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Fisheries Behavioral Ecology - Abstracts

Sogard, S.M. and B.L. Olla. 1998. Behavior of juvenile sablefish, Anoplopoma fimbria (Pallas), in a thermal gradient: balancing food and temperature requirements. Journal of Experimental Marine Biology and Ecology 222:43-58.


The vertical distribution of early juvenile sablefish, Anoplopoma fimbria (Pallas), and their behavioral responses to changing food and temperature conditions were examined in laboratory experiments. Although field collections have suggested that juveniles are obligate residents of the neuston, laboratory experiments on diel movements indicated the potential for downward movement during the day, when activity levels were high and active foraging would be likely. The relationship of downward movement to fish size (small vs. large juveniles), prior feeding success (high vs. low rations) and thermal structure of the water column (isothermal vs. varying intensities of thermal stratification) were tested. Thermal stratification inhibited downward movement, with sablefish displaying clear avoidance of cold water even when the thermal gradient was moderate (12 °C top layer and 8 °C bottom layer). However, sablefish could be induced to swim downward into cold water if food was introduced beneath the thermocline. Across all temperature treatments, occurrence in cold water significantly increased when food was present. Increasing the sharpness of the thermal gradient (top layer = 12 °C, bottom layer = 8, 6, 4 or 2 °C) did not affect the percentage of time spent in cold water, but did shorten the maximum time an individual would spend beneath the thermocline. Movement into cold water constituted a potentially lethal risk for juvenile sablefish; dives beneath the thermocline that exceeded 60 s resulted in loss of equilibrium and immediate death for small fish moving from 12 to 2 °C water. Thus, sablefish displayed behavioral trade-offs between food acquisition and the physiological risk of cold temperatures, similar to previous studies documenting balancing of foraging behavior and predation risk. Behavioral responses to thermal stratification were modified by fish size and hunger level, with larger juveniles and those held on low rations spending more time in cold water. These behavioral responses correspond with the high feeding requirements necessary to maintain the naturally rapid growth rates of this species and the transitions in habitat associated with different life history stages.


Last updated 31 March, 2009

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