New study reveals massive under-reporting of deep-sea fish catch

A new study published today in Frontiers in Marine Science used the data gathered by the Sea Around Us project in a unique effort to reconstruct the fish catch at global scale, to reveal that in the past 60+ years, the practice of towing giant fishing nets along the sea floor has caused the extraction of 25 million tonnes of fish that live 400 metres or more below sea level, leading to the collapse of many fish populations in a “boom and bust” pattern of exploitation.

The new estimates suggest that 42 per cent more fish have been caught by countries than they reported to the Food and Agriculture Organization (FAO) of the United Nations. “Our study shows that there is systematic underreporting of the real catch. This means that fisheries managers are making decisions based on incorrect data, which has dramatic consequences for marine ecosystems,” said Lissette Victorero, lead author of the paper and a PhD student at the National Oceanography Centre in the United Kingdom.

Orange roughy © Claire Nouvian / BLOOM

The study, which was published this week in Frontiers in Marine Science, examines the historical catch of 72 deep-sea fish species taken by bottom trawlers around the world, many of which are exploited to unsustainable levels and several of which are no longer commercially viable.

For example, Roundnose grenadier estimated catches for the North Atlantic were greater than 60,000 tonnes in 2001, but the stock was overfished so fast that a moratorium had to be imposed in 2006. “One of the reasons behind this collapse is the fact that trawls are not selective: they catch everything and anything, which means that young grenadiers that have not yet reached full reproductive development are caught along with adult grenadiers, decreasing the ability of the population to recover,” said Deng Palomares, co-author of the study and the Sea Around Us Project Manager at the University of British Columbia.

Prof. Les Watling, from the University of Hawaii in Mānoa, and an author of the study explained that most of the fisheries they analyzed followed a typical “boom and bust” cycle, lasting for less than a decade or two, because deep-sea fish generally have low fecundity, grow very slowly and are highly vulnerable to overexploitation. Deepwater fish also often live around seamounts and ridges that are often left bare of life when fished by deep-sea bottom trawls.

Bottom trawling of deep fish depletes the stocks but does not generate much in the way of marketable fish. Immature individuals are thrown overboard because they generally don’t meet minimum size requirements, while non-targeted species caught as bycatch are also returned dead to the sea.

The new estimates presented by Victorero and her colleagues suggest that six million tonnes of fish were discarded over the study period while deep-sea fisheries only contributed 0.5 per cent of total fisheries landings. “This means that globally their economic importance is trivial,” the paper states.

The need to keep the business going despite its financial unviability drives fleets to continuously look for public subsidies and new species to fish, particularly once they have fished out a stock or are subjected to new regulations. “So what we are seeing is a cycle in which trawlers start targeting fish that they were already dragging up as bycatch. They create new markets for them until they also exhaust that stock with regulations lagging behind,” the lead author explained.

The impact of trawling goes beyond the capture of fish populations. As they are dragged on the seabed, trawls remove sponges, corals, sea stars, sea cucumbers, and anemones, all of which play important roles as food source or habitat for fish. They destroy seamount communities and other fish homes, turning former thriving habitats into large cleared areas.

The massive under-reporting of catch “means that much more biomass of fish and habitat-forming species have been removed from the deep sea than we thought. This has altered the ecosystem in ways that we have yet to understand,” Palomares said.

Claire Nouvian, co-author of the paper and founder of the marine conservation NGO BLOOM added that these results would have to be taken into consideration by politicians when setting the total allowable catch (TAC) and quotas for deep-sea species in the European Union in the winter 2018. “The new EU deep-sea fishing regulation that we fought so hard for and that establishes a ban on deep-sea bottom trawling below 800 meters came into force in January 2017, but because deep-sea quotas are decided every two years, 2018 is the first time that the TAC will be set according to the new rules”. There will be pressure on decision makers to ensure the quotas are set against fundamental principles of both the Common Fisheries Policy and the new deep-sea fishing Regulation (n°2016/2336), which clearly impose “the long-term conservation of deep-sea fish stocks”, the application of the precautionary approach to fisheries management and the restoration of harvested populations “above levels which can produce the maximum sustainable yield.”


Publication shall be cited as: Victorero, L., L. Watling, M.L. Deng Palomares, C. Nouvian. 2018. Out of sight, but within reach: a global history of bottom-trawled deep-sea fisheries from >400 m depth. Frontiers in Marine Science. Doi: 10.3389/fmars.2018.00098.

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