The global footprint of bottom trawling is smaller than we think

The global footprint of bottom trawling is not as large as many assume and varies dramatically by region. For example, at the extremes, just 0.4% of the seabed off Southern Chile is trawled, compared to 80% of the Adriatic Sea off the coast of Italy. On South Africa’s west coast, between 12 and 14% of the continental shelf is trawled, while on the south coast, 9 to 11% of the seabed is trawled. Trawling is for hake, a South African seafood staple and the basis of a sustainable and successful export industry.

A paper that attempts to quantify the global extent of trawling was published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) earlier this month. South African scientists, Deon Durholtz, Tracey Fairweather and Rob Leslie, who work for the Department of Agriculture, Forestry and Fisheries, were part of a team of 57 fisheries scientists led by Ricardo Amoroso of the University of Washington, who used a combination of satellite tracking data and the logbooks of fishing skippers and scientific observers to collate very precise information about the extent of bottom trawling and dredging on the continental shelves of 24 ocean regions, including North America, South America, Africa, Australia and New Zealand. The study excluded some regions where the quality of data was poor, notably in Asia, but it still represents the most comprehensive and accurate study of the global footprint of bottom trawling ever conducted.

Nearly all bottom-trawling, a method of fishing that involves dragging a weighted net along the seafloor to scoop up fish, occurs on continental shelves or slopes – the relatively shallow areas off the coasts of landmasses that eventually slope down into the deep sea. These areas are mostly shallower than 500m.

Bottom trawling is used to catch about 19 million tons of fish per year – about a quarter of all the fish caught in the world – but it has been criticised for causing damage to the seabed and triggering the depletion of fish stocks that live there, partly based on very high estimates of the areas affected.

The paper Bottom trawl fishing footprints on the world’s continental shelves, rolls back some of this criticism because it uses high definition data to show that the impact of trawling is much less than previously thought. Overall, 14% of the seafloor was trawled in the areas where high resolution data were available over the 2 to 6-year study period. Europe had the highest trawling footprint, while Australia and New Zealand had footprints below 10%.

Previous studies have suggested that bottom trawling takes place over an area equivalent to half of the world’s continental shelf, even though members of the fishing industry have consistently asserted the impact is much more limited because of their targeted use of well-defined fishing grounds, rather than widespread “ploughing” of the seabed.

Importantly for South Africa, the study shows that in regions where the bottom trawling footprint is less than 10 percent of the seafloor area, fishing rates on bottom-dwelling fish stocks almost always meet international sustainability benchmarks. South Africa’s trawl footprint is very close to the 10% threshold and a “ring-fence initiative” ensures it will stay that way. The ring-fence initiative is a voluntary undertaking by members of the South African Deep-Sea Trawling Industry Association (SADSTIA) to only trawl on currently used grounds, prevent damage to lightly trawled areas and preserve natural refuges for hake. Trawling outside the ring-fenced area requires the completion of an environmental impact assessment.

“Despite the important contribution that trawling makes to global food security, it often gets a bad rap,” says Johann Augustyn, secretary of SADSTIA. “The study by Amoroso and his co-authors is important because it shows that trawling impacts a far smaller area of the seabed than was previously thought.”

Augustyn emphasised that the South African trawl fishery for hake is sustainable and well-managed, saying it is the only fishery in Africa to be certified by the Marine Stewardship Council, the world’s leading certification and eco-labelling programme for sustainable, wild-caught seafood.

“The Amoroso study is part of a larger effort known as the Trawling Best Practices Project,” he explained, “as an industry we are enthusiastically supporting this global initiative because ultimately it will publish a set of “best practices” for the methods, equipment, density and frequency of bottom trawling.”