Discard mitigation increases skate survival in the Bristol Channel

Abstract

The survival of fish discarded after being caught can be improved by simple gear-based technical measures aimed at reducing discards. We look at the effects of three different codends on the initial and short-term survival of trawl-caught skate (Rajidae), using a control codend (80 mm diamond mesh used as standard in the fishery) and two experimental codends (100 mm diamond mesh and 100 mm diamond mesh turned on the square). Both experimental nets reduced discarded numbers of fish by ~70%, with no commercial loss. This reduction in discards had an effect in reducing the total weight of the experimental codends by as much as 80%. We also placed 278 skate in onboard holding tanks for 48 h and evaluated the survival rates of fish caught in the different codends. Visual inspection of health at time zero was a good indicator of survival, because 86% of skate with a good score survived (p < 0.01). From a further 1539 skate assessed for health, we show that fish caught in the control codend have the lowest proportional good score (25%), followed by the 100 mm diamond mesh codend (34%) and the 100 mm square mesh codend (47%). The of the fish caught is related to codend weight (p = 0.01). We conclude that technical measures aimed at reducing discards have an additional benefit; they indirectly increase discard survival, and the benefits of mitigating discards through by-catch reduction devices may be a more powerful tool in fisheries management than previously thought.