MEPS 232:269-279 (2002)  -  doi:10.3354/meps232269

Trawl stress and escapee vulnerability to predation in juvenile walleye pollock: Is there an unobserved bycatch of behaviorally impaired escapees?

Clifford H. Ryer*

Fisheries Behavioral Ecology Program, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, Hatfield Marine Science Center, Newport, Oregon 97365, USA

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.

KEY WORDS: Bycatch · Predation · Stress · Trawl · Walleye Pollock · Behavior

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