Habitat use by the European eel Anguilla anguilla in Irish waters

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1 Estuarine, Coastal and Shelf Science 67 (2006) 569e578 Habitat use by the European eel Anguilla anguilla in Irish waters T. Arai a, *, A. Kotake b, T.K. McCarthy c a International Coastal Research Center, Ocean Research Institute, The University of Tokyo, , Akahama, Otsuchi, Iwate , Japan b Ocean Research Institute, The University of Tokyo, Nakano, Tokyo , Japan c Department of Zoology, National University of Ireland, Galway, University Road, Galway, Ireland Received 17 October 2005; accepted 3 January 2006 Available online 17 February 2006 Abstract The apparent use of marine and freshwater habitats by European eel Anguilla anguilla was examined by analyzing the strontium (Sr) and calcium (Ca) concentrations in otoliths of the eels collected from Irish coastal and fresh waters. The age and growth of eels were also examined using their otolith annuli. The sizes and ages of the female eels were greater than those of the males. The somatic growth rates ranged from 15 to 62 mm/year, which is typical for Ireland and other European countries. Analyses of Sr:Ca ratios along a life history transect in each otolith showed peaks (maximum more than ÿ3 ) between the core and elver mark corresponding to the period of their leptocephalus and early glass eel stages in the ocean. Outside the elver mark, the Sr:Ca ratios indicated that eels had remained in different habitats that included freshwater (average Sr:Ca ratios, 0.98e ÿ3 ) and areas with relatively high salinities (average Sr:Ca ratios, 6.73e ÿ3 ). Some individuals showed clear evidence of shifts from sea to fresh waters. These findings suggest that Irish eels have the same behavioral plasticity regarding whether or not to enter freshwater or remain in marine environments as has been recently documented in this species and several other temperate anguillid species. However, patterns of habitat use in Irish waters were somewhat different than those previously reported for other habitats. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Anguilla anguilla; age; growth; otolith microchemistry; habitat use; migration 1. Introduction The life cycle of the European freshwater eel Anguilla anguilla Linnaeus, 1758 has five principal stages: the leptocephalus, glass eel, elver, yellow eel and silver eel stages (Bertin, 1956). The spawning area of A. anguilla is in the Sargasso Sea (Schmidt, 1922, 1925). The larvae, leptocephali, drift on the Gulf Stream and are further transported by the North Atlantic Current across the Atlantic Ocean (Schmidt, 1922, 1925; Boëtius, 1985). The leptocephali presumably leave oceanic currents after metamorphosing into glass eels and then typically migrate upstream as elvers, 6e8 months after hatching (Arai et al., 2000), to grow in the freshwater * Corresponding author. address: arai@wakame.ori.u-tokyo.ac.jp (T. Arai). habitats of Europe and North Africa during the yellow stage. At ages that vary widely among individuals of both sexes, but especially for the larger older females, the yellow eels metamorphose into silver eels, which migrate downstream to the ocean to begin their journey to their spawning areas in the Sargasso Sea (Tesch, 2003). Recently, the migratory history of several species of anguillid eels have been studied using microchemical techniques that determine the ratios of strontium to calcium (Sr:Ca ratio) in their otoliths. The Sr:Ca ratio in the otoliths of fishes differs according to the time they spend in freshwater and seawater; this has also been found to be true for anguillid eels (Tsukamoto et al., 1998; Tzeng et al., 2000; Tsukamoto and Arai, 2001; Jessop et al., 2002; Arai et al., 2003a,b, 2004; Kotake et al., 2003, 2005). Early studies on the strontium incorporation into eel otoliths of Anguilla japonica showed that the Sr:Ca level in their otolith strongly correlated /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.ecss

2 570 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e578 with the salinity of the water and little affected by other factors such as water temperature, food and physiological factors (Kawakami et al., 1998). Thus, the Sr:Ca ratios of otoliths could help in determining whether or not individual eels actually enter freshwater at the elver stage and remain in freshwater, estuarine or marine environments until the silver eel stage, or whether they move between different habitats with differing salinity regimes. Otolith microchemistry studies have revealed that some yellow and silver eels of temperate Anguilla anguilla and Anguilla japonica never migrate into freshwater, but spend their entire life history in the ocean (Tsukamoto et al., 1998). Application of otolith Sr:Ca ratios to trace the migratory history of eels has also revealed otolith signatures intermediate to those