Discards survival probabilities of flatfish and rays in North Sea pulse-trawl fisheries

Samenvatting

Discards survival probability in conventional commercial 80 mm pulse fisheries was assessed for undersized plaice (Pleuronectus platessa n=558), sole (Solea solea n=274), turbot (Scophthalmus maximus n=111), brill (Scophthalmus rhombus n=90), thornback ray (Raya clavata n=99) and spotted ray (Raya montagui n=23). In total nine sea trips were performed on three commercial pulse-trawlers with three trips per trawler. Sea trips were spread out over the year to account for potential seasonal variation in discards survival. All test-fish were randomly collected from the end of the sorting belt at both the start and end of the catch-sorting process from multiple hauls per sea trip. Reflex impairment and damages were assessed for all test-fish and summarized in a vitality index score indicating fish condition. Test-fish were housed in four custom-built monitoring units installed on board. Each unit contained 16 24L tanks. Numbers of fish housed per tank were five (plaice, sole), three (turbot, brill) and one (rays) fish per tank. Tank water was continuously renewed with sea water at a rate of at least one tank volume per hour to maintain proper water quality. Survival was monitored and dead fish were removed upon detection. Upon arrival in the vessel’s home port, monitoring units were taken from board and transported to and placed in a climate controlled room to continue survival monitoring for two more weeks. The total monitoring period ranged from 15 to 18 days among test-fish depending on the day of collection at sea. In the climate controlled room, tank bottoms were covered with coarse sand and fish were fed natural food. Control-fish, fish of the same species and in good condition collected in advance at sea, were deployed during all sea trips (30-35 control plaice, 10 control sole, 3 control turbot and brill, 2 control thornback ray, 2 control spotted ray per sea trip). Control-fish were treated equally as test-fish to separate fisheries related mortality from mortality caused by the experimental procedures. Discards survival probabilities were estimated from counts of surviving fish at the end of the monitoring period.Within all species, discards survival probabilities varied among sea trips. Discards survival probability estimates and their 95% confidence intervals (95%CI) based on all sea trips combined were 14% (95%CI 11-18%) for plaice, 19% (95%CI 13-28%) for sole, 30% (95%CI 20-43%) for turbot, 13% (95%CI 7-23%) for brill and 53% (95%CI 40-65%) for thornback ray. For spotted ray discards survival probabilities of 21% and 67% were observed during two sea trips. Given the agreement with a previous and comparable study and the large number of observations collected year-round, we consider the current results for plaice and for sole to represent the actual overall survival probability of undersized plaice and sole discards in the 80 mm pulse-trawl fisheries. The discards survival probability estimates for turbot, brill and thornback ray are based on limited numbers of observations per species. These estimates should therefore be considered and treated as a first indication of the actual discards survival probability for these species in the 80 mm pulse-trawl fisheries. Collection of more data should narrow down the current 95% confidence intervals and give more precise estimates for the survival probability. We expect however that these more precise estimates lie within the current 95% confidence intervals for the survival probability estimates.In all species tested, discards survival was strongly affected by fish condition, with large differences in survival probability between fish in best and worst condition. The proportion of fish in the best condition is small and their high survival probability has little effect on the survival probability of the entire population in the catches. We therefore recommend that measures aimed at increasing discards survival focus on improving the condition of discarded fish. Since catch-processing time seems to have no effect on fish condition nor discards survival, we recommend that measures aimed at improving the condition of discarded fish focus on the fish capture process rather than catch processing.