The managed wetlands of the marsh contribute considerably to the methylation of mercury (Siegel et al. 2011), with concentrations often above those measured at sites within the Delta (Foe 2003). Additionally, positive correlations exist between tissue levels of mercury in largemouth bass, which often exceed safe-eating guidelines, and mercury concentrations of the water in the Delta (Davis et al. 2008). While mild advisories exist for consumption of white catfish captured from the Delta, none are currently promulgated specifically for Suisun Marsh in the California Department and Fish and Game's regulations booklets. Moreover, much higher mercury concentrations have been found in white catfish from other waterways relative to white catfish in the Delta (e.g., Trinity Reservoir; California Department of Water Resources 2007). Given that a significant portion of the diet of white catfish in Suisun Marsh comes from the managed wetlands, some of which consists of secondary consumers (e.g., threespine stickleback and prickly sculpin; Moyle 2002), there is potential for substantial bioaccumulation of methylmercury in the tissue of white catfish. Consequently, there could be a greater health risk to fishermen targeting white catfish in Suisun Marsh than in other areas of the San Francisco Estuary. However, there are no known studies measuring the mercury concentrations in the tissues of white catfish from Suisun Marsh; this is a hazard that should be addressed in the near-term future.

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41 Poe, T. P., H. C. Hansel, S. Vigg, D. E. Palmer, and L. A. Prendergast Feeding of predaceous fishes on out-migrating juvenile salmonids in John Day Reservoir, Columbia River. Transactions of the American Fisheries Society 120: Quinn, S. P Stomach contents of flathead catfish in the Flint River, Georgia. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 41: Rulifson, R. A., and S. A. McKenna Food of striped bass in the Upper Bay of Fundy, Canada. Transactions of the American Fisheries Society 116: Schaffter, R. G Growth of white catfish in California's Sacramento-San Joaquin Delta. California Fish and Game 83: Schroeter, R. E The temporal and spatial trends, biological constraints, and impacts of an invasive clams, Corbula amurensis, in Suisun Marsh, San Francisco Estuary. University of California, Davis, California. Schuytema, G. S., A. V. Nebeker, and T. W. Stutzman Salinity tolerance of Daphnia magna and potential use for estuarine sediment toxicity tests. Archives of Environmental Contamination and Toxicology 33: Siegel, S., P. Bachand, D. Gillenwater, S. Chappel, B. Wickland, O. Rocha, M. Stephenson, W. Heim, C. Enright, P. Moyle, P. Crain, B. Downing, and B. Bergamaschi Final evaluation memorandum, strategies for reducing low dissolved oxygen and methylmercury events in northern Suisun Marsh. Prepared for the State Water Resources Control Board, Sacramento, California. SWRCB Project Number Smale, M. A., and C. F. Rabeni Hypoxia and hyperthermia tolerances of headwater stream fishes. Transactions of the American Fisheries Society 124: Spilseth, S. A., and C. A. Simenstad Seasonal, diel, and landscape effects on resource partitioning between juvenile Chinook salmon (Oncorhynchus tshawytscha)and threespine stickleback (Gasterosteus aculeatus) in the Columbia River Estuary. Estuaries and Coasts 34: Stevens, R. E The white and channel catfishes of the Santee-Cooper Reservoir and tail race sanctuary. Proceedings of the Annual Conference of the Southeastern Fish and Wildlife Agencies 13: Thomas, M. E Monitoring the effects of introduced flathead catfish on sport fish populations in the Altamaha River, Georgia. Proceedings of the Annual Conference of the Southeastern Fish and Wildlife Agencies 47: Torrans, E. L Production responses of channel catfish to minimum daily dissolved oxygen concentrations in earthen ponds. North American Journal of Aquaculture 70: Turner, J. L Distribution and food habits of ictalurid fishes in the Sacramento- San Joaquin Delta. Pages in J. L. Turner and D. W. Kelley, editors. Ecological Studies of the Sacramento-San Joaquin Delta, part 2. California Department of Fish and Game Fish Bulletin 136. Vigg, S., T. P. Poe, L. A. Prendergast, and H. C. Hansel Rates of consumption of juvenile salmonids and alternative prey fish by norhtern squawfish, walleyes, smallmouth bass, and channel catfish in John Day Reservoir, Columbia River. Transactions of the American Fisheries Society 120:

42 Wang, J. C Fishes of the Sacramento-San Joaquin estuary and adjacent waters, California: a guide to early life histories. Interagency Ecological Program Technical Report pp. Waters, D. S., T. J. Kwak, J. B. Arnott, and W. E. Pine, III Evaluation of stomach tubes and gastric lavage for sampling diets from blue catfish and flathead catfish. North American Journal of Fisheries Management 24: Weller, R. R., and C. Robbins Food habits of flathead catfish in the Altamaha River system, Georgia. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 53: Zuur, A. F., E. N. Ieno, and G. M. Smith Analyzing ecological data. Springer Science, United States. 37

