Abstract
The deep sea is the world's largest ecosystem, with high levels of biodiversity and many species that exhibit life-history characteristics that make them vulnerable to high levels of exploitation. Many fisheries in the deep sea have a track record of being unsustainable. In the northeast Atlantic, there has been a decline in the abundance of commercial fish species since deep-sea fishing commenced in the 1970s. Current management is by effort restrictions and total allowable catch (TAC), but there remain problems with compliance and high levels of bycatch of vulnerable species such as sharks. The European Union is currently considering new legislation to manage deep-sea fisheries, including the introduction of a depth limit to bottom trawling. However, there is little evidence to suggest an appropriate depth limit. Here we use survey data to show that biodiversity of the demersal fish community, the ratio of discarded to commercial biomass, and the ratio of Elasmobranchii (sharks and rays) to commercial biomass significantly increases between 600 and 800 m depth while commercial value decreases. These results suggest that limiting bottom trawling to a maximum depth of 600 m could be an effective management strategy that would fit the needs of European legislations such as the Common Fisheries Policy (EC no. 1380/2013) and the Marine Strategy Framework Directive (2008/56/EC).
Highlights
There has been a recent global debate as to whether there is a depth beyond which fisheries cannot be expected to operate in an economically and ecologically sustainable way
We examined the trends of catch composition indices taken from scientific trawl surveys with depth to determine whether consistent patterns could be found
This is clearly relevant to the EC’s Marine Strategy Framework Directive, which requires fishing activity to be managed to meet conservation objectives, one of which is ‘‘the maintenance of biodiversity.’’ Even though a recent study in the northeast Atlantic suggested that there has been no detectable impact of deep-sea fishing on fish diversity [2], there have been significant declines in abundances of some commercially important species [6, 8], leading to commercial extinction in some cases [7]
Summary
Clarke et al use a novel technique to show depths where ecological indices and the value of catch significantly change using long-term scientific deepsea trawl data from the NE Atlantic. The results suggest that between 600 and 800 m the commercial benefits derived from fishing start to be outweighed by potentially negative ecological consequences.
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