This thesis is accepted by the Dean of Graduate Studies

Transcription

1 MOVEMENT PATTERNS AND GROWTH OF AMERICAN EELS (ANGUILLA ROSTRATA) BETWEEN SALT AND FRESH WATER, BASED ON OTOLITH MICROCHEMISTRY by Heather M. Lamson Bachelor of Science, University of Northern British Columbia, 2000 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF Master of Science In the Graduate Academic Unit of Biology Supervisors: Examining Board: D.K. Cairns, Ph.D., Department of Fisheries and Oceans, (Charlottetown, PEI) R.A. Curry, Ph.D., Canadian Rivers Institute, UNB (Fredericton) S.C. Courtenay, Ph.D., Biology UNB (Fredericton), Internal Examiner G.A. Benoy, Ph.D., Environment Canada External Examiner This thesis is accepted by the Dean of Graduate Studies THE UNIVERSITY OF NEW BRUNSWICK November, 2005 Heather M. Lamson, 2005

2 DEDICATION For all those people who can put the sliminess aside and appreciate eels for the amazing creatures that they are. ii

3 ABSTRACT Reconstructing lifelong movements of teleost fishes between salt and fresh water is possible through otolith strontium:calcium microchemistry. This method was employed to trace movements of American eels (Anguilla rostrata) captured in saltwater bays and adjoining freshwater ponds in Prince Edward Island, Canada. Three migratory contingents were identified: freshwater residents, inter-habitat shifters, and saltwater residents. A pond with a pool-and-weir salmonid fishway and drained by a low-gradient channel contained eels that had entered freshwater at all ages. Another pond with a 2.2 m vertical spillway contained only eels that had entered freshwater upon initial continental arrival. Saltwater residents dominated saltwater bays, which challenges the conventional paradigm of obligate catadromy for American eels. Growth rates increased with the amount of time eels spent in salt water. Annual growth of yellow eels that resided in salt water throughout their lives (94.0 mm yr -1 ) was double that of freshwater residents (45.2 mm yr -1 ). Eels that shifted between salinity zones had intermediate growth rates (60.5 mm yr -1 ). Population models estimating growth and escapement of silver eels from salt, brackish, and freshwater habitats need to account for divergent growth rates based on habitat salinity and realize that previously reported growth rates that have not sampled eels in salt water may be underestimated. iii

4 ACKNOWLEDGMENTS Thanks to David Cairns, my supervisor, who provided the opportunity to study this fascinating species and for his guidance throughout the project. Jay Shiao, W.N. Tzeng and Yoshiyuki Iizuka were all instrumental by performing the otolith microchemical analysis. Thanks to my co-supervisor Allen Curry and supervisory committee member Tillmann Benfey for their advice and direction. Corey Muttart, Valérie Tremblay, Robbie Moore, Noella McDonald and Mark Grimmett provided greatly appreciated field and lab assistance. Fisheries and Oceans Canada in Charlottetown supported the project financially and through the use of equipment and office space. Thanks to Brian Jessop for guidance with growth analysis and to Jay Shiao for his assistance with ageing and analysis. This study received support from the National Science Council, ROC (NSC B and B ). I am grateful to my parents and family for all their support. Finally, I would like to thank my best friend and loyal field partner even though he is afraid of eels, my black lab, Angus. iv

5 TABLE OF CONTENTS DEDICATION... ii ABSTRACT...iii ACKNOWLEDGMENTS... iv TABLE OF CONTENTS... v LIST OF TABLES... vii LIST OF FIGURES...viii 1 GENERAL INTRODUCTION Background Outline of Thesis Statement Regarding Contribution of Co-Authored Articles Literature Cited MOVEMENT PATTERNS OF AMERICAN EELS (ANGUILLA ROSTRATA) BETWEEN SALT AND FRESH WATER IN A COASTAL WATERSHED BASED ON OTOLITH MICROCHEMISTRY Abstract Introduction Methods Results Discussion Effect of obstacle type on upstream movement Movement patterns Implications for conservation v

6 2.6 Acknowledgements Literature Cited GROWTH RATES OF AMERICAN EELS (ANGUILLA ROSTRATA) IN RELATION TO HABITAT SALINITY BASED ON OTOLITH MICROCHEMISTRY Abstract Introduction Methods Growth Analysis Results Discussion Length-at-age Analysis Back-calculation Implications for Conservation Acknowledgements Literature Cited GENERAL DISCUSSION General Conclusions Summary of Findings Management Implications and Suggested Research Needs Literature Cited vi

7 LIST OF TABLES Table 2-1. Catch and catch rates of American eels in fyke nets in the Brackley- Covehead system Table 2-2. Salinity history and rate of inter-habitat shifting of American eels captured in saltwater and freshwater sites. See text for definitions Table 2-3. Habitat occupancy patterns of wild (non-stocked) yellow and silver Anguilla eels after arrival in continental waters, as inferred from otolith Sr:Ca ratios Table 3-1. Mean annual growth (mm/year) of American eels under 15 years old, estimated by otolith radius back-calculation. SW= saltwater residents, IH= inter-habitat shifters and FW= freshwater residents vii