of marine and freshwater residents of A. anguilla (Tzeng et al., 2000), A. japonica (Tsukamoto and Arai, 2001; Arai et al., 2003a,b; Kotake et al., 2003, 2005), Anguilla rostrata (Jessop et al., 2002), Anguilla australis, and Anguilla dieffenbachii (Arai et al., 2004), all of which appeared to reflect estuarine residence, or showed clear evidence of switching between different salinity environments. It thus appears that a proportion of eels move frequently between different environments during their growth phase. Therefore, because individuals of several anguillid species have been found to remain in estuarine or marine habitats, it appears that anguillid eels do not all enter into freshwater environments and that these species display more a facultative catadromy (Tsukamoto and Arai, 2001). Although Sr:Ca ratios have been studied in the otoliths of yellow and silver eels of the five species of temperate anguillid eels, there have been only several studies of this nature on these species including Anguilla anguilla. Therefore, it is not known if all populations display the same utilization of both estuarine and marine environments in addition to the typical freshwater environments in A. anguilla. To begin to address this question, we analyzed the Sr:Ca ratios in the otoliths of yellow and silver eels of A. anguilla that were caught in a bay and several rivers on the western side of Ireland. The objective of this study was to use Sr:Ca ratios to reconstruct the environmental history of the European eel Anguilla anguilla captured in Irish waters, and thereby to determine the salinity environments that each individual had experienced and to compare these data with the age and growth of each eel. This approach enables a greater understanding of the biological characteristics and apparent habitat use of the species. 2. Materials and methods 2.1. Fish A total of 75 specimens of Anguilla anguilla were sampled, using coghill nets, by electronic fishing and by hand capture in one bay and six sites of four river systems in Ireland in November 2003 (Fig. 1, Table 1). None of the sampling sites in the river systems, River Garavogue, River Moy, River Corrib, River Shannon (Killaloe, Castleconnell) and Lough Derrevaragh (upper part of the River Shannon system) were influenced by tides and their salinity was 0. All eels were captured commercially by silver eel fishing except for the Castleconnell site in River Shannon. At the sampling site in Galway Bay, which has fully saline waters, eels were collected by hand under intertidal stones and seaweeds. After measurement of total length (TL, to 1 mm), body weight (BW, to 1 g) and eye diameter (to 0.01 mm), the sex of each eel >300 mm long was determined by visual observation of the gonads according to Tesch (2003), i.e. eels having thin, regularly lobed organs were males, while individuals having more broad and folded curtain-like gonads were females. An eye index appears to be one of the best indicators of the onset of reproductive maturation in silver eels of this species (Pankhurst, 1982). We classified eels with an eye index less than 6.5 as sexually immature adults (yellow eel), and those over 6.5 as sexually mature adults (silver eels) (Table 2). The eye index was calculated according to Pankhurst (1982) as follows: Eye index ¼ ½ðA þ BÞ=4Š 2 p=tl 100 where A is the horizontal eye diameter (mm) and B is the vertical eye diameter (mm) Otolith preparation and microchemical analysis Sagittal otoliths were extracted from each fish, embedded in epoxy resin (Struers, Epofix) and mounted on glass slides. The otoliths were ground and polished, as described by Arai et al. (2004), cleaned in an ultrasonic bath, and rinsed with deionized water prior to being examined. For electron microprobe analyses, all otoliths were PtePd coated by a high vacuum evaporator. All otoliths were used for life history transect analysis of Sr and Ca concentrations, which were measured along the longest axis of each otolith from the core to the edge using a wavelength dispersive X-ray electron microprobe (JEOL JXA-8900R), as described in Arai et al. (1997, 2004). Wollastonite (CaSiO 3 ) and Tausonite (SrTiO 3 ) were used as standards. The accelerating voltage and beam current were 15 kv and ÿ8 A, respectively. The electron beam was focused on a point 10 mm in diameter, with measurements spaced at 10 mm intervals. The detection limits of Ca and Sr were 88.7 and mg/g, respectively, and their standard deviations (SD) were in the range of 0.14e0.26% and 0.22e0.24%, respectively Age estimation and data analyses Following the microchemistry analyses, the otoliths were repolished to remove the coating. Otoliths were then etched with 1% HCl for 60 s, stained with 1% toluidine blue and aged by counting the number of blue-stained transparent zones, as reported in Arai et al. (2003a,b, 2004). The ages given in this study are up to the last annuli and do not include any additional age of less than one year. The mean positions of the transparent zones for all eels were calculated

