Seasonal distribution and movements of coastal cod (Gadus morhua L.) in Placentia Bay, Newfoundland

Sequential acoustic surveys and mark-recapture experiments were used to exa mine seasonal movements and distribution of coastal cod in Placentia Bay, N ewfoundland (NAFO stock sub-division 3Ps). Cod were located in Placentia Ba y in all seasons of 1997 and early 1998. Acoustically-measured densities an d abundances were highest during spring-time spawning (>1 fish m(-2)). The degree of aggregation was highest in spring and fall (99% of fish in 8-12% of Surveyed area), and low in summer (17-19%). Cod moved shallower after th e spawning period, and occupied an increasingly narrow range of depths from spring to fall. In contrast, the range of temperatures occupied broadened over this period, as did available temperatures. Stomach contents indicated that cod were feeding on pelagic zooplankters (hyperiid amphipods) in late summer and fall. We conclude that local movements of cod are more related to depth and feeding behaviour than to temperature. Cod tagged primarily on spawning grounds in Placentia Bay were recaptured mostly within the bay (8 7%), where fishing effort was high. Recaptures of marked cod per unit weigh t of fishery catch indicated that 10-30'% may move in spring and summer int o the adjacent stock area (3L). The majority of tagged cod recaptured durin g the following spring was caught within Placentia Bay, suggesting a return migration. Almost all cod recaptured at long distances (>100 km, and up to 600 km) from tagging sites were large (>50 cm). Smaller fish were recaptur ed mostly within the bay, and closer to tagging sites. Our results are cons istent with the hypothesis of an ontogenetic threshold to long-distance mig ratory behaviour in cod. Evidence of repeat spawning, year-round residence, and return migrations from the bay, all suggest the existence of a Placent ia Bay coastal cod stock. (C) 2000 Elsevier Science B.V. All rights reserve d.