The science of sustainable seafood, explained

The global impact of bottom trawling visualized with data

Dragging nets across the seafloor to catch fish has been a point of controversy recently. Bottom trawling provides about a quarter of all wild-caught seafood but comes with the environmental impact of disturbing the seafloor. It’s scientists’ job to measure the environmental impact so policy-makers and managers can best balance food production and environmental impact.

Five years ago, an international team of researchers published a paper (open access) describing a new way to quantify the environmental impact of bottom trawling on seafloor habitat. Now, in a paper published today in PNAS, those same researchers have applied the method to 24 regions around the world and reported on the impact on the plants and animals that live on the seafloor.

How do you quantify the impact of bottom trawling?

The 2017 paper created an equation to quantify the relationship between population growth and recovery rates of impacted species, size and frequency of trawls, and other measurables like gear types. It produces a relative benthic status (RBS) a unit between 0 – 1 where 0 is totally depleted and 1 is untrawled.

An RBS score of .95 could be interpreted as: the seafloor habitat is 95% of its untrawled state.

With the RBS equation, impact on species would be much easier to predict with relatively common data—all that would be needed is basic species composition information and data on the amount of trawling.

This was an exciting development for scientists and policy makers to finally have a quantified baseline with which to make management decisions. Debates over how much depletion is acceptable could finally take place with holistic RBS reports instead of individual species reports.

What is that status of the oceans’ seafloor?

Now, in Pitcher et al. 2022 (open access), researchers report the RBS of 24 large marine regions from around the world. In the figure below, you can see how different regions compare. 15 out of the 24 regions had an RBS above .90; places like Australia, New Zealand, the U.S., Chile, and South Africa all scored well, while Europe was a mixed-bag. The Adriatic Sea had the lowed RBS of the measured regions.

Figure 1 from Pitcher et al. 2022. Global map showing the impact of bottom trawling.
Figure 1 from Pitcher et al. 2022 shows the RBS of 24 different regions around the world. The pie charts show the proportional area of each region, i.e. the regions are colored by their most common RBS, but the entire region is not necessarily uniform in impact.

The results are unsurprising and add to the big pile of evidence that shows that effective fishery management produces sustainable seafood. “The results show that effectively managed and sustainable trawl fisheries are associated with regions having high seabed status of 0.95 or more.” Said lead author Dr. Roland Pitcher, “regions that had low seabed status scores were places where fish stocks typically are over-exploited and have ineffective management regimes.”

Missing from the paper are regions without sufficient data, like much of Asia, where data used in this paper was not available. Evidence suggests that bottom trawling impacts in those regions are high.

However, according to Dr. Ray Hilborn (a co-author on the paper and founder of this website), “This research is a critical step in moving towards an overall estimate of the global impact of trawling, and understanding the steps required to improve fisheries management, reduce exploitation, improve stock sustainability and the status of the seabed environment.”

Picture of Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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