Tactics and demographic attributes in the European eel Anguilla anguilla in the Gironde watershed, SW France

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, tip 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 size-at-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.