It turns out that allowing trawl nets to drag deep below the ocean, and other forms of deep-sea fishing that scoop up or kill many species, is a real threat to marine environments and vulnerable species. It also doesn't benefit fishermen. That is, researchers from the University of Glasgow and elsewhere explain in a recent study that as fishermen explore deeper waters, they are causing more damage with no additional benefit to themselves. Their work, published in Current Biology, supports regulations being contemplated in Europe that will ban trawling below 600 meters if passed. 

"The most notable thing to consider about our findings is that the trend in catch composition over the depth range of 600 to 800 meters shows that collateral ecological impacts are significantly increasing while the commercial gain per unit effort is decreasing," Joanne Clarke, of the University of Glasgow, said in a statement.

According to the study, fish living deeper in the ocean are more vulnerable because they live longer and produce less offspring than shallow-water species. However, these deep-sea fish are vital for taking in large amounts of carbon dioxide annually.

By examining data collected from trawl surveys, researchers David Bailey of the University of Glasgow, and Francis Neat of Marine Scotland Science, determined a suitable depth limit. Trawling is a method of fishing that involves pulling a fishing net through the water behind a boat. Their data was collected from various locations in the northeast Atlantic between 1978 and 2013, and was from depths ranging from 240 to 1,500 meters.  

According to their study, between the depths of 600 and 800 meters there was an increase in biodiversity, as well as an increase in the ratio of discarded to commercial biomass and the ratio of sharks and rays to commercial biomass. This means that an increased number of species not needed for commercial use were also swept up in the trawls. 

"Depth limitations are often labelled as a 'blanket' measure, unsophisticated and poorly thought out," Clarke said in the release. "In this case, however, it appears that there would be some very specific conservation benefits to a depth limit at around 600 meters."

Discussions regarding the proposed ban in Europe are expected to begin again in September, and researchers said they are, "very confident that the work will be brought to the attention of the relevant people and at a critical time."

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