Journal Article

Laboratory evidence for behavioural impairment of fish escaping trawls: a review

Clifford H. Ryer

in ICES Journal of Marine Science

Published on behalf of ICES/CIEM

Volume 61, issue 7, pages 1157-1164
Published in print January 2004 | ISSN: 1054-3139
Published online January 2004 | e-ISSN: 1095-9289 | DOI:
Laboratory evidence for behavioural impairment of fish escaping trawls: a review

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  • Marine and Estuarine Biology


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It is now widely accepted that for some species a proportion of the undersized fish escaping trawl codends die as a direct result of stress, with 10% to 30% mortality commonly cited. It has also been suggested that there may be indirect or behaviourally mediated mortality; fish that encounter and escape the trawl, only to experience stress-induced behavioural deficits and succumb to predators in the hours or days afterwards. The goal of this review was to evaluate the plausibility of this behaviourally mediated, yet unobserved mortality. Three laboratory studies utilizing cod (Gadus morhua), walleye pollock (Theragra chalcogramma), and sablefish (Anoplopoma fimbria) have assayed for behavioural impairment in fish following application of stressors designed to simulate entrainment and escape from trawls. Where impairments in anti-predator capabilities occurred, it was determined that trawl-stressed fish exhibited reduced swimming speed, reduced shoal cohesion, and reduced predator vigilance compared to control fish. Although stressed fish appeared to rapidly recover their ability to avoid being eaten by predators, measurements of more subtle aspects of escapee behaviour suggest that impairments may persist for days after stressor application. Although these studies demonstrate that more investigation is required, when combined with a more extensive literature demonstrating that a variety of stressors can impair fish anti-predator behaviour, it is reasonable to conclude that many fish species escaping trawl codends will likely suffer behavioural deficits that subject them to elevated predation risk. As such, there is probably mortality associated with trawl fisheries that is generally unrecognized, unmeasured, and unaccounted for in current stock assessment models. Further, these studies demonstrate that behavioural competency needs to be considered in the design and implementation of by-catch reduction devises and strategies.

Keywords: by-catch; predation; stress; trawl

Journal Article.  4921 words.  Illustrated.

Subjects: Environmental Science ; Marine and Estuarine Biology

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