The science of sustainable seafood, explained

Bottom Trawling Sustainability 101

If you’ve ever eaten cod, flounder, hake, or hoki, you’ve likely eaten a fish caught via bottom trawl, a fishing method defined by towing a net along the sea floor. Bottom trawling is one of the most common forms of fishing but has been increasingly scrutinized for its destructive potential.

“Potential” is the keyword: poorly-managed bottom trawl fisheries can have significant impacts on the plants and animals living on (and in) the seafloor, while well-managed ones are often sustainable by any metric.

In this post, we compare good vs. bad trawl management and evaluate the four main ways to assess the impact of bottom trawling:

  1. target catch
  2. bycatch
  3. habitat impact
  4. carbon emissions/climate change impact.

Underpinning the entire article is the assertion that bottom trawling is food production—responsible for 26% of wild-caught seafood.

What does good or bad fishery management look like?

Well-managed fisheries monitor stock status, restrict overfishing, protect sensitive species and habitat, and enforce rules. Good management stays up to date with new science, data, and information, and quickly adapts regulations to changing conditions. In other words, effective fishery management requires money.

Poorly-managed fisheries do the opposite of well-managed ones, but they are often a symptom of limited funding and poor governance. For example, most trawling in Southeast Asia and Western Africa is poorly-managed and considered unsustainable. A 2020 systemic review of trawling in Southeast Asia reported that trawl fleets are often overcapacity (too many boats) with few livelihood alternatives for fishermen or boat owners; most fisheries are overfished; local and regional management do not have the scientific capacity to monitor stocks nor the enforcement capacity to deter illegal fishing.

Wealthier regions generally manage their fisheries well, but some, like many areas around the Mediterranean, struggle to maintain sustainable trawl fisheries.

Potential environmental impacts of bottom trawling

Understanding how bottom trawling works is key to understanding its environmental impacts. Bottom trawls are towed along the seafloor to harvest creatures that live on the bottom of the ocean or just above it. Nets generally have some kind of contraption to keep the net open and round wheels below to help them roll on the seafloor. There are several kinds of bottom trawls, typically defined by the way in which the net is held open at depth. Most bottom trawling is done by otter trawls, where doors (called otter doors) act as wings to hold the net open. Beam trawls use a large metal bar to hold the net open. Scallops and other bivalves are often targeted with dredges, a type of bottom trawl that has teeth to sink into the substrate.

Drawings of four different types of bottom trawls with a boat on the surface towing a net on the bottom.
From left to right, otter trawl, beam trawl, demersal seine trawl, and dredges. From Oberle et al. 2017.

As the boat pulls from above, the net skims along the bottom and catches creatures in its path. Some nets have contraptions that dangle on the bottom of the net, like ‘tickler chains’ to agitate fish into swimming up, slightly off the seafloor, to ensure they make it into the net. After anywhere from fifteen minutes to several hours, the net is hauled back to the surface and the catch is sorted on board.

Target catch

Bottom trawlers target demersal species—those that live on the sea floor. Common demersal fish, called groundfish, are cod and cod-like fish i.e., pollock, haddock, and hake in the Gadiformes order; flatfish like sole or flounder in the Pleuronectiformes order; and many rockfish from the Scorpaeniformes order.

Managed groundfish fisheries are typically sustainable. A 2021 paper titled, Global status of groundfish stocks found that, on average, groundfish biomass was well above target thresholds and fishing pressure was below. Fisheries with high biomass and low fishing pressure will maintain healthy populations in perpetuity.

Further, over half of the bottom trawl fisheries from that paper are certified sustainable by the Marine Stewardship Council (MSC) which includes sustainability measures beyond target catch (like ecosystem impacts). However, the paper excluded unmanaged fisheries as, by definition, basic data are missing.

