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 Published and unpublished data from 6 sites (Canada, United States, Sweden, France, Taiwan and Japan) sampled across the geographical range of each eel species were compiled.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 patterns of habitat use and their relative proportions were investigated for 270 eels Anguilla anguilla sampled in coastal areas, the estuary and the river of the Gironde watershed, by measuring Sr and Ca concentrations in the otolith. Sr:Ca values outside the elver mark discriminated residence in the same habitat until capture (freshwater, estuary, or sea) from a switch of habitat following 3 patterns: a shift from freshwater to estuarine, seasonal movements within the limits of the estuary, or a switch of habitat from brackish to freshwater (downstream nomads). Results show a gradient of residency from freshwater (100%) to estuary (44%) to the sea (24%). The most abundant (30%) alternative to residence in an estuary and its adjacent coastal site were the downstream nomads, which concentrated in coastal areas and the lower part of the estuary after leaving freshwater. Overall, up to 50% of the eels analysed had spent a period in freshwater. Fish growth rates in freshwater were lower than anywhere else in the watershed. Back-calculation of fish sizeat-age showed that, of all eels passing through freshwater, residents were slow growers, while downstream nomads were fast growers. The latter migrate to the estuary at Ages 2+ and 3+, when their size is greater than estuarine residents. Results revealed that lower estuarine and marine habitats are colonised by yellow rather than glass eels. This suggests that competition with estuarine residents and downstream nomads prevents precocious settlement of glass eels in estuary and marine habitats.

To enable a relevant interpretation of otolith strontium : calcium (Sr/Ca) variations in terms of habitat shifts of eels, the Sr/Ca-salinity relationship in eel otoliths was validated. Downstream and upstream migrations of young eels were reproduced in the laboratory by transferring groups of fish every 2 months between aquaria filled with water coming from the Dordogne river (salinity = 0), the upper Gironde estuary (salinity = 5), the lower Gironde estuary (salinity = 25) and the coast (salinity = 30), which represented the salinity gradient observed in the Gironde-Garonne-Dordogne watershed. Ontogenetic changes in otolith Sr/Ca were assessed in two groups of control fish that were kept in one of either two constant salinities (fresh water or seawater). X-ray electron microprobe (wavelength dispersive spectrometry, WDS) analyses of Sr/Ca ratios in the otoliths showed that the change of aquarium was recorded as a Sr/Ca increase (downstream migration) or a Sr/Ca decrease (upstream migration). No ontogenetic effect was detected in otoliths of control fish outside glass eel marks in either group of fish. The electron microprobe (WDS) analysis of the Sr/Ca life (transected in several otoliths of eels caught in the Gironde-Garonne-Dordogne watershed) showed that some of them were migrant eels that had experienced one major habitat shift during their continental life.

–  A cooperative effort gathered a large European length‐at‐age data set (N = 45,759, Lat. 36S–61N Long. 10W–27E) for Anguilla anguilla, covering one century. To assess the effect of global warming during the last century and habitat effects on growth, a model was fitted on the data representing the conditions met at the distribution area scale. Two GLMs were designed to predict eel log(GR): one model was fitted to the whole data and the other was fitted to the female data subset. A model selection procedure was applied to select the best predictors among sex, age class, five temperature parameters and six habitat parameters (depth, salinity and four variables related to the position in the catchment). The yearly sum of temperatures above 13 °C (TempSUP13), the relative distance within the catchment, sex, age class, salinity class and depth class were finally selected. The best model predicted eel log(GR) with a 64.46% accuracy for the whole data and 66.91% for the female eel data. Growth rate (GR) was greater in habitats close to the sea and in deep habitats. TempSUP13 variable had one of the greatest predictive powers in the model, showing that global warming had affected eel growth during the last century.

The reconstruction of individual life histories from chemical otolith measures is stated as an unsupervised signal-processing issue embedded in a Bayesian framework. This computational methodology was applied to a set of 192 European eel (Anguilla anguilla) otoliths. It provided a robust and unsupervised analysis of the individual chronologies of habitat use (either river, estuary, or coastal) from Sr:Ca measures acquired along an otolith growth axis. Links between Sr:Ca values and habitat, age, and season and the likelihood of the transitions from one habitat type to another were modelled. Major movement characteristics such as age at transition between habitats and time spent in each habitat were estimated. As a straightforward output, an unsupervised classification of habitat use patterns showed great variability. Using a hidden Markov model, 37 patterns of habitat use were found, with 20 different patterns accounting for 90% of the sample. In accordance with literature, residence behaviour was observed (28% of the eels). However, about 72% changed habitat once or several times, mainly before age 4. The potential application of this method to any other measures taken along an otolith growth axis to reconstruct individual chronologies gives a new insight in life history tactics analysis.