Fisheries Behavioral Ecology - Abstracts
Ryer, C.H. 2002. Trawl stress and escapee vulnerability to predation
in juvenile walleye pollock: Is there an unobserved
bycatch of behaviorally impaired escapees? Marine Ecology Progress Series 232:269-279.
Abstract
Recent studies suggest that mortality of undersized fishes escaping through trawl
codends may range from 0 to 100%, with mortalities of 10 to 30% being common. These values may
be low, as they do not account for fishes which become behaviorally compromised by their passage
through the trawl and ultimately succumb to predators. The goal of this study was to simulate in the
laboratory the stressors associated with trawl passage and determine if they degrade the behavioral capabilities of juvenile walleye pollock Theragra chalcogramma to avoid predation. In the first of 2
experiments, groups of Age 1 yr+ walleye pollock were subjected to 3 treatments: (1) controls: no
stressor; (2) swim/escape: forced swimming for 90 min at 0.33 m s-1 in a towed net, followed by
escape through 8 cm square mesh; (3) swim/crowd/escape: forced swimming followed by 3 min of
crowding, followed by escape. To evaluate the effect of these treatments on pollock behavior, a sablefish Anoplopoma fimbria (48 to 53 cm) was placed in an observation arena with the group and pollock
anti-predator behavior was quantified. Beginning immediately after simulated trawling and for
up to 24 h afterwards, pollock exposed to both trawl-stressor treatments were less likely to avoid the
predator than controls, allowing it to approach closer. They were also less able to form a cohesive
shoal, and in the case of the swim/crowd/escape treatment, swam more slowly than control fish. To
determine if trawl-stressed fish are more vulnerable to predation, in a second experiment I mixed
control and swim/crowd/escape pollock together and then subjected them to predation by a 48 to
60 cm lingcod Ophiodon elongatus, observing the behavior and enumerating the number of pollock
consumed in each treatment. Lingcod concentrated attacks upon solitary individuals or those straggling
behind the shoal, were more likely to lunge at pollock that did not move away when
approached, and were more successful the closer the pollock at lunge initiation. As a result, trawl-stressed
pollock were consumed in greater numbers than controls. On the basis of these results, it is
reasonable to expect that juvenile walleye pollock passing through trawls suffer behavioral deficits,
subjecting them to elevated predation risk. If this is a generic effect, these results suggest that there
may be a significant bycatch associated with many commercial trawl fisheries which is generally unrecognized, unmeasured, and unaccounted for in current stock-assessment models.
Last updated
26 April, 2007
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