8 LIST OF FIGURES Figure 2-1. Brackley and Covehead Bays, Prince Edward Island (a) and associated freshwater impoundments: McCallums Pond (b) and Cass and Marshalls Ponds (c) Figure 2-2. Schematic profiles of pond outlets, showing the vertical spillway at McCallums Pond (a), the concrete salmonid fishway at Cass Pond (b) and the low-gradient channel at Marshalls Pond (c) Figure 2-3. Mean within-year Sr:Ca ratios vs. age for American eels from five sites. For inter-habitat shifters (eels that moved between habitat types at least once after age 1), lines represent individual eels. For eels that remained resident in either salt or fresh water after age 1, symbols represent means, with N indicating the number of eels Figure 2-4. Frequency distribution of Sr:Ca ratios measured in otoliths of age 1+ American eels captured in saltwater (a) and freshwater (b) sites Figure 2-5. Rate of movement between salt and fresh water of eels sampled in Cass Pond which shifted between habitat types. Sample size (number of eelyears) is given at the top of the graph Figure 2-6. Allocation of time in habitat categories during each year, by eels which shifted habitat at least once after age 1. Samples are from Covehead Bay (a), Cass Pond (b) and Marshalls Pond (c) Figure 2-7. Percentage of sampled eels which showed movements out of the habitat type where they were captured, based on Sr:Ca analysis. The habitat type given in each panel is the habitat where captured. All species includes A. marmorata, A. australis and A. dieffenbachii. See Table 2-3 for data sources Figure 3-1. Brackley and Covehead Bays, Prince Edward Island and associated freshwater impoundments: McCallums Pond and Cass and Marshalls Pond Figure 3-2. a) Length-at-age of eels captured in saltwater Brackley and Covehead Bays and freshwater McCallums, Cass, Marshalls and Parsons Ponds and b) saltwater residents, freshwater residents, and freshwater residents under 15 years of age fitted with linear regression lines using an independent variable model Figure 3-3. Relationship between total length (mm) and maximum otolith radius (mm) Figure 3-4. Back-calculated lengths-at-age of saltwater residents, freshwater residents and inter-habitat shifters (mean +1 standard error) Figure 3-5. a) Mean annual growth rate vs. time spent in salt water and b) Growth per year vs. percent of that year spent in salt water (F= p<.0001) of American eels under 15 years old in the Brackley-Covehead Watershed.. 67 viii

9 1 GENERAL INTRODUCTION 1.1 Background The American eel (Anguilla rostrata) is a member of the family Anguillidae, which is comprised of eel species that follow a similar life cycle. Anguillid eels spawn in the open ocean and eggs hatch into a larval form called leptocephali. The leptocephali metamorphosize into the glass eel stage while drifting with currents towards continental waters. Glass eels typically begin to show pigmentation as they arrive in continental waters where they are called elvers. Once fully pigmented, they are termed yellow eels and will spend the majority of their lives feeding and growing across a range of habitats. Upon maturity, a silvering of the skin prepares the silver eel for the oceanic migration back to their spawning area (Tesch 2003). The catadromous life history of spawning in the ocean and rearing in fresh water is commonly assigned to anguillid eels which are often termed freshwater eels. However, catadromy is a misleading descriptor for this genus because growth is not restricted to fresh water. Residency in fresh water is not obligatory for European eels, A. anguilla (Limburg et al 2003), Japanese eels (A. japonica) (Tsukamoto and Arai 2001), short finned eels (A. australis) (Arai et al. 2004) or American eels (A. rostrata) (Morrisson et al. 2003). Various patterns of movement across salinity gradients exist for yellow eels and include freshwater or saltwater residence and movements between salt and fresh water one or more times. 1

10 Various methods have been used to track yellow eel movements between salinity zones, including tagging (Oliveira 1997) and telemetry (Helfman et al. 1984). These methods, however, are limited in their ability to track individuals over long periods of time (Secor et al. 1995). The recent introduction of Sr:Ca otolith microchemistry methods have advanced anguillid eel research with the ability to determine lifetime movements across salinity boundaries. Fish otoliths (ear stones) are composed primarily of calcite (CaCO 3 ), which forms annual rings much like those of a tree due to slower growth during winter than summer. During otolith formation, strontium (Sr) can substitute for calcium (Ca) due to similar ionic charges and radii. The ratio of Sr to Ca is approximately 100 times greater in salt water than in fresh. When a fish resides in salt water, this higher ratio is reflected in the deposition of Sr in its otoliths (Casselman 1982). Sr:Ca otolith microchemistry, by taking incremental measurements of Sr and Ca along a transect that dissects the annual rings of the otolith, has the power to uncover the past salinity history of a fish. The American eel is one of two anguillid species that occur in the Atlantic Ocean (along with the European eel), and is distributed along the Atlantic coast during its yellow eel stage from Greenland to northern South America. There is much variability in traits across the species range though, based on our present knowledge, there is no genetic difference among the young arriving on continental shores (Avise et al. 1986, 1990). 2

11 American eel movements between salt and fresh water have been characterized by Sr:Ca otolith microchemistry in three previous studies (Jessop et al. 2002, Morrisson et al. 2003, Cairns et al. 2004) These studies all sampled eels in fresh or brackish water and determined that movement between fresh and brackish water is highly variable during the yellow eel life stage. American eel abundances have declined since the mid 1980 s (Jessop 2000). Possible causes for the reduction in numbers include adverse oceanic conditions, habitat loss, excessive fishing pressure and obstruction to eel passage upstream. Dams and other obstacles can prevent or impede migration and adversely affect eel populations (Legault 1988, Feunteun et al. 2003). Eels are relatively weak swimmers and although they have been observed to move through grass and perform other migrational feats, obstacles generally impede movements. Previous results from Sr/Ca analysis on eels in a watershed with an impounded stream in eastern Prince Edward Island indicate that dams impede normal movements between fresh and salt water (Cairns et al. 2004). The first objective of this study was to reconstruct movement patterns of American eels between salt and freshwater habitats in a small watershed in Prince Edward Island, Canada, based on samples collected in full strength saltwater bays and freshwater ponds. As previous Sr:Ca studies on American eel movement and growth have only sampled eels from freshwater and brackish water sites, this was the first Sr:Ca study to examine American eels captured in salt water. Secondly, this study examined the effect of various types of impoundment structures on eel movement between fresh and salt water. 3

