International study measures the impact of trawling on the seabed

In contrast to commonly held views that bottom trawling transforms large portions of the seabed into an underwater desert, the impacts are highly variable and recovery times for plants and animals disturbed by trawling depend on the type of gear used and a range of environmental variables.

These are some of the findings of an international study published in Proceedings of the National Academy of Sciences in July. The study was conducted by a team of 16 researchers who analysed data from trawl fisheries around the globe in an attempt to quantify the impacts of trawling.

Bottom trawlers catch at least 20% of global marine fish catches and consequently trawling plays an essential role in providing food for millions of people. In South Africa, deep-sea and inshore trawlers catch approximately 150 000 tons of hake and other deep-sea species per year. The catch is supplied fresh, or processed and packaged, to seafood markets at home and abroad. A number of low value species caught by these fisheries − such as horse mackerel, snoek and angelfish − are a valuable source of good quality animal protein, particularly in the Western Cape.

Data gathered from the South African trawl fishery for hake was utilised in the trawling impacts study, said Johann Augustyn, secretary of the South African Deep-Sea Trawling Industry Association, SADSTIA.

“Most of the data used in the study comes from fisheries in the eastern United States and Western Europe, but we are pleased to have been included in the study,” he said.

“Understanding the ecosystem consequences of trawling is important to SADSTIA because it will help us to work with the Department of Agriculture, Forestry and Fisheries to introduce appropriate management measures that will reduce negative impacts on the seabed.”

South African trawlers make use of the “otter trawl” system in which a cone-shaped net is held open by two “otter boards” − more commonly referred to as “trawl doors” − while it is pulled along the seabed by a powerful stern trawler. Hake and other fishes living on or close to the seabed are herded into the net by the disturbance created.

According to the trawling impacts study, out of the four trawl methods analysed, otter trawling causes the least damage to the seabed, removing 6% of plants and animals per drag and penetrating the seabed down to 2.4cm on average, in contrast to hydraulic dredges (used to catch bivalve shellfish like mussels and clams) that penetrate the seabed to 16.1cm and remove 41 percent of plants and animals per drag.

“There’s a common perception that you trawl the bottom and the ecosystem is destroyed,” said Ray Hilborn, Professor at the University of Washington in Seattle, United States and one of four co-authors who designed the analysis.
Hilborn was quoted by the University of Washington.

“This study shows that the most common kind of trawling, otter trawling, does not destroy the marine ecosystem, and places that are trawled once a year really won’t be very different from places that are not trawled at all.”
But, says Hilborn, the widespread use of otter trawls means that the footprint of this trawling method is much greater than other trawl methods.

“While otter trawling has the least impact per trawl pass, it is the most widely used of all the bottom fishing gear types and hence its effects are more widespread than are those of more specialized fishing gears, such as hydraulic dredges,” he said.

The study is one part of a larger effort to catalogue the effects of different types of bottom trawling worldwide. This is known as the Trawling Best Practices Project which Hilborn leads with co-authors Michael Kaiser of Bangor University and Simon Jennings of the International Council for the Exploration of the Seas in Denmark.

Ultimately, the team aims to publish a set of fishing-industry “best practices” for the methods, equipment, density and frequency of bottom trawling.

Meanwhile, in South Africa, SADSTIA has collaborated with the University of Cape Town, the South African National Biodiversity Institute, the South African Environmental Observation Network and the Department of Agriculture, Forestry and Fisheries in a five-year study that is using a submersible camera and a benthic grab to closely examine and compare the state of the seabed in three lanes that have been closed to trawling, and two lanes in between them where trawling is allowed.

The seabed experiment enters its fifth and final year in 2018 when a final batch of photographs and samples will be taken. Data analysis and the publication of results will follow.

Read more about the trawl impacts study: