Roundnose grenadier Corypahenoides rupestris at 1300m at Rosemary Bank. Image credit: Marine Scotland

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Members of the Institute’s Marine Science Group have published evidence that supports a clearly defined depth limit of 600 metres for deep-sea fishing in Europe.

Reporting in the journal Current Biology, the study by PhD student Jo Clarke and Dr David Bailey together with Dr Francis Neat of Marine Scotland comes as the European Council considers controversial new legislation to manage deep-sea fisheries, including a ban on trawling below 600 metres.

The research suggests that trawling deeper causes greater ecological damage, despite a reduced benefit to fishermen.

“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,” says Joanne Clarke, a PhD student at the University of Glasgow.

Studies have already shown that deep-sea fish species are more vulnerable, due to the fact that individuals tend to live for a long time while having relatively few offspring in comparison to shallow-water species.

Evidence has also shown that deep-sea bottom trawling has taken its toll, with implications for the deep-sea ecosystem and for the climate. Deep-sea fish lock-away large amounts of carbon per year.

The question asked by the researchers was: Should there by a depth limit, and if so, at what depth?

Study authors David Bailey and Francis Neat identified a way to find out by looking at how the trends in catch composition change with depth. They used data from trawl surveys collected between the depths of 240 and 1,500 metres in the northeast Atlantic. These surveys used different gear types at various locations between 1978 and 2013.

The data analysis revealed a clear transition in catches at depths of 600 to 800 metres, including a significant increase in biodiversity, the ratio of discarded to commercial biomass, and the ratio of sharks and rays to commercial biomass. As the ecological impacts increased, the commercial value per unit of effort decreased—trawling at greater depth requires more fuel and pulls up non-commercial fish species that are discarded.

“Depth limitations are often labelled as a ‘blanket’ measure, unsophisticated and poorly thought out,” Clarke says. “In this case, however, it appears that there would be some very specific conservation benefits to a depth limit at around 600 metres.”

The discovery that their evidence backed the latest depth-limit proposal came as a surprise to the researchers.

“We had no prior reason to expect that our findings would suggest an appropriate depth,” Clarke says. “We went into this analysis to test whether any depth appeared particularly suitable for a depth limit, but with no expectation that this would be the case.”

European Council discussions on the matter are expected to begin again in September and, while there are many other factors at play, the researchers say they are “very confident that the work will be brought to the attention of the relevant people and at a critical time.”

‌The work was part of the IBIS EU project—a partnership between the Loughs Agency, Queen's University Belfast, and the University of Glasgow, supported by the EU's INTERREG IVA Programme.

First published: 27 August 2015

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