Behaviour of Atlantic cod, a marine fish predator, during Atlantic salmon post-smolt migration

Abstract

Acoustic telemetry was used to determine the behavioural strategies of Atlantic cod (Gadus morhua) during Atlantic salmon (Salmo salar) post-smolt migration within a Norwegian fjord (Eresfjord). In all, 38 adult cod captured in the inner fjord were tagged with acoustic transmitters in 2008 and 2009, and their behaviour was determined using a fixed hydrophone array. Cod tended to aggregate in the innermost part of the fjord, occupying distinct demersal home territories, showing horizontal movements consistent with foraging. Tidal influences were not observed; cod spent more time near the surface at night. Cod behaviour during peak post-smolt migration differed from that before and after migration. First, cod tended to have more-focused spatial distributions during peak post-smolt migration, consistent with them not having to forage so far when prey were available in abundance. Second, some half the cod were detected nocturnally (but rarely during daylight) near the river mouth during peak post-smolt migration, consistent with them feeding on nocturnally migrating post-smolts. Third, cod were more common near the surface, consistent with them feeding on post-smolts migrating through near-surface waters. These patterns, however, were not shown by all the cod, suggesting that this opportunistic feeder was also preying on other species.