Figure 4 from Hilborn et al. 2021. Trends in groundfish global means of: relative abundance, B/BMSY (top row) ; and relative fishing pressure, U/UMSY (bottom row) from 1970 to 2018. Distributions and mean trends across stocks are shown separately by order.
Figure 4 from Hilborn et al. 2021. Trends in groundfish global means of: relative abundance, B/BMSY (top row) ; and relative fishing pressure, U/UMSY (bottom row) from 1970 to 2018. Distributions and mean trends across stocks are shown separately by order.

Shrimp and bivalves (mostly scallops) make up the other significant bottom trawl fisheries; target catch is generally above target thresholds for most of the shellfish species you find on a local grocery store shelf.

The Japanese scallop fishery is the largest in the world; it has been MSC certified since 2013. The U.S. is the second largest producer—all U.S. scallop fisheries are sustainable. The U.K has a sizeable scallop industry as well with a recent stock assessment showing most populations are healthy and fishing is sustainable.

Of course, there is more to fishery sustainability than target biomass and fishing pressure. Though many wild-caught shrimp populations are healthy, shrimp fisheries struggle with bycatch sustainability. There are certainly sustainable wild-caught shrimp (e.g. Gulf of Mexico, West Coast pink, Northern Prawn), but smaller net mesh size inherently leads to more bycatch. We wrote up a whole guide to shrimp sustainability:

Bottom trawler bycatch

Because life on the bottom of the ocean is much more mixed than a monospecies school of fish in the midwater, bottom trawls often catch several species at once. For some bottom trawl fisheries, this is by design—most or all species brought on board are desirable in the right market. But bottom trawling often yields catches of non-target species that fishermen are incentivized (or, in some rare regulatory environments, required) to throw overboard to keep room for the target catch. This is when bycatch is an issue. Non-target species thrown overboard are a waste of potential food and many creatures do not survive the catch-and-release ordeal—a pointless loss of life.

However, so long as bycatch and discard species are within biological limits, fishing with high bycatch rates is sustainable. Good management monitors bycatch populations and limits its impact. In many cases, effectively managed bottom trawl fisheries are heavily constrained in their ability to catch target species by quota limits on non-target, bycatch species. For example, the bottom trawl fishery in the Bering Sea faces significant restrictions due to quota limits on halibut.

In Southeast Asia and other regions, hardly anything is considered bycatch or discarded—there are local markets for nearly everything pulled up from the seafloor. Many species, however, are not at healthy biological limits.

Bottom trawling has an average bycatch rate of 31-55%, but is highly dependent on the type of trawl gear, target species, and individual fishery; for example, some shrimp fisheries produce bycatch rates over 80%, while many scallop and clam fisheries are under 5%.

Reducing trawler bycatch

One of the best ways to reduce bycatch is to improve fishing gear so non-target species aren’t hauled to the surface in the first place.

For example, the adoption of ‘excluders’ in many U.S. fisheries have led to significant decreases in bycatch. Sea turtle excluders adopted by shrimp trawlers in the 1990s directly led to a significant recovery in sea turtle populations. The Alaskan groundfish fishery has relatively low bycatch rates (5-7%), but still managed to cut halibut bycatch by 50% over the last decade using halibut excluders.

An infographic showing how turtle excluder devices work.
How turtle excluder devices work. From NOAA

Gear improvements are starting to incorporate more electronics like deep-sea cameras. Cameras with a live feed allow fishermen to open and close nets to improve selectivity. Some nets now have remote-operated trap doors, so even if a trawl net catches non-target species, they can be efficiently released from the net at the seafloor.

Management and regulation also have a major role to play in bycatch mitigation. A 2022 paper showed that adaptive, dynamic management can reduce bycatch by 57%. Communication is key, and new satellite internet and cell phone technology make it much easier than even five years ago. Fishermen and managers can digitally map areas of high bycatch and communicate more frequently about bycatch rates and species. On-board camera monitoring ensures accurate accounting and gets information to managers much quicker.

The continuing adoption of ‘move on’ rules, which require fishing boats to move on from a fishing area if they hit a bycatch quota is also helping to reduce bycatch. Bycatch will never be completely eliminated, but discarding may be regulated away. In 2018, the EU banned discarding at sea, requiring fishermen to bring everything caught back to shore to be utilized in some way.

