Variation in snow crab entry ratio among sections of footrope and its effect on trawl net catching efficiency
2015
Fujita, K. (National Research Institute of Fisheries Engineering, Fisheries Research Agency, Kamisu, Ibaraki (Japan)) | Watanabe, T. | Ito, M. | Tokai, T.
When evaluating catching efficiency of trawl nets for snow crabs, assessing crab behavior around the footrope is crucial. Using two underwater video cameras, we captured footage at three sections of footrope with different thicknesses and attack angles in relation to the towing direction: the belly, the lower wing gusset, and the wing sections. Based on the footage, we calculated the entry ratio (ratio of snow crabs entering the trawl net to the overall number of snow crabs in contact with the footrope). Our results indicated that the entry ratio was 0.122 at both the belly and the lower wing gusset sections and 0.530 at the wing section, suggesting that footrope thickness and/or attack angle in relation to towing direction affects the entry ratio. The total number of crabs that entered was estimated using these entry ratios and corresponded well to the number of individuals captured at the cod end. Considering the variation among entry sections, the catching efficiency of the trawl net used was estimated to range from 0.311 to 0.361. These results demonstrate that calculating the entry ratio at each section of the footrope separately allows for a more accurate estimation of catching efficiency.
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