Seasonal activity and depth distribution of resident yellow-phase American eels (Anguilla rostrata) in a large fluvial ecosystem

Abstract

The American eel (Anguilla rostrata) is a catadromous species occupying diverse habitats, but little is known about the specific activities of this elusive nocturnal fish. Mobile radio and acoustic telemetry were used to locate 33 transmitter-implanted resident yellow eels (729 ± 22 mm TL), acquiring 1613 locations from 2014 to 2017 in a 21-km2 slow-flowing section of the upper St. Lawrence River. Measurements of distance moved, water depth, and temperature were used to study activity, homing, and site fidelity. Movement was greatest in spring, late April (422 m, median semi-monthly), after emergence from winter dormancy, from their deepest habitat (3.71 ± 0.06 m) to their shallowest (1.74 ± 0.17 m). Activity was low in early summer and early fall (69 m). As temperature reached a maximum during midsummer (24.3 ± 0.29◦C), most eels became more active (106 m) and moved deeper (2.46 ± 0.28 m), probably following prey fish. Activity increased in mid-to-late fall (277 m), decreased considerably at ≲8◦C when settling into their winter habitat, and ceased at ≲4◦C. Temperatures of ~10◦C (8.1–11.1◦C) corresponded with the greatest seasonal activity and spring/fall commercial hoop-net catches. These spatial and temporal habitat insights can help focus sampling and assessment procedures, as well as habitat suitability modelling.