Bottom trawling impacts on seafloor habitat

Anything a bottom trawl contacts is likely going to be disturbed, but lasting damage depends on the nature of the plants and animals living on the seafloor. For example, in mud or sand habitats, most species are adapted to disturbances as mud and sand are easily impacted by storms and currents. Bottom trawling in those habitats may kill or damage a percentage of the plants and animals living there, but most recover rapidly unless the area is frequently trawled. On the other hand, bottom trawling over sensitive habitats like deep-water soft coral and sponges will take decades or centuries to regenerate.

Essentially, five factors determine the impact of bottom trawling on the seafloor:

  • How often a trawl contacts the seafloor
  • How deep the gear penetrates the substrate
  • The frequency of trawling
  • The depletion rate (what percent of benthic invertebrates are killed by each trawl)
  • The recovery rate of native organisms after a trawl

Most trawling is done over gravel, sand, or mud, with depletion rates ranging from 4.7-26.1% according to a 2023 review of bottom trawl fisheries. Recovery rates vary from roughly 29-68% per year, depending on the substrate. Compared to terrestrial food production, where depletion rates are close to 100%, and recovery takes centuries, bottom trawling can certainly be considered sustainable.

Managing trawling impacts on the seafloor comes down to giving the fished area time to recover and closing or restricting areas with sensitive habitats. However, gear improvements and new technology are also helping. For example, the Alaskan groundfish fisheries recently funded the development of new nets that contact the bottom significantly less than previous generations. The trawl doors don’t contact the bottom at all and the sweeper lines (that connect the net to the tow line) have been designed to vibrate in such a way to agitate the fish to swim off the bottom and into the net that no longer needs to contact the bottom. Some bottom trawlers are using electrical pulses in the same way—to scare fish off the seafloor and into a net just above the seafloor.

Nets without doors and automated winches to raise and lower a trawl net along the contours of the seafloor are also being developed in Canada:

Infographic showing an automated winching system for bottom trawling.
Katchi Tech is developing an automatic winching system to raise and lower a door-less trawl net along the contours of the seafloor to avoid contact with the bottom.

Notwithstanding bottom trawling’s gear advancements, good management has been shown to preserve seafloor habitat effectively. A 2022 paper reviewed the status of the seafloor in managed areas around the world and found that most of the seafloor was largely intact. Using a relative benthic score (RBS) that roughly translates to percent-intact, researchers mapped most of the seafloor in managed bottom trawl fisheries and found that for most areas, over 80% of the biota on the bottom are intact:

Chart showing relative depletion from bottom trawling in several regions around the world.
Depletion level of benthic flora and fauna in regions where trawl data is available. From Pitcher et al. 2022 and reproduced in Hilborn et al. 2023.

The Adriatic Sea has had management issues for decades, but most of the rest of the world’s managed bottom trawl fisheries are not very impactful on benthic ecosystems.

Lack of management that allows intense bottom trawling is a problem, particularly in impoverished areas that can’t afford to manage their fisheries or enforce regulations. The regions in the figure above mostly border wealthier countries that can afford to manage their fisheries effectively (the Adriatic Sea notwithstanding). Choosing seafood from countries that manage their fisheries well is the best way to ensure you are eating sustainably:

Bottom trawling as carbon emissions

Towing a heavy net along the ocean floor takes a lot of diesel-powered energy. Bottom trawling produces an average of 4.65kg of CO2 per kg of seafood produced, one of the more carbon-intensive forms of fishing. This is about double the carbon impact of chicken, but only a quarter of beef.

Table 2 from Hilborn et al. 2023 shows the average, minimum, and maximum fuel used to capture one metric ton of fish for different fishing gear types and the amount of CO2 released per kg landed. Data from Parker and Tyedmers 2015.
Table 2 from Hilborn et al. 2023 shows the average, minimum, and maximum fuel used to capture one metric ton of fish for different fishing gear types and the amount of CO2 released per kg landed. Data from Parker and Tyedmers 2015.

