The survival of skates (Rajidae) caught by demersal trawlers fishing in UK waters

Skates and rays are a common component of mixed demersal fisheries, and large quantities may be discarded. Given their biological vulnerability, understanding the fate of these elasmobranchs is of management concern. Estimates of discard survival are needed for modelling the possible benefits of management measures. In this study, the focus is on the Bristol Channel skate fishery, where on-board holding tanks were used to assess the short-term rates of survival of trawl-caught skates (Rajidae). From monitoring the survival rates of 162 fish kept in specially designed on-board holding tanks for periods of up to 72 h, the short-term rate of survival was 55%. Visual inspection of "health" at time zero was a good indicator of survival, because 79% of skates with a poor health score did not survive. Mortality rates for fish of moderate health and good "health" were 16% and 5%, respectively. This information allows one to predict the consequences of fishing practice on discard survival using a larger dataset on fish scored for health before tagging and release. The proportion in poor condition on capture is positively correlated with estimated codend weight, so technical modifications to fishing gear aimed at reducing unwanted by-catch would increase the survival of discarded skates. Combined with information on discarding rates in the study area, the results indicate that discard mortality removes almost as many fish from the skate stock as are landed commercially. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.