Seasonal activity, depth distribution, and microhabitat associations of resident yellow-phase American eels (Anguilla rostrata) in the upper St. Lawrence River

Abstract

The American eel (Anguilla rostrata) is an endangered species that inhabits fresh, brackish, and oceanic waters. Eels are elusive; little is known about their seasonal movements, depth distribution, and microhabitat associations in fresh water. Radio and acoustic telemetry was used in the slow-flowing upper St. Lawrence River (1.5-by-12-km section) to study 33 transmitter-implanted large yellow-phase resident eels from September 2014 to June 2017. Semi-monthly minimum movements peaked the last part of April (450 m), first part of May (393 m), and last part of October (283 m). Eels were inactive and dormant from the last part of December to the first part of April, when they emerged and moved into shallow water (< 2.5 m) as water temperatures approached 10 ºC. Eels moved slightly deeper in summer as temperatures peaked, likely following prey-fish movements, started moving deeper in October as temperatures dropped below 8 ºC, and settled for the winter in December at approximately 3.8 m. Telemetry allowed precise locating of eels (approximately 1,500), enabling detailed examination of microhabitat associations. Substrate properties, vegetative biomass, and cover were analyzed throughout the study area to describe habitat that was available (34 sites), selected (31), and never frequented (non-selected, 6). Two general ecotypes were found: one that associated with soft substrate with vegetative cover, mainly the green algae Chara vulgaris, and the other with hard substrate and large rock cover. The soft substrate ecotype dominated (88%); sediment was significantly softer, quantified by an impact penetrometer, and higher in organic content (> 14 %) compared with available and non-selected habitats. Site fidelity among seasons and years was remarkable: four eels consistently used the same rock cover from spring to fall, but all eels overwintered in soft substrate at sites that were slightly harder and had significantly lower organic content (P = 0.026) and Chara biomass (P = 0.007) than summer and fall sites. Discriminant analyses determined that most of the variance in distribution of eels is related to substrate type, followed by vegetative cover, primarily Chara. Telemetry revealed that eels had consistent and very predictable seasonal activity patterns and specific microhabitat associations and preferences.