43 Table 1. Size statistics of white catfish sampled for gut contents (n = 304). Measurement Mean Range Standard Deviation Standard Length (mm) Mass (g)

44 Table 2. Identity of and codes for diet items in NMDS ("N" = native, "I" = introduced, and "?" denotes taxa whoe status is unknown). Phylum Diet Item Scientific Name Diet Item Common Name Code Native/Introduced Chlorophyta Enteromorpha spp. green alga Entero? Bryozoa Bryozoa moss animals Bryozoa? Aneelida Lumbricus terrestris nightcrawler Lumbri I Annelida Oligochaeta aquatic earthworm Oligo N/I Annelida Polychaeta pileworm Poly N/I Bivalvia Corbula amurensis overbite clam Camur I Arthropoda Dytiscidae water beetle Dytisc N Arthropoda Daphnia magna water flea Dmagna N Arthropoda Talitridae beach hopper Talitrid I Arthropoda Americorophium spinicorne tube-dwelling scud Aspin N Arthropoda Americorophium stimpsoni tube-dwelling scud Astim N Arthropoda Corophium alienense tube-dwelling scud Calie I Arthropoda Gammarus daiberi scud Gam I Arthropoda Eogammarus confervicolus scud Econ I Arthropoda Gnorimosphaeroma insularae aquatic pillbug Ginsular N Arthropoda Hyperacanthomysis longirostris opposum shrimp Hypera I Arthropoda Neomysis kadiakensis opposum shrimp Nkad N Arthropoda Neomysis mercedis opposum shrimp Nmer N Arthropoda Rithropanopeus harrissii Harriss mud crab Rharriss I Arthropoda Exopalaemon modestus Siberian prawn Emodest I Arthropoda Crangon franciscorum California bay shrimp Cfrancis N Vertebrata Gasterosteus aculeatus threespine stickleback STBK N Vertebrata Cottus asper prickly sculpin SCP N Vertebrata Tridentiger bifasciatus shimofuri goby SG I 39

45 40 Table 3. Seasonal proportions of standardized mass and frequency of occurrence for all diet items consumed by white catfish (n = 263; Tr. = less than 1%). Those taxa listed in bold were used in the NMDS analysis. Phylum Diet Item Percent Standardized Mass (Percent Frequency of Occurrence) Winter Spring Summer Autumn All Seasons Chlorophyta Unidentified chlorophytes 1(2) Tr.(3) Tr.(1) Enteromorpha spp. 10(18) 9(14) 18(10) 17(21) 11(14) Angiosperm Rubus armeniacus 3(3) Tr.(Tr.) Bryozoa 2(7) 1(4) Tr.(3) 2(8) 1(5) Nematoda Nematoda Tr.(4) Tr.(5) Tr.(1) Annelida unidentified annelid Tr.(1) Tr.(Tr.) Hirudinea Tr.(1) Tr.(Tr.) Lumbricus terrestris 7(6) 5(3) Oligochaeta 3(7) 1(14) Tr.(4) 1(15) Tr.(10) Polychaeta 1(32) 2(33) 5(16) Tr.(10) 2(24) Platyhelminthes Tr.(1) Tr.(Tr.) Mollusca Corbicula fluminea Tr.(7) Tr.(3) Tr.(1) Tr.(2) Corbula amurensis 3(11) Tr.(7) 1(3) Tr.(3) 1(5) Arion spp. Tr.(1) Tr.(Tr.) Lymnaeidae Tr.(2) Tr.(1) Tr.(1) Physidae Tr.(1) Tr.(1) Tr.(1) Arthropoda unidentified arthropoda Tr.(1) Tr.(Tr.) Diplopoda Tr.(1) Tr.(1) Collembola Tr.(1) Tr.(Tr.) unidentified Insecta Tr.(1) Tr.(Tr.) Siphlonuridae Tr.(1) Tr.(Tr.) Heptageniidae Tr.(1) Tr.(Tr.) Gomphidae Tr.(1) Tr.(Tr.) Corixidae Tr.(4) 1(13) Tr.(3) Tr.(3) 1(7) Sialidae Tr.(1) Tr.(Tr.) Crambidae Tr.(1) Tr.(Tr.) Hydrophilidae Tr.(1) Tr.(Tr.) Dytiscidae 2(7) Tr.(3) Tr.(1) Muscidae Tr.(1) Tr.(Tr.) Chironomidae Tr.(11) Tr.(18) Tr.(4) Tr.(5) Tr.(11) Tipulidae Tr.(1) Tr.(Tr.) unidentified Crustacea Tr.(1) Tr.(Tr.) Ostracoda Tr.(11) Tr.(4) Tr.(1) Tr.(3) Tr.(4) Harpacticoidea Tr.(1) Tr.(3) Tr.(2) Calanoida Tr.(7) Tr.(2) Tr.(2) Cyclopoida Tr.(4) Tr.(3) Tr.(1) Tr.(5) Tr.(3) Daphnia magna 1(32) 4(21) 12(13) 4(14) Cumacea Tr.(4) Tr.(16) Tr.(3) Tr.(3) Tr.(9) Tanaidae Tr.(1) Tr.(1) Tr.(5) Tr.(1) Hyallelidae Tr.(1) Tr.(Tr.) Talitridae Tr.(6) 5(5) 1(8) 1(5)

