Habitat use patterns of 3 species of temperate eels, Anguilla anguilla, A. japonica and A. rostrata, were investigated using otolith strontium:calcium ratio life history transects. Published and unpublished data from 6 sites (Canada, United States, Sweden, France, Taiwan and Japan) sampled across the geographical range of each eel species were compiled. Sr:Ca patterns indicated that the 3 species displayed similar patterns of habitat use. In all sites, patterns of habitat use consisted of either residency in one habitat (fresh, brackish, or marine) or movements between habitats. One movement pattern consisted of either a single change or 2 changes of habitat from fresh to brackish waters, or from brackish water to freshwater. Seasonal movements between fresh and brackish waters were observed for all 3 species. When only a single habitat switch event was detected, it occurred between 3 and 5 yr of age. Occurrence of eels with no freshwater experience was demonstrated, but such eels accounted for a smaller proportion of the overall sample than eels with some (even brief) freshwater experience. Contrary to the common convention that these are obligate catadromous species, we must now consider them as facultative catadromous, with far more flexibility in habitat use. The most variable parameter among study sites was the relative proportion, rather than the diversity, of lifetime spent in the various habitat use patterns. Eels found at higher latitudes exhibited a greater probability of remaining in the lower reaches of watersheds in brackish water. Diversity of habitat use appears to be a common strategy of temperate eel species, and, as a life history tactic, is under environmental control.KEY WORDS: Habitat use · Anguilla spp · Otolith Sr:Ca Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 308: [231][232][233][234][235][236][237][238][239][240][241] 2006 shelf of Europe and North Africa. The American eel A. rostrata also spawns in the Sargasso Sea and travels 2000 to 3000 km into North American waters while the Japanese eel A. japonica spawning area is located west of the Mariana Islands (NW Pacific) and its nursery grounds extend through continental waters of East Asia. Tropical species have shorter migrations (Marui et al. 2001). Leptocephalus larvae metamorphose into glass eels as they arrive at continental shelves (Bertin 1951, Tesch 2003. Glass eels become pigmented elvers as they penetrate estuaries, rivers and streams and complete their growth phase in a large choice of habitats (lagoons, estuaries, marshes, rivers, lakes and streams). The growth phase typically lasts from 3 to 15 yr and is followed by a second metamorphosis into silver eel, a pre-pubertal stage. Silver eels achieve their sexual maturation as they swim back to their spawning grounds (Van den Thillart et al. 2004).Although the presence of yellow eels (juveniles) in brackish and marine waters has long been known, the dominating paradigm has been that eel growth phase was restricted t...

The European eel (Anguilla anguilla L.) has been a prime example of the panmixia paradigm because of its extraordinary adaptation to the North Atlantic gyral system, semelparous spawning in the Sargasso Sea and long trans-oceanic migration. Recently, this view was challenged by the suggestion of a genetic structure characterized by an isolation-by-distance (IBD) pattern. This is only likely if spawning subpopulations are spatially and/or temporally separated, followed by non-random larval dispersal. A limitation of previous genetic work on eels is the lack of replication over time to test for temporal stability of genetic structure. Here, we hypothesize that temporal genetic variation plays a significant role in explaining the spatial structure reported earlier for this species. We tested this by increasing the texture of geographical sampling and by including temporal replicates. Overall genetic differentiation among samples was low, highly significant and comparable with earlier studies (F ST Z0.0014; p!0.01). On the other hand, and in sharp contrast with current understandings, hierarchical analyses revealed no significant inter-location genetic heterogeneity and hence no IBD. Instead, genetic variation among temporal samples within sites clearly exceeded the geographical component. Our results provide support for the panmixia hypothesis and emphasize the importance of temporal replication when assessing population structure of marine fish species.

Environmental histoly of the European eel Angu~lla angullla (L ) collected from Swedish bracklsh waters a n d lakes was studled by examining strontium (Sr) a n d calcium ( C a ) in their otoliths w~t h wavelength d~s p e r s i v e X-lay s p e c t~o m e t r y on a n electron mlcroprobe T h e Sr/Ca ratios In the otollths changed w~t h both ontogenetic development a n d s a l i n~t y of the h a b~t a t A peak Sr/Ca ratlo of approximately 2 X 10-L was observed in the m a n n e phase of the otol~ths corresponding to the p e r~o d w h e n a leptocephalus metamorphosed into a glass eel T h e ratios decreased to 5 72 (+ 0 32) X 10 at yellow eel stage w h e n the fish lived in brackish water a n d to 3 7 (t 0 20) X 10.' In fresh water T h c ratios were s~g n~f i c a n t l y different between brackish and fresh water ( p < 0 00001) Several relat~vely h~g h e~ Sr bands w e r e found In otol~ths of eel from bracklsh waters but not In the eels from fresh watel Salinity probably influences Sr deposition mole than does t e m p e r a t u~e 01 somatic g~o w t h rates KEY WORDS: European e e l . O t o l~t h . Strontium/calcium ratlos . Elemental X-ray m a p . Environmental factor M~g r a t o r y h~s t o r y . Electron m~c r o p r o b e

Strontium (Sr) and calcium (Ca) contents in the otoliths of yellow and silver European eels [Anguilla anguilla (L.)] collected from coastal waters of the Baltic Sea and a freshwater lake in Sweden were examined by wavelength dispersive X-ray spectrometry with an electron microprobe. The mean Sr/Ca ratios from elver check to otolith edge were signi®cantly higher for the eels from coastal waters (5.39 1.09&) than for those from the lake (0.71 0.89&). The evidence indicates that European eels in the Baltic Sea do not necessarily migrate into freshwater streams during the growth phase.

The main energetic stores at the silver eel stage were studied by analysing muscle fat concentrations and hepatosomatic indices in female silver eels from various habitats in Sweden. Muscle fat concentrations varied both within and between localities and lean eels with muscle fat concentrations <20% occurred at all study sites. Furthermore, no correlation could be found between muscle fat content and internal or external maturation indices, neither was the relative liver size related to the maturation process, as the correlation between the hepatosomatic and gonadosomatic indices was very weak. Consequently, it was concluded that silvering and the spawning migration may begin also at low muscle fat concentrations. However, most of the energy reserve is stored as muscle fat in eel, and it is highly unlikely that female silver eels with such low fat contents, as were observed occasionally in this study, will ever recruit to the next generation. Therefore, it is suggested that the maturation process in eel is more flexible than previously recognized, and that this process might be temporarily arrested and feeding resumed during the first part of the migratory phase. 1997 The Fisheries Society of the British Isles