12 The American eel is a long-lived species, spending 4 to 43 years in continental waters during the yellow eel life stage. Residency time to migration is closely linked to growth rate, as maturation and the silvering process depend on size, not age (De Leo and Gatto 1995). Most studies of eel growth in North America have looked at eels in fresh water, and have shown slow growth. Studies that have compared freshwater and brackish or saltwater growth (Morrison et al. 2003, Jessop et al. 2004, Cairns et al. 2004) have shown that growth rates of American eels that spend most of their time in fresh water are lower than those captured in waters of higher salinity. These findings indicate that data on eel growth may be skewed to show overall slower growth rates due to the preponderance of growth studies in fresh water. As growth rates are critical for population assessments, it is imperative that salt water growth data be included in the biological framework. Furthermore, it is important to include growth in saltwater because most fisheries in eastern North America target eels in salt or brackish bays and estuaries. The third objective of this study was to relate growth rates to movement patterns between salt and fresh water and determine if growth is faster in fresh or saltwater habitats. I hypothesized that growth increases with the relative time spent in saltwater habitats. To test this hypothesis I related back-calculated growth at each age to the salinity the eel resided in as determined from Sr:Ca analysis. This was the first study to determine growth rates in relation to life history movement patterns of American eels captured in both saltwater and freshwater sites. 4

13 1.2 Outline of Thesis Chapter 1 includes a general introduction, and outline of the thesis. Chapter 2 examines the movement patterns of American eels between salt and fresh water. Otolith microchemistry was used to reconstruct the continental movement patterns of eels in a small watershed in Prince Edward Island consisting of adjoining saltwater bays, and inflowing watercourses with obstructions of various types at head of tide. The number of times eels shift between salt and fresh water was examined and related to the site of capture. The paper examines the colonization of upstream habitats in a small watershed and the implications of various types of obstruction to upstream movement. This paper, titled Movement patterns of American eels (Anguilla rostrata) between fresh and saltwater in a small coastal watershed, based on otolith microchemistry was submitted to Marine Biology in July Chapter 3 determines and compares growth rates of eels that inhabit fresh and salt water. Growth was compared between eels that resided in fresh or salt water or moved between the habitats as established from otolith microchemistry. The paper proposes that growth rates of American eels are a function of the percent of time spent in either salt or fresh water, and will be higher during years spent in salt water than in fresh water. Backcalculated lengths were used to determine growth rates. 5

14 The paper titled Growth rates of American eels (Anguilla rostrata) in relation to habitat salinity, based on otolith microchemistry will be submitted to Marine Ecology Progress Series. Chapter 4 summarizes the major findings of the study and how this research is useful for conserving the species. Furthermore, it provides recommendations for future work in this field. 1.3 Statement Regarding Contribution of Co-Authored Articles Heather Lamson was responsible for the logistical aspects of the research, data analysis and manuscript preparation. David K. Cairns (the supervisor) was responsible for securing funding, and providing advice on data interpretation and for overseeing preparation of article manuscripts. Jay C. Shiao, Yoshiyuki Iizuka, and Wann-Nian Tzeng analyzed otoliths for Sr:Ca ratios and also provided input into the analysis and article manuscripts. 1.4 Literature Cited Arai, T., Kotake, A., Lokman, P.M., Miller, M.J. and Tsukamoto K Evidence of different habitat use by New Zealand freshwater eels Anguilla australis and A dieffenbachii, as revealed by otolith microchemistry. Mar Ecol Prog Ser 266:

15 Avise, J.C., Helfman, G.S., Saunders, N.C. and Hales, L.S Mitochondrial DNA differentiation in North Atlantic eels: population genetics consequences of an unusual life history pattern. Proceedings of the National Academy of Sciences, USA 83: Avise, J. C., Nelson, W.S., Arnold, J., Koehn, R.K., Williams, G.C. and Thorsteinsson, V The evolutionary genetic status of Icelandic eels. Evol. 44: Cairns, D.K., Shiao, J.C., Iizuka, Y., Tzeng, W.N., and MacPherson, C.D Movement patterns of American eels in an impounded watercourse, as indicated by otolith microchemistry. North Amer. J. Fish. Manage. 24: Casselman, J.M Chemical analyses of the optically different zones in eel otoliths. Proc N. Am. Eel Conf., pp De Leo, G.A., and M. Gatto A size and age structured model of the European eel (Anguilla anguilla). Can. J. Fish. Aquat. Sci. 52: Feunteun, E., Laffaille, P., Robinet, T., Briand, C., Baisez, A., Olivier, J.-M., and Acou, A A review of upstream migration and movements in inland waters by anguillid eels: towards a general theory. In Eel biology. Edited by K. Aida, K. Tsukamoto, and K. Yamauchi. Springer, Tokyo. pp

16 Helfman, G.S., Bozeman, E.L., and Brothers, E.B Comparison of American eel growth rates from tag returns and length-age analyses. US Fish. Bull. 82: Jessop, B.M Estimates of population size and instream mortality rate of American eel elvers in a Nova Scotia River. Trans. Am. Fish. Soc. 129: Jessop, B.M., Shiao, J.C., Iizuka, Y., and Tzeng, W.N Migratory behaviour and habitat use by American eels Anguilla rostrata as revealed by otolith microchemistry. Mar. Ecol. Prog. Ser. 233: Jessop, B.M., Shiao, J.C., Iizuka, Y, and Tzeng, W.N Variation in the annual growth, by sex and migration history, of silver American eels Anguilla rostrata. Mar. Ecol. Prog. Ser. 272: Legault, A Le franchissement des barrages par l'escalade de l'anguille: étude en Sèvre Niortaise. Bull. Fr. Pêche. Piscic. 308:1 10. Limburg, K.E., Wickstrom, H., Svedang, H., Elfman, M., and Kristiansson, P Do stocked freshwater eels migrate? Evidence from the Baltic suggests "yes." Amer. Fish. Soc. Symp. 33:

17 Morrison, W.E., Secor, D.H., and Piccoli, P.M Estuarine habitat use by Hudson River American eels as determined by otolith strontium:calcium ratios. Amer. Fish. Soc. Symp. 33: Oliveira, K Movements and growth rates of yellow-phase American eels in the Annaquatucket River, Rhode Island. Trans. Amer. Fish. Soc. 126: Secor, D.H., Henderson-Arzapalo, A., and Piccoli, P.M Can otolith microchemistry chart patterns of migration and habitat utilization in anadromous fishes? J. Exp. Mar. Biol. Ecol. 192: Tesch, F.W The eel. Blackwell Science, Oxford. Tsukamoto, K., and Arai, T Facultative catadromy of the eel Anguilla japonica between freshwater and seawater habitats. Mar. Ecol. Prog. Ser. 220:

18 2 MOVEMENT PATTERNS OF AMERICAN EELS (ANGUILLA ROSTRATA) BETWEEN SALT AND FRESH WATER IN A COASTAL WATERSHED BASED ON OTOLITH MICROCHEMISTRY [Manuscript submitted for publication to Marine Biology July 2005) 2.1 Abstract Otolith strontium:calcium ratios were used to trace lifetime movements of American eels (Anguilla rostrata) captured in saltwater bays and adjoining freshwater ponds in Prince Edward Island, Canada. A pond with a pool-and-weir salmonid fishway and a pond drained by a low-gradient channel contained eels that had entered fresh water at all ages, but a pond with a 2.2 m vertical spillway contained only eels that had entered fresh water in the elver year. Saltwater residents were the dominant migratory contingent in saltwater bays (85% of 39), which overturns the paradigm of obligate catadromy for this species. Freshwater residency was the sole pattern found in the pond with the vertical spillway (100% of 12) and the majority contingent in the pond with the low-gradient channel (54% of 24). Inter-habitat shifting was the dominant migratory contingent in eels sampled from the pond with the pool-and-weir fishway (85% of 20). Resident eels were established in salt and freshwater habitats by the year after their arrival in continental waters. Eels that shifted between habitats increased their rate of inter-habitat 10

19 shifting with age. The high degree of plasticity in habitat use found in this study is consistent with worldwide Anguillid patterns as revealed by Sr:Ca. 2.2 Introduction Anguillid eels are viewed as textbook catadromous species, spawning in the open sea, migrating to fresh water to rear and returning to the ocean to complete their life cycle. There is, however, great variability in continental habitat use, and anguillid eels in this phase may be found in habitats ranging from full strength salt water to fresh water (Daverat et al. 2004; Morrison et al. 2003). Analysis of strontium:calcium ratios of eel otoliths has greatly expanded our knowledge of eel movement patterns, showing that some eels settle and remain in salt, brackish or fresh water during their continental lives, while others shift among these habitats (Morrison et al. 2003; Tzeng et al. 1997, 2000; Tsukamoto and Arai 2001). In European (Anguilla anguilla) and Japanese (A. japonica) eels, Sr:Ca data have revealed that some eels spend their entire lives in salt water (Tsukamoto et al. 1998; Arai et al. 2003). These species can thus no longer be regarded as obligate catadromous fish, but instead use catadromy as a facultative life history option. Sr:Ca studies on American eels (A. rostrata) at three locations have shown a variety of inter-habitat movement patterns. However, none of these investigations has sampled eels in full-strength salt water. Yellow American eels are commonly found in salt water, 11

20 but it has not been demonstrated that American eels can complete their life cycles in the marine environment. Secor (1999) noted that estuarine and diadromous fishes commonly exhibit varying movement patterns among salt, brackish and fresh water. He termed these groups "migratory contingents." Tosi et al. (1988) and Édeline and Élie (2004) found that glass eels sampled from marine waters showed distinct salinity preferences in laboratory trials, some choosing salt water while others preferred fresh. This implies that membership in migratory contingents may be determined before arrival at the coast. Eels which attempt to ascend rivers may encounter natural or artificial obstacles. Eels under 10 cm long are able to creep up wet vertical surfaces (Legault 1988). Hence dams or waterfalls where water trickles down vertical walls may impose an age-dependent barrier to migration, with upstream movement possible only for young eels (Cairns et al. 2004). We used the Sr:Ca technique to investigate the ontogeny, frequency and directions of American eel movements between salt and fresh water in a small watershed in eastern Canada. To permit the examination of effects of obstacle type on movement patterns, we chose a study area consisting of saltwater bays and adjacent freshwater ponds which were formed by dams of three different types: earthen dam with a vertical spillway, concrete dam with a salmonid fish ladder and earthen dam with a low-gradient outlet channel. Patterns of movement between salinity zones, as indicated by Sr:Ca profiles, were used to test the following predictions: 12

21 1. Ponds formed by dams with vertical water drops (salmonid fish ladder, vertical spillway) will contain only eels that entered at a small size, but the pond drained by a low-gradient channel will contain eels that entered at all sizes. 2. Eels will show a variety of migratory contingents, including residence in salt water, residence in fresh water, and movements between these habitats. 3. Some eels will show saltwater residence only, indicating an exclusively marine life cycle. 4. Eels choose their migratory contingents upon arrival in continental waters, so that colonization of salt water and of adjacent freshwater ponds occurs simultaneously. 2.3 Methods This study was conducted on the north shore of Prince Edward Island in Brackley and Covehead Bays and associated ponds (Fig. 2-1). Both bays have full-strength salt water (>28 ppt) and their combined watersheds total 81 km 2. Four streams entering these bays are blocked by dams at head of tide, forming freshwater impoundments. Water exits McCallums Pond to Brackley Bay by falling vertically 2.2 m from a wooden spillway set in an earthen dam (Fig. 2-2). Cass Pond on Covehead Bay has a 5-chamber pooland-weir salmonid fishway through which water drops 0.9 vertical m over a horizontal distance of 12.2 m. Chambers are 1.8 m wide, m long and m deep. Water also leaves Cass Pond over a vertical concrete spillway 5 m wide. Marshalls Pond drains into Covehead Bay by a low-gradient channel with a rocky bottom that falls 5.0 vertical m over a horizontal distance of 303 m (1.7% slope). Parsons Pond connects to Covehead Bay via a culvert with wooden baffles on its floor that are intended to aid 13