However, like all trawling impacts, carbon emissions are highly dependent on individual fishery characteristics like how often fishing gear is contact with the seafloor and how deep the gear penetrates. More fuel is required to pull fishing gear through substrate. However the key determinant of fuel use is—surprise—how well the fishery is managed. When stocks are healthy and fishermen do not race to catch fish first (i.e. a catch share system is implemented), fuel use can be very low. For example, Alaskan bottom-trawl fisheries produce just 1.17 kg of CO2 for every kg of food (about half of the average chicken). The fishery is one of the best-managed and most profitable fisheries in the world. Profits have allowed the fleet to modernize with highly efficient engines and modern ships that significantly reduce carbon emissions.

Nearly all of seafood’s carbon emissions come from fuel use so bottom trawling emissions (and fishing emissions in general) will continue to improve as boats modernize.

What about the headlines that say bottom trawling produces as much carbon emissions as all of air travel?

Those headlines came from a 2021 paper that has since been debunked. The carbon calculations from that paper were based on false assumptions about how carbon is stored in seafloor sediments. The original authors claimed that carbon on the top layer of the seafloor would be considered a carbon sink and stored if it wasn’t disturbed by trawling. However, nearly all carbon on the seafloor is naturally remineralized into seawater by microbes and organisms that live there. The authors erroneously omitted that fact in their models.

Rather than modeling carbon, scientists in 2022 actually went out and measured carbon storage on the seafloor in trawled vs untrawled areas. They found no significant differences—sometimes there was even more carbon in the trawled areas as trawls remove organisms that remineralize carbon!

We’ve covered this science story since the beginning. Our overall explainer on all of it is here. Rest assured, there is little evidence that bottom trawling significantly contributes to climate change outside of fuel use.

Bottom trawling as contentious food production

Comparing bottom trawling to other forms of fishing and land-based food production is relevant and necessary for understanding the conservation and sustainability context. It is not the least impactful form of food production, but it is certainly better than several alternatives. Well-managed bottom trawl fisheries can be some of the lowest-impact foods on the planet and many around the world are improving. Of Seafood Watch’s 380 bottom trawl ratings, 75% are rated as “Best Choice or Good Alternative.”

Food produced via bottom trawling shares the other typical environmental advantages of seafood over terrestrial-produced proteins: no freshwater or fertilizer inputs, no runoff or pollution concerns, and no land-use changes where native ecosystems are turned into farmland.

Generally, seafood is one of the best proteins you can eat for the planet, but with so many different species and fishing methods, there is a lot of variation in impact. Bottom trawling uses more fuel than most other forms of fishing, but impacts depend on the species being fished and management effectiveness. Healthy and abundant populations require less fuel to harvest so ensuring biological sustainability significantly contributes to overall sustainability.

Nevertheless, calls to ban bottom trawling have grown louder.

Advocacy can be shortsighted when it comes to banning things. Advocating against something without a viable alternative rarely leads to a net improvement. A ban on U.S.-caught swordfish led to increased swordfish demand in other countries (with worse management) causing more harm to swordfish overall. Environmental impacts need to be reduced, not displaced. Banning trawling would be yet another example of environmental impact displacement instead of environmental problem-solving.

Anti-fishing advocates need to ask, what is the counterfactual? What is the alternative to fishing? Trawling produces 20 million tons of animal protein—where will the global food supply make that up? Replacing bottom trawled protein with a typical livestock mix of 30% beef, 33% pork, and 37% chicken would require land clearing equivalent to half the Amazon rainforest.

The solution is not to ban bottom trawling but to build management capacity to transform poor or unmanaged fisheries into well-managed ones. Well-managed bottom trawl fisheries are perfectly sustainable—the science says so.

Picture of Max Mossler

Max Mossler

Max is the managing editor at Sustainable Fisheries UW.

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