Increasing the survival of discards in North Sea pulse-trawl fisheries

Samenvatting

Measures to increase discard survival in the 80 mm pulse-trawl fisheries were assessed under commercial fishing conditions using plaice (Pleuronectes platessa) as model species. Measures tested were a water filled hopper (8 sea trips), short hauls (90 instead of 120 min, 4 sea trips) and a knotless cod-end (1 sea trip) with undersized plaice. In total nine sea trips were performed with three different commercial pulse-trawlers. Sea trips were spread over the year to account for potential seasonal variation in discards survival. Additional trials with a knotless cod-end (one sea trip) and water filled hopper (2 sea trips) were performed with undersized sole. Effects were assessed by comparing survival to conventional conditions (dry hopper, conventional haul duration, conventional cod-end) for which data were collected from the same or subsequent hauls.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. Test-fish were housed on-board in custom-built monitoring units containing 16 (24L) tanks with five fish each tank. Tank water was continuously renewed with sea water at a rate of at least two tank volumes 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 road transported to the laboratory to continue survival monitoring for two more weeks. Total monitoring period ranged from 15 to 18 days among test-fish depending on the day of collection at sea. In the laboratory tank bottoms were covered with coarse sand and fish were fed natural food. In total 558 plaice from conventional fisheries (ca. 60 per sea trip) were collected, 478 plaice for the water filled hopper treatment (ca. 60 per sea trip), 200 plaice from short hauls (ca. 40 and 60 each in two sea trips) and 60 plaice from the knotless cod-end.Control-fish of the same species, in good condition and not exposed to the fishing gear were deployed during all sea trips (circa 30 control plaice and circa 15 control sole per sea trip). Control-fish were handled and tagged as test-fish to separate fisheries related mortality from mortality caused by the experimental procedures.Discards survival probabilities and their 95% confidence intervals (CI) were estimated from counts of surviving fish at the end of the monitoring period. For all sea trips combined, no significant effect of a water filled hopper on plaice discards survival probability could be detected with 16% (95%CI 12-19%) for the conventional dry hopper and 20% (95%CI 15-25%) for the water filled hopper. Within the individual sea trips, a significantly higher survival probability for plaice discards from the water filled hopper was found for three sea trips. In three other sea trips a lower survival of plaice discards was detected for the water filled hopper, although the difference with the dry hopper was not significant. Given this observation, it cannot be entirely excluded that the water filled hopper can also have a negative effect on discards survival. For sole discards the effect of a water filled hopper was tested during two sea trips only, yielding a higher survival probability of sole discards for the water filled hopper (14%, 95%CI 10-21%) compared to the dry hopper (5%, 95%CI 2-10%).Deployment of a water filled hopper results in a shift towards a better condition of the discarded fish. Despite this effect, the total proportion of fish in good condition within catches remained small. We therefore recommend to prioritize measures aimed at improving fish condition in the trawl to increase discards survival chances.For all sea trips combined, no effect of short (90 instead of 120 min) hauls on discards survival probability could be detected: survival probabilities for plaice discards were equal at 11% (95% CI 8-15%) for both short and conventional hauls. No effect of a knotless cod-end on plaice and sole discards survival probability could be detected.In conclusion, deployment of a water filled hopper does not result in higher survival probability for plaice discards than a conventional dry hopper in year-round pulse-trawl fisheries. However, it is clear6 of 39 | Wageningen Marine Research report C038/18that for individual trips the deployment of a water filled hopper can result in an increase of survival chances of discarded plaice, but as it seems only under certain specific, yet to be established, conditions. In addition, it cannot be excluded at this point that under certain conditions a water filled hopper may have a negative effect on discards survival. For sole a positive effect of the water filled hopper on discards survival was detected. However, since sole was tested during two sea trips only, the current findings may not be representative for year-round fisheries and the positive effect may be specific for the conditions that prevailed during the two trips. Survival probability of plaice and sole discards cannot be increased by reducing haul duration from 120 to 90 min or using a knotless cod-end.