Movement Patterns of American Eels in an Impounded Watercourse, as Indicated by Otolith Microchemistry

Abstract

Otolith Sr:Ca ratios were used to determine migrational histories of American eels Anguilla rostrata in an eastern Canadian coastal stream that is impounded at the head of tide. American eels sampled from the estuary were smaller (41.0 ± 9.3 cm (mean ± SD), N = 324) and younger (ages 1–4) than those in a freshwater pond (65.0 ± 7.5 cm, N = 265; ages 11–26) 2.5 km upstream of the head of tide. The Sr:Ca ratios permitted identification of three migrational contingents. In 7 of 13 (54%) American eels taken in the estuary, Sr:Ca ratios declined to 4.76 ± 1.22 × 10⁻³, indicating direct migration from the sea to estuarine settlement sites. In the 6 other American eels (46%), Sr:Ca ratios indicated an approach to freshwater soon after arrival in continental waters, followed by settlement in the estuary where ratios stabilized at 4.50 ± 1.23 × 10⁻³. In all 15 American eels sampled from the freshwater pond, Sr:Ca ratios indicated settlement in freshwater in the elver year and no subsequent change of habitat salinity (postelver ratio, 0.64 ± 0.66 × 10⁻³). The lack of evidence for postelver movements between freshwater and brackish water contrasts with literature findings that such movements are common in systems where dams do not impede access to freshwater. Dams may hamper normal American eel movements between rivers and estuaries, even in watercourses in which some American eels are able to colonize freshwaters.