Migratory behaviour and habitat use by American eels Anguilla rostrata as revealed by otolith microchemistry

The environmental history of American eels Anguilla rostrata from the East River, Nova Scotia, was investigated by electron microprobe analysis of the Sr:Ca ratio along transects of the eel otolith. The mean (±SD) Sr:Ca ratio in the otoliths of juvenile American eels was 5.42 × 10–3 ± 1.22 × 10–3 at the elver check and decreased to 2.38 × 10–3 ± 0.99 × 10–3 at the first annulus for eels that migrated directly into the river but increased to 7.28 × 10–3 ± 1.09 × 10–3 for eels that had remained in the estuary for 1 yr or more before entering the river. At the otolith edge, Sr:Ca ratios of 4.0 × 10–3 or less indicated freshwater residence and ratios of 5.0 × 10–3 or more indicated estuarine residence. Four distinct but interrelated behavioural groups were identified by the temporal changes in Sr:Ca ratios in their otoliths: (1) entrance into freshwater as an elver, (2) coastal or estuarine residence for 1 yr or more before entering freshwater, and, after entering freshwater, (3) continuous freshwater residence until the silver eel stage and (4) freshwater residence for 1 yr or more before engaging in periodic, seasonal movements between estuary and freshwater until the silver eel stage. Small (<70 mm total length), highly pigmented elvers that arrived early in the elver run were confirmed as slow growing age-1 juvenile eels. Juvenile eels that remained 1 yr or more in the estuary before entering the river contributed to the production of silver eels to a relatively greater extent than did elvers that entered the river during the year of continental arrival.