Researchers studying the impacts of deep-sea fishing have arrived at a clearly defined depth limit for trawling in Europe: 600 meters (2,000 feet). Deeper than that, ecological impacts increase while catch values decrease. The findings, published in Current Biology this week, come just as the European Union is considering contentious legislation to manage deep-sea fishing and limit trawling, the practice of harvesting seafood by dragging giant nets along the seafloor.
The deep sea is Earth’s largest ecosystem. While biodiversity levels are high down there, many species have unique life history traits that make them especially vulnerable to exploitation by overfishing. Compared to shallow-water species, they tend to live longer, breed slower and have fewer offspring, for example. Previous studies have found that bottom trawling takes a toll on ecosystems, and since deep-sea fish sequester large amounts of carbon every year, there are also huge implications for our warming world.
To see if there should be a depth limit – and if so, where – a team led by the University of Glasgow’s Joanne Clarke studied how catch composition trends change with depth. The researchers collected data from trawl surveys conducted between depths of 240 and 1,500 meters (800 and 5,000 feet) at various locations in the northeastern Atlantic between 1978 and 2013.
They found a clear transition in catches at depths of 600 to 800 meters (2,000 to 2,600 feet), including major increases in biodiversity, the percentage of (threatened) sharks and rays, and the ratio of discarded biomass to commercial biomass. At this depth range, "collateral ecological impacts are significantly increasing while the commercial gain per unit effort is decreasing," Clarke said in a statement. "Going deeper causes greater and greater damage for a reducing benefit to fishermen." Below 400 meters (1,300 feet), New Scientist reported, 18 new species are caught in the nets every 100 meters (330 feet).
"Depth limitations are often labeled as a 'blanket' measure, unsophisticated and poorly thought out," Clarke added. "In this case, however, it appears that there would be some very specific conservation benefits to a depth limit at around 600 meters."