The science of sustainable seafood, explained

How are benthic invertebrates impacted by bottom trawling?

A new paper out in Fish & Fisheries measures the environmental impact of bottom trawling, showing that benthic impacts can be limited with sound science and management. Bottom trawling can be highly destructive, so measuring, monitoring, and managing its effects are crucial for sustainable fisheries.

During a bottom trawl, ships sink nets to the seafloor and drag them along the bottom. Some bottom trawls, like scallop dredges, dig several inches into the floor. Others, like otter trawls, roll on top with the nets held open by heavy metal doors that either float just above the seafloor or skim along the bottom. What is caught by a bottom trawl depends on the design of the net, size of the holes, and whether the trawl digs into the seafloor or not. Nets are often designed to let certain species escape, like the turtle exclusion devices that are standard in the U.S. The environmental impact of bottom trawling is dependent on the seabed being fished and the mix of species on the bottom.

If the seafloor contains slow-growing species like corals and sponges, bottom trawling impacts can be extensive, killing species that may take decades to regenerate. However, most bottom trawling is over sand and mud, where benthic biota regeneration is much faster.

Bycatch is another concern. Unlike species in the water column that form large single-species schools, many different fish live in close proximity on the bottom of the ocean. A bottom trawl is not discerning and scoops them all up. With good management and a healthy market, the bycatch can be accounted for, however. Several groundfish fisheries operate profitably because of multi-species landings, not despite them.

However, bottom trawling’s potential to impact benthic ecosystems has led certain NGOs to oppose it unilaterally. A common claim is that even bottom trawling over sand or mud leaves a wake of destruction and irreparably damages the seabed.

But the science is not on their side: new research shows that bottom trawling over sand and mud probably has minimal long-term impact on seafloor biodiversity. A group of scientists led by Dr. Tessa Mazor combined three types of data to model how non-target benthic creatures recover after a bottom trawl disturbance in different areas.

In this post: I explain the data, how it was combined, and what it means for bottom trawling and fishery management around the world.

The first data set gives location data for bottom trawls around the world. Some of the data come from on-board observers that mark GPS locations of trawls, but much of the location data in Mazor et al. 2020 comes from satellite tracking ship locations via vessel monitoring systems (VMS). With location data, scientists can map exactly where and how intensively the bottom was trawled. Refined data from this mapping was only published two years ago in Amoroso et al. 2018, a paper we covered when it came out.

Researchers then went out into the field to collect quantitative surveys across study regions to see how bottom trawling was impacting benthic biodiversity (the paper specifically focused on benthic invertebrates, including things like worms and crustaceans that play a crucial role in the food web).

The third type of data input was using a computer model to estimate the growth and reproductive rates of various seafloor animals on different substrates. Decades of data collected from the bottom of the ocean were compiled into a model published last year.

The authors claim that combing the trawl location data, invertebrate survey data, and the benthic regeneration model “will enable environmental managers to identify which regions and taxa are at greatest risk of unsustainable trawling regimes.” You can see the results below.

Figure 3 from Mazor et al. 2020. Trawling footprints around the world.
Map of mean benthic status across 13 case‐study. For each region, n is the total number of benthos groups assessed. Pie charts represent the proportion of benthos groups with a particular benthos status—colored according to the overall mean benthos status pie chart. Red means the group's abundance is between 90% and 95% of what it would have been in the absence of trawling, orange, between 94.5% and 96.4%, and so on. From Mazor et al. 2020.

The figure shows that over most of the study regions there are few kinds of invertebrates depleted more than a few percent, except in Europe. However, what is shown is mean status. The most depleted species was found to be depleted by 14% due to trawling. From the paper:

It is important to note that we have only considered eight common taxonomic classes and have not included biogenic habitats or most types of colonial organisms (e.g., bryozoans, porifera, and hydrozoans).

Also, individual sites within these regions may be much more impacted. In general (as shown in Amoroso et al. 2018), most trawl effort concentrates on a few areas, and much of the bottom remains untrawled.

You may also notice that most of the study areas are off the coast of wealthy countries. Collecting the kind of monitoring data needed for this sort of analysis is expensive. Wealthier countries can also afford to spend more on fishery management, so it is often better. A good management scheme will already restrict bottom trawling in sensitive areas, so the results in North America and Oceania are not surprising (though Europe is). 

The lack of data in several parts of the world is highly concerning. Without the money and capacity to monitor and manage their natural resources, many areas of the world are almost certainly practicing destructive, unsustainable bottom trawling. NGOs should focus on building scientific and management capacity in these places rather than pushing for counterproductive unilateral bans. Mazor et al. 2020 is strong evidence that when managed well, bottom trawling can be sustainable.

Max Mossler

Max Mossler

Max studied environmental perception & policy in grad school. He thinks a lot about how other people think about the planet. He is the managing editor at Sustainable Fisheries UW.

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