Fisheries Behavioral Ecology - Abstracts
Ryer, C.H. 2004. Laboratory evidence for behavioural impairment of
fish escaping trawls: a review. Journal of Marine Science 61:1157-1164.
Abstract
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.
Last updated
26 April, 2007
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