22 fish movement. The sole unimpounded stream in the Brackley-Covehead system runs through the bed of the former McMillans Pond, whose dam washed out in Eels were fished by fyke net in Brackley and Covehead Bays, associated ponds and the stream running through McMillans pond bed in May-November Eels were anaesthetized with clove oil, measured for total length, weighed and frozen until the otoliths were removed. One otolith per eel was subjected to microchemical analysis. Otoliths were embedded in epofix resin, then ground and polished until the primordium was exposed. The otoliths were then carbon coated under a high vacuum evaporator prior to analysis with an electron probe microanalyzer (EPMA). Using a JEOL JXA-8900R system equipped with wavelength dispersive X-ray spectrometers, Sr and Ca concentrations were measured at 10 μm intervals from the primordium to the otolith edge. Beam conditions were an acceleration voltage of 15 kv, a current of 3nA and a 5 X 4 μm rectangular scanning beam. A synthetic aragonite (CaCO 3 ) and strontiantite (SrCO 3 NMNH R10065) were used for standard calibrations. Sr concentrations were measured for 80 s at Sr Lα peak positions and 20 s at both the lower and upper sides of the baseline. Ca was measured for 20 s at the Ca Kα peak and for 10s at both sides of the baseline. After Sr:Ca ratio analysis the otolith was polished to remove the carbon layer and etched for 1 to 2 min with 5% EDTA to reveal annular rings for age determination. 14

23 Sr:Ca reference levels were established to identify occupancy of fresh, salt and interhabitat transition waters, on the basis of eels from the present study that occupied only one habitat type after age 1 (see Results). A five-point running mean was used to smooth short-term fluctuations in Sr:Ca ratios that are likely due to otolith surface flaws or analytic artifacts (Kotake et al. 2003). Smoothed Sr:Ca ratios that were less than the mean +2 SD of smoothed measurements of age 1+ eels from McCallums Pond were considered to indicate freshwater occupancy (F). Smoothed Sr:Ca ratios that were greater than the mean -2 SD of smoothed measurements of 15 saltwater resident age 1+ eels from Brackley Bay were considered to indicate saltwater occupancy (S). Sr:Ca ratios intermediate between these reference levels were considered to indicate transition between habitat types (T). Eels were categorized as freshwater residents, saltwater residents or inter-habitat shifters on the basis of habitat occupancy patterns. An eel was categorized as a freshwater resident if its combined smoothed Sr:Ca ratios after age 1 were >97% F and <3% S, or >92% F, <8% T and 0% S. Saltwater residents included eels whose post age 1 Sr:Ca ratios were >97% S and <3% F, or >92% S, <8% T and 0% F. Eels that fell into neither of these categories were termed inter-habitat shifters. For each eel the proportions of smoothed measurements in F, S and T were calculated within each year, and then summed across all years to give eel-years for each category. The mean Sr:Ca value within each year for each eel was also calculated from smoothed measurements. Changes in an eel s smoothed Sr:Ca ratio between F and S after age 1 15

24 were considered to indicate inter-habitat shifts. Shifts did not include transfers to T. Shifts were only counted if they were at least four Sr:Ca sample points (40 μm) past the year one marker. 2.4 Results We caught eels at all fishing locations except McMillans pond bed (Table 2-1). We stopped fishing at Parsons Pond before obtaining sufficient eels for analysis due to excessive lethal bycatch of white perch (Morone americana). Otoliths from 95 eels captured in Brackley and Covehead Bays and McCallums, Cass and Marshalls Ponds were analysed for Sr:Ca ratios. Sr:Ca ratios peaked during the oceanic phase, fell as eels arrived in continental waters and then showed a variety of patterns (Fig. 2-3). Overall frequency distributions of smoothed Sr:Ca ratios in age 1+ eels were bimodal in samples from both salt and freshwater sites (Fig. 2-4). Modal values were <0.5 x 10-3 and 5 x The high mode dominated saltwater samples and the low mode dominated freshwater samples. Some Sr:Ca trajectories of individual eels were stable near one of the two modes, while others varied widely (Fig. 2-3). All 12 eels from McCallums Pond showed low and stable Sr:Ca ratios, with a mean of 0.57 x 10-3 (SD=0.61 x 10-3 ). We considered these eels to be freshwater residents (F), and set the upper boundary for freshwater occupancy to be the McCallums mean + 2 SD (1.8 x 10-3 ). In Brackley Bay, 15 eels showed high and stable Sr:Ca ratios (mean=5.24 x 10-3, SD=1.28 x 10-3 ) and one eel showed a variable ratio. We considered the 15 Brackley eels with high and stable ratios to be 16

25 saltwater residents (S), and set the lower boundary for saltwater occupancy to be their mean ratio - 2 SD (2.7 x 10-3 ). Smoothed ratios between 1.8 x 10-3 and 2.7 x 10-3 were considered to indicate inter-habitat transition (T). The proportion of eels aged 1+ that were freshwater residents, saltwater residents and inter-habitat shifters differed significantly among ponds (G-test, G=43.4, P<0.001) but not between bays (G=1.92, P>0.05) (Table 2-2). Saltwater residents dominated samples from both Brackley (93.8%) and Covehead Bays (78.3%). All eels sampled from McCallums Pond were freshwater residents. Most (85%) eels sampled from Cass Pond were inter-habitat shifters. Marshalls Pond samples were 54.2% freshwater residents and 33.3% saltwater residents. When habitat use was measured in eel-years, a significant difference was found between the bays (G=12.31, P<0.01), though salt water use dominated samples from both bays (Brackley 98.9%; Covehead 87.5%). Freshwater use was the sole pattern in McCallums Pond samples. Eels from Cass Pond spent slightly more time in fresh water (52.5%) than in salt water (43.9%) and Marshalls Pond eels spent the majority (79.7%) of their time in fresh water. Proportion of time spent in the various habitats differed significantly among ponds (G=119.3, P<0.001). Eels from all sites except McCallums Pond showed inter-habitat shifts (Table 2-2). Mean rate of shifting did not differ between samples in the bays (Brackley 0.12 shifts per eel, Covehead 0.35 shifts per eel; ANOVA F=1.02, P>0.05). Rate of shifting differed among pond samples (ANOVA, F=28.2, P<.0001), with highest rates found in Cass Pond (1.75 shifts per eel). 17