3 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e Fig. 1. Map showing the collection sites (numbered) of the European eel Anguilla anguilla in Irish waters. There are shown only rivers studied in the present study. Numbers adjacent to symbols indicate fished sites. and correlated to elemental analysis points. The relative ages at particular elemental analysis points could then be assigned. The growth rate for each individual was calculated by dividing the TL of individuals minus 70 mm, which is the mean sizes of the glass eel when they recruit to coasts (Svedang et al., 1996) following the formula: growth rate ¼ (TL ÿ 70)/Age Statistical analyses Differences among data were tested first by analysis of variance (ANOVA) and then with Scheffe s multiple range tests for pairwise comparisons. The significance of the correlation coefficient and regression slope were tested by Fisher s Z-transformation and an analysis of covariance (ANCOVA) (Sokal and Rohlf, 1995). 3. Results 3.1. Biological characteristics The eye index of Anguilla anguilla collected from seven sites ranged from 2.7 to 10.8 (Table 2). Sex in all eels collected from Galway Bay could not be differentiated, and thus these eels were identified as yellow eels (Table 2). Based on the result and the Pankhurst criteria (1982), there were in total 37 yellow and 37 silver eels (Table 2), and one female from River Garavogue that could not be identified as either yellow or silver stage (Table 2). Total lengths of Anguilla anguilla collected from seven sites ranged from 129 to 805 mm (Table 1). The total lengths of the yellow eels ranged from 326 to 407 mm with a mean SD of mm for males; for females, they ranged from 477 to 725 mm with a mean of mm. The total lengths of the silver eels ranged from 334 to

4 572 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e578 Table 1 Biological characteristics of 75 specimens used for otolith microchemistry analyses. Numbers for each sampling location correspond to Fig. 1 Growth rate (mm/year) Age (years) Body weight (g) Total length (mm) Sampling location Sex No. of fish examined Mean SD Range Mean SD Range Mean SD Range Mean SD Range 1. River Garavogue, Sligo Male e e e e21.9 Female e e e e River Moy, Mayo Male e e e e36.3 Female e e e e River Corrib, Galway Male e e e e46.2 Female e e e e Galway Bay Undifferentiated e e e e River Shannon, Killaloe Male e e e e28.8 Female e e e e River Shannon, Castleconnell Not determined e e e e Lough Derrevaragh Male e 118 e 16 e 22 e Female e e e e mm with a mean SD of mm for males; for females, they ranged from 344 to 805 mm with a mean of mm. There were significant differences in the total lengths between the sexes for both yellow and silver eels (ANOVA, df ¼ 17e35, p < e0.0001). However, no significant differences occurred in the total lengths between yellow and silver eels in either sex (ANOVA, df ¼ 20e32, p > 0.5). The body weights of Anguilla anguilla collected from seven sites ranged from 2 to 955 g (Table 1). The body weights of the yellow eels ranged from 68 to 98 g with a mean of g for males; for females, they ranged from 182 to 651 g with a mean of g. The body weights of the silver eels ranged from 64 to 154 g with a mean of g for males; for females, they ranged from 100 to 955 g with a mean of g. There were significant differences in the body weights between the sexes for both yellow and silver eels (ANOVA, df ¼ 17e35, p < 0.01e0.0001), but not between yellow and silver eels in each sex (ANOVA, df ¼ 20e32, p > 0.5). Close linear relationships appeared between total length and body weight for each sex in either yellow or silver eels (ANCOVA, df ¼ 29e35, p < e ), except for yellow eel males due to the limited number of specimens (three specimens) (Fig. 2). The ages of Anguilla anguilla collected from seven sites based on the number of annual rings in their otoliths ranged from 2 to 24 years (Table 1). The ages of A. anguilla yellow eels ranged from 6 to 13 years with a mean SD of years for males; for females, they ranged from 11 to 22, with a mean of years. The ages of the silver eels ranged from 8 to 19 years with a mean SD of years for males; for females, they ranged from 9 to 24 years with a mean of years. The ages of the males of both yellow eels and silver eels were significantly less (ANOVA, df ¼ 17e35, p < 0.05) on average than those of females. However, no significant differences occurred in the ages between yellow and silver eels in either sex (ANOVA, df ¼ 20e32, p > 0.5). A close linear relationship occurred between age and total length in female yellow eels (ANCOVA, df ¼ 29, p < 0.05), while no linear relationship existed in male yellow eels or in silver eels of either sex (ANCOVA, df ¼ 5e35, p > 0.05) (Fig. 3). There was also a close significant linear relationship between age and body weight in male silver eels (ANCOVA, df ¼ 35, p < 0.05), while no linear relationship existed in yellow eels of either sex or in female silver eels (ANCOVA, df ¼ 5e35, p > 0.05) (Fig. 3). The somatic growth rates of Anguilla anguilla collected from seven sites ranged from 14.5 to 62.0 mm/year (Table 1). The growth rates of A. anguilla yellow eels ranged from 25.9 to 46.2 mm/year with a mean of mm/year for males; for females, they ranged from 26.9 to 49.5 mm/ year with a mean of mm/year. The growth rates of the silver eels ranged from 14.5 to 36.3 mm/year with a mean of mm/year for males; for females, they ranged from 16.9 to 54.9 mm/year with a mean of mm/year. The growth rates differed significantly between yellow and silver eels in males and between sexes

5 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e Table 2 Developmental stage of 75 specimens used for otolith microchemistry analyses nd: not determined. Numbers for each sampling location correspond to Fig. 1 Sampling location Sex No. of fish examined Eye index Mean SD Range Developmental stage 1. River Garavogue, Sligo Male e9.1 Silver 5 Female e10.8 Silver 4 nd 1 2. River Moy, Mayo Male e10.6 Yellow 1 Silver 4 Female e8.2 Yellow 3 Silver 2 3. River Corrib, Galway Male e7.9 Yellow 2 Silver 3 Female e8.6 Yellow 4 Silver 5 4. Galway Bay Undifferentiated 10 nd Yellow River Shannon, Killaloe Male e10.5 Silver 5 Female e10.2 Silver 5 6. River Shannon, Castleconnell nd e5.1 Yellow Lough Derrevaragh Male Silver 1 Female e7.0 Yellow 7 Silver 3 No. of fish in silver eels (ANOVA, df ¼ 20e35, p < 0.01e0.001), but not between yellow and silver eels in females or between sexes in yellow eels (ANOVA, df ¼ 17e32, p > 0.5) Otolith microchemistry The Sr:Ca ratios in the transects along the radius of each otolith showed the same common feature of a high ratio near the center of the otolith in all specimens; outside the otolith core, however, there were generally three different patterns (Fig. 4). All otolith specimens had a central core region with high Sr:Ca ratios with a maximum of more than ÿ3 (Fig. 4) surrounded by an elver mark that could be observed with a light microscope. The radius of the elver mark in Anguilla anguilla ranged from 124 to 196 mm with a mean SD of mm. The high Sr:Ca ratios in the central core region during the leptocephalus stage may be derived from the large amounts of gelatinous extracellular matrix that fill their bodies until metamorphosis (Arai et al., 1997). This material is composed of sulfated glycosaminoglycans (GAG), which are converted into other compounds during metamorphosis (Pfeiler, 1984). The drastic decrease in Sr at the outer region in both river and seawater samples after metamorphosis to glass eels, may occur because these sulfated polysaccharides have an affinity to alkali earth elements, and are particularly high in Sr, suggesting that a high Sr content in the body has a significant influence on otolith Sr content through the saccular epithelium in the inner ear, and the sudden loss of Sr-rich GAG during metamorphosis probably results in the lower Sr concentration in otoliths after metamorphosis (Arai et al., 1997). Outside of the high Sr core, there was considerable variation in the Sr:Ca ratios in the otoliths of some of the eels of both species. In Anguilla anguilla, the change in Sr:Ca values outside the elver mark was generally divided into three types corresponding to the elver, yellow and silver stages (Fig. 4): (1) constantly low values generally ranging between about 0.79 and ÿ3 (mean values in each site: 0.98e ÿ3 ) (64 specimens from all sites except for Galway Bay), (2) relatively high values generally ranging between about 6.73 and ÿ3 (mean: ) with no apparent movement into freshwater (nine specimens from Galway Bay), and (3) values that change between high and low values at various distances outside the elver mark within an overall range of 3.87e ÿ3, with a single movement from one high salinity habitat (5.64e ÿ3 ) to a low salinity habitat (0.86e ÿ3 ) (two specimens; one from River Corrib, the other from Galway Bay). The most interesting individual of the third type (Galway Bay specimen, age 3 years yellow eel) apparently lived in seawater for 1e5 years (River Corrib specimen, age 15 years silver eel) after recruitment, and then moved to freshwater for the remainder of its life up to capture. There were 64 specimens of A. anguilla that showed the type (1) pattern of low Sr:Ca values in their otoliths, which is apparently indicative of long-term residence in freshwater habitats after upstream migration during the elver stage. Nine specimens showed type (2) values, indicating that they had experienced high salinity during their growth phase. Two others showed type (3) evidence of remaining in areas with relatively high salinity for several years before entering freshwater habitats. We compared growth rates according to the life history types as estimated Sr:Ca ratios in otoliths between type (1) and type (2); we could not include type (3) due to its small sample size. Significant differences in growth rates were found between Galway Bay (type 2) and Castleconnell site in River

