Fish density and size alter Pacific halibut feeding: implications for stock assessment
Laboratory experiments were conducted with Pacific halibut Hippoglossus stenolepis to test the hypothesis that responsiveness to food cues is density‐ and size‐ dependent. Tanks holding one, two and four fish were video‐taped continuously before and after introduction of an olfactory cue and whole baits (squid). Pre‐cue indices of activity did not vary with fish density. After olfactory stimulation, per capita activity was directly related to density, with no significant increase in activity by single fish. Numbers of baits located, attacked and consumed were directly related to fish density, and average times to first attack varied from 12 s in groups of four fish to >15 min in single fish. The latency period between location and attack decreased significantly with fish density. When large (43–55 cm, total length, L T) and small (31–38 cm L T) Pacific halibut were tested together in pairs and groups of four fish, small individuals located baits first in 61% of the trials. Large fish, however, consumed all of the baits (except one) in trials where both large and small fish responded. Social facilitation in Pacific halibut feeding will result in a non‐linear relationship between catch per unit effort and population density, and the size composition of fish captured may be influenced by interference competition.
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Document Type: Regular Paper
Publication date: 2004-06-01