26 The number of shifts from salt to fresh water per eel-year for inter-habitat shifters sampled in Cass Pond increased significantly with age (logistical regression t-ratio = 2.04, P<0.05; Fig. 2-5). Rate of shifting from fresh to saltwater did not differ significantly with age (t-ratio=0.95, P>0.05). Overall rate of shifting between habitats increased with age (t-ratio=2.08, P<0.05). Fig. 2-6 plots the percent of time spent in salt, transition and fresh waters vs. age by inter-habitat shifters. For shifters captured in Covehead Bay, percent of time in salt water increased, and percent of time in fresh water decreased, with age (salt: Spearman R=0.406, P<0.05; fresh: Spearman R= , P<0.05; n=29 eel-years). For shifters from Cass Pond, percent of time in saltwater decreased, and percent of time in freshwater increased, with age (salt: Spearman R=-0.305, P=0.003; fresh: Spearman R=0.263, P=0.001; n=29). Percent time allocated to salt and fresh habitats by shifters captured in Marshalls Pond did not show significant trends with age (salt: Spearman R=0.318, P>0.05; fresh: Spearman R= , P>0.05; n=21). 2.5 Discussion Effect of obstacle type on upstream movement Eels cannot swim directly against strong currents, but those smaller than 10 cm can creep up damp vertical walls (Legault 1988). We therefore expected that eels would colonize ponds with vertical water drops (McCallums, Cass) only at young ages, while colonization of Marshalls Pond, which drains by a low-gradient channel, would occur at all ages. This was confirmed for McCallums Pond, where Sr:Ca ratios showed that all 18

27 captured eels had entered freshwater in their elver year (Fig. 2-3). The dam at this site, with its 2.2 m vertical water drop, evidently posed an obstacle to upstream migration of post-elver eels. Contrary to prediction, eels of all ages moved between salt water and Cass Pond, indicating that the pool-and-weir salmonid fishway at that site did not impede upstream migration. Eels of all ages also moved between salt water and Marshalls Pond, showing, as expected, that that pond's low-gradient channel could be readily ascended by eels Movement patterns This study identified three migratory contingents of American eels. Saltwater residents dominated samples in two saltwater bays (84.6% of 39). Freshwater residents were the sole contingent found in McCallums Pond, which is accessible only to eels in their first continental year. Inter-habitat shifters were found in all study sites except McCallums Pond. Shifters comprised of 85% of samples in Cass Pond and 12.5% in Marshalls Pond, whose dams did not impede inter-habitat movement. Sr:Ca life history data are now available for the continental phases of naturally-reared Anguilla eels of six species at 39 locations worldwide (Table 2-3, Fig. 2-7). These studies show that plasticity of habitat usage is the norm among eels. For all species combined, 27 of 39 (69%) samples contained eels that had shifted from the habitat where they were captured. Fidelity to habitat type appears to be commonest in freshwater, where one half (7 of 14) of samples showed fresh water residency only. The exclusivity of this contingent at many locations may be due to long distances to other 19

28 salinity zones (Hudson River, USA, Morrison et al. 2003; Pearl River, China, Tzeng et al. 2003a), or to dams which impede upstream movements of eels at post-elver ages (Cairns et al. 2004, this study). Most samples of eels from brackish and salt water contained eels that had used other habitats (brackish: 13 of 15, 87%; salt: 7 of 10, 70%). Movement patterns shown by American eels in the Brackley-Covehead system reflect the high plasticity of Anguillid habitat use worldwide. Sr:Ca studies have demonstrated that some European and Japanese eels never leave salt water (Tsukamoto et al. 1998; Arai et al. 2003). This study is the first to measure Sr:Ca ratios of American eels sampled in salt water. Our finding that most eels from Brackley and Covehead Bays had an exclusive saltwater Sr:Ca profile indicates that American eels can likewise complete their life cycle in the sea. The catadromy paradigm for the American eel is thus overturned. Like European and Japanese eels, American eels must now be considered as species where catadromy is a facultative life history option. The high representation of exclusive saltwater residency in eels from bay samples (84.6%) also suggests that non-catadromy may be an important and common pattern for American eels. Tosi et al. (1988) and Édeline and Élie (2004) found that European glass eels have distinct individual salinity preferences. This implies that young eels separate into migratory contingents upon arrival on the coast, with salt-seeking eels remaining in marine waters while fresh-seekers ascend into fresh waters. The freshwater ponds in the Brackley-Covehead system are adjacent to saltwater bays, and all are within 5 km of the 20

29 open sea. Given a salinity preference that is activated on arrival in coastal waters, and an upstream ascent rate of 0.6 km/day (White and Knights 1997), we hypothesized that there would be little time lag between invasion of salt and fresh waters. Consistent with expectation, saltwater and freshwater residents were established in their respective habitats by age 1 (Fig. 2-3). Early movement to settlement areas may have an adaptive basis. Because of their superior climbing ability, young eels can overcome vertical barriers to upstream migration (such as the dam at McCallums Pond) and reach habitats that are inaccessible at older ages. Eels may choose among salinity zones, and they may also choose between sedentary and mobile lifestyles (Feunteun et al. 2003). Some eels in the Brackley-Covehead system shifted between habitats only once and then remained in the new habitat. This could be interpreted as searching for suitable habitat, and then settling there when they find it (Fig. 2-3). However, others shifted continuously, suggesting that nomadic behaviour is an inherent property of some eels Implications for conservation American eel abundances have declined since the mid 1980 s (Jessop 2000). Possible causes for the reduction in numbers include adverse oceanic conditions, habitat loss, excessive fishing pressure and obstruction to eel passage upstream. Dams and other obstacles can prevent or impede migration and adversely affect eel populations (Legault 1988, Feunteun et al. 2003). 21