6 574 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e Discussion Fig. 2. Relationships between body weight and total length of Anguilla anguilla collected in Irish waters in November, All specimens for each sex (top), yellow eels for each sex (middle) and silver eels for each sex (bottom) are shown. The asterisk (*) indicates statistical significance. Shannon (type 1) and between Galway Bay (type 2) and male eels in River Garavogue (type 1) (ANOVA, df ¼ 13e18, p < 0.01e0.05), but no significant differences existed for the other 53 of 55 combinations (ANOVA, df ¼ 9e19, p > 0.05). The silver eels of Anguilla anguilla examined during this study showed the same sexual dimorphism in size and body weight that is present in other temperate anguillid species. The female silver eels in this study were significantly larger than the males, and this is typical for this species (Panfili et al., 1994; Poole and Reynolds, 1996; McCarthy et al., 1999) and for other temperate species such as Anguilla rostrata (Oliveira, 1999), Anguilla japonica (Kotake et al., 2003, 2005), Anguilla australis and Anguilla dieffenbachii (Jellyman et al., 2001; Arai et al., 2004). Based on a number of previous studies, Tesch (2003) concluded that the TL of European silver eels during their downstream migration was typically 350e460 mm in males and 500e610 mm in females. At that life history stage, their ages were 2e15 years (6 years in average) in males and 4e20 years (8.7 years in average) in females. In Irish waters, the TL and age of silver eels were as mentioned above (Table 1). Therefore, the mean growth rates of A. anguilla examined in several Irish waters of 15e62 mm/year during this study are typical for Ireland (14e46 mm/year; Moriarty, 1983; Poole and Reynolds, 1996) and for other European countries such as Germany (48 mm/year; Berg, 1985), Norway (62 mm/year; Vollestad and Jonsson, 1986), Poland (41 mm/year; Nagiec and Bahnsawy, 1990), and France (53 mm/year; Panfili et al., 1994). There were no significant positive correlations between age and TL for either sex in silver eels or between age and BW for female silver eels, although a significant positive correlation was found between age and BW for male silver eels. Furthermore, no significant differences occurred in age, TL and BW between yellow and silver eels in each sex. A large variation in age, TL and BW, and considerable overlap in age and size of yellow and silver eels indicated that the eels did not start their downstream migration at a certain fixed age or body size. However, the Australian shortfin eel Anguilla australis needs to attain a minimum size and an age prior to migration; both these criteria are extremely variable among populations; and there is considerable overlap in size and age for becoming silver and yellow shortfin eels (De Silva et al., 2002). In the European eel there is no critical size or age when they become silvery, nor are size and age at maturity positively related (Svedang et al., 1996). These results indicate that the mass of anguillid eels could have no bearing on their readiness to undertake the spawning migration. The analyses of patterns of variation of the Sr:Ca ratios in the otoliths from the Anguilla anguilla examined during this study indicated that a variety of environmental salinities had been experienced in the habitats that were occupied during the growth phase of these individuals. The otolith microchemistry of these eels indicated that most of them had entered freshwater relatively quickly after recruitment and had stayed in freshwater until maturation. Nine eels from Galway Bay appeared to have remained in relatively high salinities up until maturation. In addition, a few individuals of this species showed evidence of shifts from one salinity level to another, and at least two specimens showed clear evidence of two such shifts. This type of variability in otolith microchemistry and evidence of marine residency also has been found among

