The bottom trawl fishery for sole is severely disrupting the North Sea ecosystem. In terms of number of sea creatures, 99 per cent of what is caught in trawl nets is discarded marine life, ranging from small seahorses to thornback rays over a meter long. A significant proportion of this bycatch, which is thrown back into the sea, does not survive. As a result, millions of sea creatures die every week. This emerges from an analysis of monitoring data from Dutch cutter fishermen conducted by environmental economists Ben Vollaard and Aart de Zeeuw of Tilburg University.
For the first time, this monitoring data, collected as part of the European Fisheries Policy, has been used to determine how much of the discards do not survive. Bovenkant formulierThe authors combined the monitoring data with what is known about survival chances of discarded catch based on earlier studies. The analysis reveals that for every flatfish sold, 38 other sea creatures perish, including 18 fish and 20 other marine animals. The majority consist of brittle stars, juvenile flatfish, sea urchins, crabs, and lobsters.
Low survival rates for fishThe stress of being compressed in the net for up to two hours, the sudden pressure changes during the net’s retrieval, and the sorting process on board prove fatal for many discarded sea creatures. Survival rates are particularly low for discarded fish - approximately 90 percent do not survive. This has a significant impact on the North Sea ecosystem, as these fish have not yet had the chance to reproduce.
On average, the weekly catch of a single flatfish trawler involves about 1.2 million other sea creatures being caught in the net, of which 420,000 die. Currently, there are around 50 flatfish trawlers operating in the North Sea.
Dead bycatch in focusThe authors aim to provide a clearer understanding of the scale of unwanted bycatch in fisheries. Most attention is typically focused on commercially important species like sole and plaice, rather than the broader marine life. The disruption to other sea creatures caused by fishing occurs out of sight, at sea. The authors have developed a simple measure to make this disruption more visible: the number of discarded sea creatures that do not survive the catch - or ’dead bycatch’ - per fish sold. They apply this measure to sole fishing, which is the main source of income for Dutch fishermen in the North Sea.
Due to fraud involving nets, unwanted bycatch may be even higher than estimated. According to earlier research by one of the study’s authors, many sole fishermen reduce the mesh size of their nets by up to half of what is legally allowed. Fishermen benefit from this practice because it prevents small sole (known as "sliptong") from escaping the nets.