30 Prince Edward Island has over 800 artificial impoundments (MacFarlane 1999), and most streams are blocked by one or several dams. Our results suggest that eels navigate dams equipped with pool-and-weir salmonid fishways or low-gradient channels. Ponds formed by dams with vertical spillways can be colonized, but only when eels are small and can climb vertical surfaces. The declines in eel abundance indices have prompted efforts to devise management schemes that would assure adequate escapement to the spawning grounds (Richkus and Whalen 2000, ICES 2003). Such schemes must recognise contributions of escaping silver eels from unfished, as well as fished areas. In much of eastern North America eel exploitation is restricted to coastal and estuarine waters. Our results suggest that fishing in marine waters may also affect eel populations in nearby fresh waters, due to movement between the two habitats. Neither marine populations nor those of adjacent fresh water are discrete. Population models that estimate escapement of silver eels from salt, brackish and freshwater habitats must account for these movements. The relative importance of marine and fresh waters to the eel's life cycle, and patterns of migration between these habitats, are vital issues in eel biology and conservation. The Sr:Ca technique will find wide employ as the key to their understanding. 2.6 Acknowledgements This study received support from the National Science Council, ROC (NSC B and B ). We thank Angus McLennan, Corey Muttart, Valérie 22

31 Tremblay, Noella McDonald and Robbie Moore for assistance in the laboratory and field, and Mark Grimmett for measuring chemical concentrations in water samples. Tillmann Benfey and Allen Curry provided valuable advice at all stages and improved the manuscript with their comments. 2.7 Literature Cited Arai, T., Kotake, A., Lokman, P.M., Miller, M.J., and Tsukamoto, K Evidence of different habitat use by New Zealand freshwater eels Anguilla australis and A. dieffenbachii, as revealed by otolith microchemistry. Mar. Ecol. Prog. Ser. 266: Arai, T., Kotake, A. Ohji, M., and Miller, M.J Occurrence of sea eels of Anguilla japonica along the Sanriku Coast of Japan. Ichthyol. Res. 50: Cairns, D.K., Shiao, J.C., Iizuka, Y., Tzeng, W.N., and MacPherson, C.D Movement patterns of American eels in an impounded watercourse, as indicated by otolith microchemistry. North Amer. J. Fish. Manage. 24: Daverat, F., Élie, P., and LaHaye, M Première caractérisation des histoires de vie des anguilles (Anguilla anguilla) occupant la zone aval du bassin versant Gironde-Garonne-Dordogne: apport d'une méthode de microchimie. Cybium: 28(suppl. 1):

32 Édeline, É., and Élie, P Is salinity choice related to growth in juvenile eel Anguilla anguilla? Cybium 28(suppl. 1): Feunteun, E., Laffaille, P., Robinet, T., Briand, C., Baisez, A., Olivier, J.-M., and Acou, A A review of upstream migration and movements in inland waters by anguillid eels: towards a general theory. In Eel biology. Edited by K. Aida, K. Tsukamoto, and K. Yamauchi. Springer, Tokyo. pp Gross, M.R Evolution of diadromy in fishes. Amer. Fish. Soc. Symp. 1: Ibbotson, A., Smith, J., Scarlett, P., and Aprahamian, M Colonization of freshwater habitats by the European eel Anguilla anguilla. Freshwater Biol. 47: ICES Report of the thirteenth session of the joint EIFAC/ICES working group on eels. EIFAC Occasional Paper no. 36. Jessop, B.M Migrating American eels in Nova Scotia. Trans. Amer. Fish. Soc. 116: Jessop, B.M Estimates of population size and instream mortality rate of American eel elvers in a Nova Scotia River. Trans. Am. Fish. Soc. 129:

33 Jessop, B.M., Shiao, J.C., Iizuka, Y., and Tzeng, W.N Migratory behaviour and habitat use by American eels Anguilla rostrata as revealed by otolith microchemistry. Mar. Ecol. Prog. Ser. 233: Jessop, B.M., Shiao, J.C., Iizuka, Y, and Tzeng, W.N. Submitted. Migration between freshwater and estuary of juvenile American eels Anguilla rostrata as revealed by otolith microchemistry. Mar. Ecol. Prog. Ser. Kotake, A., Arai, T., Ozawa, T., Nojima, S., Miller, M.J., and Tsukamoto, K Variation in migratory history of Japanese eels, Anguilla japonica, collected in coastal waters of the Amakusa Islands, Japan, inferred from otolith Sr/Ca ratios. Mar. Biol. 142: Kotake, A., Arai, T., Ohji, M., Yamane, S., Miyazaki, N., and Tsukamoto, K Application of otolith microchemistry to estimate the migratory history of Japanese eel Anguilla japonica on the Sanriku coast of Japan. J Appl Icthyol 20: Legault, A Le franchissement des barrages par l'escalade de l'anguille: étude en Sèvre Niortaise. Bull. Fr. Pêche Piscic. 308: Limburg, K.E., Wickstrom, H., Svedang, H., Elfman, M., and Kristiansson, P Do stocked freshwater eels migrate? Evidence from the Baltic suggests "yes." Amer. Fish. Soc. Symp. 33:

34 MacFarlane, R.E An evaluation of the potential impacts of some Prince Edward Island impoundments on salmonid habitat. M.Sc. thesis, Acadia University, Wolfville, Nova Scotia. Morrison, W.E., Secor, D.H., and Piccoli, P.M Estuarine habitat use by Hudson River American eels as determined by otolith strontium:calcium ratios. Amer. Fish. Soc. Symp. 33: Richkus, W.A., and Whalen, K Evidence for a decline in the abundance of the American eel, Anguilla rostrata (LeSueur), in North America since the early 1980s. Dana 12: Secor, D.H Specifying divergent migrations in the concept of stock: the contingent hypothesis. Fish. Res. 43: Shiao, J.C., Iizuka, Y., Chang, C.W., and Tzeng, W.N Disparities in habitat use and migratory behavior between tropical eel Anguilla marmorata and temperate eel A. japonica in four Taiwanese rivers. Mar. Ecol. Prog. Ser. 261: Tosi, L., Sala, L., Sola, C., Spampanato, A., and Tongiorgi, P Experimental analysis of the thermal and salinity preferences of glass-eels, Anguilla anguilla (L.), before and during the upstream migration. J. Fish. Biol. 33:

35 Tsukamoto, K., and Arai, T Facultative catadromy of the eel Anguilla japonica between freshwater and seawater habitats. Mar. Ecol. Prog. Ser. 220: Tsukamoto, K, Nakai, I., and Tesch, W.V Do all freshwater eels migrate? Nature 396: Tzeng, W.N., Iizuka, Y., Shiao, J.C., Yamada, Y., and Oka, H.P. 2003a. Identification and growth rates comparison of divergent migratory contingents of Japanese eel (Anguilla japonica). Aquaculture 216: Tzeng, W.N., Severin, K.P., and Wickstrom, H Use of otolith microchemistry to investigate the environmental history of European eel Anguilla anguilla. Mar. Ecol. Prog. Ser. 149: Tzeng, W.N., Shiao, J.C., and Iizuka, Y Use of otolith Sr: Ca ratios to study the riverine migratory behaviors of Japanese eel Anguilla japonica. Mar. Ecol. Prog. Ser. 245: Tzeng, W.N., Shiao, J.C., Yamada, Y., and Oka., H.P. 2003b. Life history patterns of Japanese eel Anguilla japonica in Mikawa Bay, Japan. Amer. Fish. Soc. Sym. 33:

36 Tzeng, W.N., Wang, C.H., Wickstrom, H., and Reizenstein, M Occurrence of the semi-catadromous European eel Anguilla anguilla in the Baltic Sea. Mar. Biol. 137: White, E.M., and Knights, B Dynamics of upstream migration of the European eel, Anguilla anguilla (L.), in the Rivers Severn and Avon, England, with special reference to the effects of man-made barriers. Fish. Manage. Ecol. 4:

37 Table 2-1. Catch and catch rates of American eels in fyke nets in the Brackley- Covehead system. Site Number Number Number of of caught eels gear-days per caught gear-day Brackley Bay Covehead Bay McCallums Pond McMillans pond bed Cass Pond Marshalls Pond Parsons Pond

38 Table 2-2. Salinity history and rate of inter-habitat shifting of American eels captured in saltwater and freshwater sites. See text for definitions. Salinity history by number a Salinity history by eel-years b Number of inter-habitat Number Fresh water Inter-habitat Salt water Fresh Transition Salt Number shifts per eel of residents shifters residents Eel-years % Eel-years % Eel-years % with edge Mean SD Range eels Number % Number % Number % mismatch c Brackley Bay Covehead Bay McCallums Pond Cass Pond Marshalls Pond a Proportion of eels with different salinity histories did not differ significantly among bays (G=1.92, P>0. 05), but differed significantly among ponds (G=43.4, P=0.005). b Proportion of eel-years with different salinity histories differed significantly among bays (G=12.31, P< 0.05) and ponds (G=119.3, P=0.005). c Eels with edge mism atch are those whose otolith edge Sr:Ca ratio indicated fresh water but which were captured in salt water, and those whose otolith edge Sr:Ca ratio indicated salt water but which were captured in fresh water. 30

39 Table 2-3. Habitat occupancy patterns of wild (non-stocked) yellow and silver Anguilla eels after arrival in continental waters, as inferred from otolith Sr:Ca ratios. Eels sampled in salt water Eels sampled in brackish water Sampling Mig. a N Percent occupancy history b Sampling Mig. a N Percent occupancy history b Sampling Mig. a N Eels sampled in fresh water Percent occupancy history b Study location habitat S SB B BF F SBF habitat S SB B BF F SBF habitat S SB B BF F SBF Ref. c A. rostrata Gulf of St. Lawrence Estuary N Impoundment N Gulf of St. Lawrence Coastal bays N Impoundments N Nova Scotia River N Nova Scotia River M Nova Scotia River N Hudson River Estuary N River N A. anguilla Sweden Coastal waters N Sweden Coastal waters, estuaries N Sweden Estuaries M Baltic Sea exit Coastal waters M Germany Open sea M,N River N France Bay N Estuary N River N A. japonica Japan Bay N Japan Bays M,N Japan Coastal waters M Japan d Bay M Japan Bay M Japan River N Japan Coastal waters M,N Estuaries N River M,N East China Sea Open sea M Pearl R., China River M Taiwan Estuary M,N River M,N Taiwan Estuary M.N River Taiwan d Estuary M Taiwan d Estuary N A. marmorata Taiwan Estuary M,N River M,N A. australis New Zealand Coastal lagoon M A. dieffenbachii New Zealand Coastal lagoon M a M - on spawning migration, N - not on spawning migration S -salt, B - brackish, F - fresh c References: 1 - Cairns et al. 2004, 2 - this study, 3 - Jessop et al. 2002, 4 - Jessop et al. submitted, 5 - Morrison et al. 2003, 6 - Tzeng et al. 1997, 7 - Tzeng et al. 2000, 8 - Limburg et al. 2003, 9 - Tsukamoto et al. 1998, 10 - Daverat et al. 2004, 11 - Kotake et al. 2004, 12 - Arai et al. 2003, 13 - Kotake et al. 2003, 14 - Tzeng et al. 2003a, 15 - Tzeng et al. 2003b, 16 - Tsukamoto and Arai 2001, 17 - Shiao et al. 2003, 18 - Tzeng et al. 2002, 19 - Arai et al d Type B occupancy history may also include types SB, BF, and SBF