7 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e Fig. 3. Relationships between age and total length (left) and between age and body weight (right) of Anguilla anguilla collected in Irish waters in November All specimens for each sex (top), yellow eels for each sex (middle) and silver eels for each sex (bottom) are shown. The asterisk (*) suggests statistical significance. Anguilla japonica from various localities in Japanese coastal waters (Tsukamoto and Arai, 2001; Arai et al., 2003a,b; Kotake et al., 2003, 2005) and a river system in Taiwan (Tzeng et al., 2002). Otolith analyses of the yellow- and silver eel stages of A. anguilla also have shown evidence of marine residency in the North Sea and Baltic Sea (Tsukamoto et al., 1998; Tzeng et al., 2000). Similarly, exclusive marine residency has been inferred for the American eel, Anguilla rostrata (Jessop et al., 2002) and New Zealand eels, Anguilla australis and Anguilla dieffenbachii (Arai et al., 2004). The occurrence and evolution pathway of the migratory diversity of the anguillid eels is not clear, but it has been considered to be due to genetics or environmental adaptation (Nordeng, 1983; Gross, 1985). The European eel is considered

8 576 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e578 Fig. 4. Plots of the Sr:Ca ratios in the otoliths measured along line transects from the core (0 mm) to the edge of the otoliths of all the specimens collected in Irish waters. Mean ages at each distance from the core in all eels examined in this study and life history stages for the life history transect are shown. to be a panmictic population (Daemen et al., 2001). Whether the divergent migratory contingents of eels have different genotype structures needs to be examined. However, there is a widely held view that life histories in salmonid fishes are selected for and adapted to maximize the production of progeny (Schaffer and Elson, 1975; Gross, 1985). In evolutionary terms, the persistence of migration needs to be seen in relation to the balance of advantages obtained and the costs incurred from migration by the population/species. Advantages include such aspects as increased food supply, avoidance of potentially harmful environmental conditions and/or a movement to more favorable ones, the occupation of habitats that have specific or specialized habitat requirements, and the availability of more living space. Costs of migration include mortalities resulting from migration itself and changed environmental conditions that may be intolerable (McDowall, 1988).

9 T. Arai et al. / Estuarine, Coastal and Shelf Science 67 (2006) 569e Gross (1987) proposed that diadromy occurs when the gain in fitness from using a second habitat minus the migration costs of moving between habitats exceeds the fitness from staying in only one habitat. When glass eels migrate from offshore seawater to upstream freshwater for habitat and feeding, they have to overcome the osmotic pressure of a saline environment. If they stayed in the estuary habitat, their osmoregulatory cost would be lower than that in either freshwater or seawater. Estuaries have function as a nursery and feeding grounds for the juveniles of many fish species (Lenanton, 1982). Many commercially important fish can be present, because estuaries provide suitable food resources as well as shelter, absence of turbulence, and a reduction of predation (Blaber et al., 1985). These conditions may confine the eel to the estuarine waters; hence, estuary-dependent eels are predominant in the European eel Anguilla anguilla (Tzeng et al., 2000), Japanese eel Anguilla japonica (Tsukamoto and Arai, 2001; Arai et al., 2003a,b; Kotake et al., 2003, 2005), American eel Anguilla rostrata (Jessop et al., 2002) and New Zealand eels, Anguilla australis and Anguilla dieffenbachii (Arai et al., 2004). In A. anguilla in Ireland, however, the ecological implications for habitat use are somewhat different than in other regions because the estuarine living space is limited by high altitude land patterns in many coastal areas. Thus, most eels might be compelled to grow in either freshwater or coastal seawater. Although we did not examine silver eels collected in coastal seawater, typical freshwater resident eels were predominant and, interestingly, a few of the eels we examined from Ireland s water did shift habitats from seawater to freshwater during their lifetime. Furthermore, somatic growth rates did not differ between freshwater and seawater resident eels in most eels in the present study. Accordingly, these conditions may result in the majority of Irish eels entering the estuaries of the larger, productive, river systems being attracted to the freshwater habitats. Consequently, because of the availability of extensive mesotrophic and eutrophic lake habitats in many Irish river systems, freshwater-dependent eels are more abundant in this country. To test this hypothesis, analyses of the otolith Sr:Ca ratios of silver-phase eels during their spawning migration in the Irish coastal waters need to be made, and their degree of migratory type should be compared. Acknowledgements The authors would like to thank Ms M. Morrissey, Mr E. MacLoughlin and the commercial fishermen who helped us obtain our eel samples. This work was supported in part by Grants-in-Aid Nos and from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. 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