46 41 Phylum Diet Item Percent Standardized Mass (Percent Frequency of Occurrence) Winter Spring Summer Autumn All Seasons Arthropoda Crangonyx floridanus Tr.(4) Tr.(1) Tr.(1) Americorophium spinicorne 13(54) 23(88) 19(60) 2(54) 20(71) Americorophium stimpsoni Tr.(25) 2(25) 2(25) Tr.(3) 2(22) Corophium alienense Tr.(7) Tr.(4) Tr.(11) 2(10) Tr.(7) Gammarus daiberi 13(64) 5(72) 14(46) Tr.(28) 6(57) Eogammarus confervicolus 50(61) 25(64) 6(15) Tr.(8) 22(40) unidentified isopods Tr.(1) Tr.(1) Gnorimosphaeroma rayi Tr.(4) Tr.(Tr.) Gnorimosphaeroma insularae Tr.(7) 1(19) 6(16) Tr.(13) 1(16) Gnorimosphaeroma oregonensis Tr.(4) Tr.(Tr.) Synidotea spp. Tr.(4) Tr.(2) Tr.(2) Tr.(3) Tr.(2) Hyperacanthomysis longirostris Tr.(18) Tr.(13) 1(20) Tr.(3) Tr.(14) Neomysis kadiakensis 1(21) 1(21) 1(12) 1(15) Neomysis mercedis Tr.(4) 2(9) Tr.(5) unidentified decapod Tr.(1) Tr.(Tr.) Procambarus clarkii Tr.(1) Tr.(Tr.) Rithropanopeus harrissii Tr.(2) 1(2) Tr.(2) Exopalaemon modestus Tr.(2) Tr.(3) 12(18) 1(4) Crangon franciscorum Tr.(4) Tr.(1) 1(5) Tr.(2) Vertebrata unidentified vertebrate Tr.(7) Tr.(2) Tr.(3) Tr.(3) Tr.(3) Cyprinodontiformes Tr.(1) Tr.(Tr.) Gastersoteus aculeatus (eggs) 1(2) 1(1) Gasterosteus aculeatus 7(18) Tr.(5) 3(13) 5(11) Cottus asper 1(3) 4(1) 2(5) 1(2) Tridentiger bifasciatus Tr.(7) 8(7) Tr.(3) 41(18) 9(7) Acanthogobius flavimanus Tr.(1) Tr.(Tr.) Engraulis mordax Number of fish sampled (number of fish with food) (fisherman's bait) 12(1) 2(Tr.) (23) (128) (82) (30) 304 (263)

47 Table 4. Pearson correlation coefficients for NMDS axes (n = 259). Variable Axis 1 Axis 2 Axis 3 Temperature Salinity Secchi depth DO Diversions/skm Mean slough depth

48 Table 5. Statistics for parameters for multiple regression model testing effects of size, DO, and temperature on the proportion of fish in the diet of white catfish that were piscivorous (n = 61; "*" = significant). Parameter Parameter Estimate t-ratio Probability > t Temperature DO * Standard length Temperature X DO Temperature X standard Length DO X standard length Temperature X DO x standard length

49 Table 6. Statistics of the effects of presence/absence of fish in the diet and season on standard length of white catfish (n = 302; "*" = significant). Predictor Sum of squares F-ratio Probability > F Season <0.0001* Fish in diet * Season X fish in diet

50 Table 7. Correlation coefficients and statistics for seasonal otter-trawl CPUE values and seasonal number of fish per white catfish sampled (n = 4). Species Correlation P-value Prickly sculpin Shimofuri goby Threespine stickleback

51 Figure 1. Suisun Marsh and sites sampled by the Suisun Marsh Fish Study, from which white catfish for this study were taken. 46

52 Figure 2. General management scenarios for managed wetlands in Suisun Marsh (taken from California Department of Water Resources 2001). 47

53 Figure 3. Seasonal proportions of the total standardized diet mass for five major diet-item groups of white catfish in Suisun Marsh. 48