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‘Boom and bust’ cycle of deep-sea trawling unsustainable, study finds

  • Researchers have built a global picture of deep-sea fish catches from bottom trawling from 1950 to 2015.
  • Deep-sea trawling can be extremely destructive for fish populations, while providing minimal economic benefits, the study found.
  • Researchers also found that large quantities of fish caught in the deep sea go unreported.

Fishing in the remote waters of the deep sea isn’t easy. But with technological advancements, fishing boats have pushed farther and deeper into the oceans, frequently using bottom trawl gear — giant fishing nets weighted down with metal attachments that drag along the seafloor — to scoop up huge amounts of fish from depths of up to 2,000 meters (6,560 feet).

Deep-sea trawling, however, can be extremely destructive for fish populations, while providing minimal economic benefits, researchers have found. Catches from deep-sea trawling are also grossly underreported, the researchers conclude in a new study published in Frontiers in Marine Science.

“Considering that the search for fishable resources, though partly ‘buffered’ by some recent regulatory positions … is progressively moving deeper and deeper, the picture that emerges from this study is definitely worrying,” Antonio Pusceddu, professor of ecology at the University of Cagliari, Italy, who was not involved in the study, told Mongabay.

Pacific sleeper shark. Image by NOAA.

Bottom-trawling in the deep sea likely unsustainable

Between 1950 and 2015, bottom trawls caught about 25 million tons of 72 fish species at depths greater than 400 meters (1,310 feet), the researchers found. Many of these fisheries followed a “boom and bust” pattern, with fish harvests first thriving, then quickly crashing.

This shows that deep-sea bottom-trawled fisheries are not sustainable, lead author Lissette Victorero, a doctoral student at the National Oceanography Centre, U.K., told Mongabay. “Most of them last only a decade or two. Additionally, this cycle is so rapid that the management and the scientific knowledge about the [target] species is lagging. Hence, the fishery is inappropriately regulated and the stock becomes rapidly over-exploited leading to a collapse,” she said.

Past research has shown that in the deep sea, fish and other organisms tend to live longer, and grow and reproduce more slowly. This suggests that fisheries could rapidly and indiscriminately overexploit deep-sea species, the researchers say, without giving them sufficient time to bounce back.

However, the overall catch from deep-sea bottom-trawled fisheries over the 65-year study period contributed to less than 0.5 percent of global fisheries.

“Certain deep-sea fisheries, such as the orange roughy [Hoplostethus atlanticus] in New Zealand, are highly valuable for a very small number of companies, in this case maybe two or three,” said co-author Les Watling, professor of biology at the University of Hawaiʻi at Mānoa. “But the environmental damage is extensive so we have to wonder whether those few companies should be allowed to benefit when the damage is so severe.”

Orange roughy caught in a trawl from off New Zealand. Image by Claire Nouvian/Bloom Association.

Deep-sea fish catch data hugely underreported

What is particularly alarming, the researchers say, is that large quantities of fish caught in the deep portions of the ocean go unreported.

To arrive at their final estimates, the researchers combined information from two sources: marine fish landings officially reported by countries to the Food and Agriculture Organization of the United Nations (FAO) from 1950 to 2015, and a set of estimates for unreported, bycatch and discarded catch data reconstructed by the Sea Around Us project, a research initiative at the University of British Columbia.

The study found that the FAO database, which includes only what countries report to the organization, underreported deep-sea, bottom-trawled catch data by about 43 percent. The total fish catch for the study period, according to the FAO database, was 14 million tons. But when combined with the Sea Around Us dataset, the researchers arrived at an estimate of 25 million tons.

One reason for this underreporting, Victorero said, is that trawlers end up catching everything in their way, including immature individuals that do not meet the desirable size requirements, or species that have no commercial value. But countries may be reporting only what was kept and landed at the dock, and not what was discarded. In fact, Victorero’s team estimated that deep-sea fisheries discarded some 6 million tons of fish during the study period.

The official landings reported to the FAO also do not account for commercially valuable fish that may have been illegally caught outside of existing official quotas, Victorero said.

Furthermore, during the earlier days of deep-sea fisheries, the FAO did not require the reporting of many deep-sea species. Without any set quotas, whenever new fisheries were found, there would be a “gold rush” for resources, Victorero said, particularly around seamounts and ridges where many deep-sea fish aggregate to feed and spawn.

Imants G. Priede, professor emeritus at the University of Aberdeen, U.K., who was not part of the study, agreed that the underreporting of official data was alarming. “The study clearly shows that the industry has been based on successively depleting one resource after another leaving behind a barren sea floor which may take decades or centuries to recover,” he said.

Summit of a seamount in the northwest Atlantic that has been fished with a bottom trawl. Image by NOAA/IFE/URI.

Accuracy of fish catch datasets

The study, however, cautions that even the Sea Around Us datasets might contain uncertainties, which may affect final estimates of unreported fish catch. “The method of estimating [unreported catches] can vary from country to country, so it is possible that the accuracy of the estimates will also vary according to country,” the researchers write in the paper.

Divya Karnad, founder of InSeason Fish, an India-based sustainable seafood program, and consultant with the Bay of Bengal Intergovernmental Organization, FAO, illustrated some of the uncertainties using India as an example.

The Sea Around Us project’s estimate for unreported catch from India’s small-scale fisheries is based on several assumptions and extrapolations, Karnad said, which is “not the same as having rigorously collected scientific data that allows us to quantify catch from a fishery.”

Karnad, who was not involved in the study, added that while the list of deep-sea, bottom-trawled fish species compiled in the study is an important one, it seemed biased toward temperate and polar regions. “Tropical deep water species, such as Bramble sharks, which are landed from India’s deep water fisheries, [are] missing from the list,” she said.

Despite the limitations, the researchers write that the FAO data combined with the Sea Around Us reconstructions constitute “the only data compilation available for estimating reported and unreported landings and discards for deep-sea fisheries.”


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