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Fisheries Behavioral Ecology Program - Newport Laboratory

Assessing Probability of Discard Mortality in Alaska Crab Species

collected crabs
Figure 1.  Crabs collected in trawls were assessed for reflex impairment and injuries to the exoskeleton, then tagged and held in deck boxes to monitor delayed mortality.

Tanner crab
Figure 2.  Six reflex actions were identified in Tanner crabs during preliminary studies in Kodiak.  These were reliable predictors of stress, and the suite of reflexes can be assessed in less than 30 seconds.

Delayed mortality associated with discards of both crabs and fishes from fishing operations has ordinarily been observed through tag and recovery studies or prolonged holding in deck tanks, and there is need for a more efficient and rapid method for assessing probability of mortality.

Over the last two summer seasons RACE biologists from Newport, Seattle, and Kodiak (i.e., Allan Stoner and Michael Davis, Craig Rose and Carwyn Hammond, and Eric Munk) have been partners in an effort to evaluate bycatch mortality in snow crab, Tanner crab, and red king crab. During the first year (2007) shipboard experiments conducted in the Bering Sea were aimed at determining whether reflex behaviors in Alaska crabs (primarily Chionoecetes spp.) can be used to predict mortality caused by encounters with trawl gear in the Bering Sea groundfish fishery.

Tanner crab and snow crab collected with bottom trawls near the Pribilof Islands were evaluated for reflexes and injuries, then tagged and held in deck boxes for up to 11 days to track delayed mortality (Fig. 1). Six reflex actions involving movements of walking legs, chelae, eyes, and mouth parts (Fig. 2) were combined to calculate a reflex impairment index for each individual.

Logistic regression revealed that reflex impairment indices provided excellent predictors of delayed mortality for both snow crabs (91% correct predictions) and Tanner crabs (79.5% correct predictions) (Fig. 3). The relationships were independent of crab gender, size, and shell condition and predicted mortality in crabs with no obvious external damage. An assessment can be made in less than 30 seconds. Relationships between reflex impairment and mortality provide substantial improvement over earlier mortality predictors that depend upon detecting injuries to the exoskeleton, and the new approach will help to increase the scope and replication of fishing and handling experiments.

The results of this study and the general usefulness of Reflex Action Mortality Predictors (RAMP) for crabs are discussed in the October 2008 issue of Fishery Bulletin. RAMP approach should be equally valuable for a wide range of crustaceans.

figure 3, see caption
Figure 3.  Surface plots showing the probabilities of mortality for Tanner crabs (C. bairdi) and snow crabs (C. opilio) with varied levels of reflex impairment and injury.  Reflex impairment alone provided an excellent predictor of mortality in snow crabs, while injuries to the exoskeleton had a significant but smaller impact on the logistic regression model for Tanner crabs.

Given that RAMP has proven to be a good tool for studying bycatch-related mortality in Alaska crabs, we conducted a field experiment in the Bering Sea during 2008 to explore how different trawl configurations, primarily sweeps and footropes, might be improved to decrease crab mortality associated with trawl contact. During the same cruise we conducted experiments to evaluate whether reflex actions can be used to predict freeze-related mortality in Chionoecetes species. The results of these two studies are currently under analysis and both look promising.

By Allan Stoner

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