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Fisheries Behavioral Ecology - Abstracts

Sogard, S.M. and B.L. Olla. 1993. Effects of light, thermoclines and predator presence on vertical distribution and behavioral interactions of juvenile walleye pollock, Theragra chalcogramma Pallas. Journal of Experimental Marine Biology and Ecology 167:179-195.


The vertical distribution of pelagic juvenile stages of fish can be altered by a variety of environmental stimuli. In laboratory experiments, we tested the influence of light intensity, thermal stratification, and predator presence on vertical distribution in juvenile walleye pollock (Theragra chalcogramma Pallas). In addition, we measured activity levels and group cohesion (schooling tendency). Experiments were conducted over a broad size range (37-126 mm), allowing us to test for ontogenetic variability in behavioral responses. Juvenile pollock displayed distinct responses to predator presence and thermal stratification, shifting downward and upward, respectively, relative to their average vertical position under baseline experimental conditions. When pollock were exposed to the two conflicting stimuli of a stratified water column plus a predator, their average depth was midway between the mean positions observed with each stimulus alone. The fishes response to bright light was less clear. Increased light levels did not significantly alter the average vertical position of pollock in the water column, but did result in increased use of the cold water layer under stratified conditions. There was an ontogenetic shift in pollock behavior. Smaller fish were less active, had a smaller range of movement through the experimental tank, and were less likely to dive through the thermocline in a stratified water column. Smaller fish also demonstrated less group cohesion, having a lower tendency to aggregate or school compared with larger fish. These behavioral patterns are potentially useful in explaining distribution patterns of juvenile Walleye pollock in the field.


Last updated 31 March, 2009

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