An Estimation Of The Cod (Gadus Morhua) Catch In The Bottom Gill Nets Lost Along The Southern Coast Of Sweden

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According to a survey of the Swedish cod gillnet fleet operating in the Baltic Sea, 3.6 – 3.8 gill nets are lost per vessel every year, corresponding to 156 – 165 km of netting. An experiment with deliberately lost nets showed some catching efficiency left still after 27 months. A considerable amount of nets are retrieved by trawlers, but still some nets continue fishing for a long time. In this paper an attempt is done to estimate the unaccounted cod mortality caused by gillnets lost by the Swedish fishing fleet. In the model used, cod catch was assumed to depend on 1) the length of lost nets in two categories (newly lost and accumulating), 2) the relative catching efficiency of the lost nets (from the experiment mentioned above) in the same two categories, 3) CPUE during the corresponding period for the gillnet fleet and 4) the retrieval (by trawlers) rate of lost nets. Of the total number of nets used during a 28-month period, 157 000 km, the model estimated 145 – 158 km to have been lost. Two retrieval rate scenarios were applied, giving 24 and 100 km nets left and continuing fishing (no lost nets assumed at the start). Total catch of cod by those nets during the 28-month period was estimated to 76 and 162 tonnes (with large confidence intervals), compared to 8445 tonnes landed by the fleet during the same period. Considering the total gillnet fishery in the Baltic, everything should be made to avoid this unaccounted cod mortality.