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

• Main Authors: Schram, Edward, Molenaar, Pieke
• Format: Report
• Language: English
• Published: Wageningen Marine Research 2018
• Subjects: Life Science; Scophthalmus maximus; Turbot
• Online Access: https://research.wur.nl/en/publications/discards-survival-probabilities-of-flatfish-and-rays-in-north-sea; https://doi.org/10.18174/449707

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. ...