Estimating handling mortality due to air exposure: Development and application of thermal models for the Bering sea snow crab fishery

The fishery for snow crabs Chionoecetes opilio in Alaska occurs during the winter, and handling mortality may be high due to cold-air exposure. Heat budget models that include convection, evaporation, long-wave radiation, and solar radiation were developed for snow crabs. Model performance was evaluated by comparing observed and predicted temperatures of live crabs exposed to cold air, The model performed well, predicting temperatures of the body, legs, and eyes within 1.5 degrees C. Legs and eyes cooled faster than the body and may be more susceptible to cold-air exposure. Weather variables were ranked in the following order of importance: (1) air temperature, (2) solar radiation, (3) wind speed, (4) humidity, and (5) cloud cover. Clear, cold, breezy nights are the most detrimental conditions for crabs. Hourly weather records from a site near the fishing grounds were used in conjunction with the thermal model to estimate cooling rates for crabs every hour during actual and hypothetical seasons. The probability of damage for various cooling rates was estimated based on previously published laboratory data. Discard handling damage rates were highly variable, ranging from 0% to 30% over 21 actual fishing seasons. Damage rates for hypothetical seasons that varied in length, start day, and daily fishing hours were calculated. Damage rates were reduced by delaying the start day (15 January) until 1 March as well as by limiting fishing time to daylight hours. Combining a later starting day with limited fishing hours resulted in 90% of the hypothetical seasons having damage rates of 10% or less. Generally, longer seasons had lower damage rates, especially when fishing hours were limited. Although developed for cold-weather Alaska fisheries, this approach can easily be adapted to other fisheries where handling mortality due to temperature changes during capture is a concern.