ICJES Journal of Marine Science
Click on header image to visit SustainableFisheries-UW.org
New review shows bottom trawling is sustainable (when well-managed)
Max Mossler - July 20, 2023 (click for link to SustainableFisheries-UW.org)
Seafood produced by bottom trawling can have a lower environmental impact than chicken or pork, according to a new review paper published yesterday. Writing in the ICES Journal of Marine Science, Hilborn et al. 2023 argues that banning bottom trawling would increase negative environmental impacts by increasing terrestrial protein production.
Hilborn et al. 2023, reviewed dozens of papers about bottom trawling impact, including stock sustainability, bycatch, ecosystem impact, and carbon footprint. Though bottom trawling is generally the most impactful kind of fishing, well-managed bottom trawl fisheries produce food with a much lower environmental impact than any terrestrial animal protein.
A review paper summarizes the current knowledge on a particular topic by combing through and presenting conclusions from recent publications. In this case, Hilborn et al. 2023 reviewed the existing literature on the environmental impacts of bottom trawling and summarized four major impacts:
Sustainability of target species
Impact on benthic ecosystems
Bycatch and discards
Carbon emissions.
The key to reducing impacts and sustaining fisheries is management. Bottom trawling can be a low-impact form of food production in places with effective management. Bottom trawling can be highly destructive in areas with little capacity for environmental management (like many developing nations in Asia).
In this post, we summarize the findings from the four major impacts, discuss what effective bottom trawling management looks like, and compare the environmental impact of bottom trawling to other forms of food production.
Sustainability of target species
Collectively, bottom trawling produces 26% of wild-caught seafood. Target species often include ‘groundfish’ (as they mostly live on the seafloor) like cod, pollock, hake, and many flatfish species. On average, groundfish populations around the world are increasing and above target levels; see Figure 1 below. Hilborn et al. 2023 reports that 50% of groundfish fisheries in the RAM legacy database (monitored via stock assessment) are certified by the Marine Stewardship Council—the most robust international sustainability standard.
Figure 1. The trend in global groundfish abundance relative to management target (solid black line). The thinner, vertical lines show the range of 50% of stocks. From Hilborn et al. 2023
This is not to say all populations are doing well—many stocks in the Mediterranean are below target levels due to overfishing, a failure of management, not necessarily the fishing gear.
However, nearly all groundfish fisheries worldwide are monitored via stock assessment. Consistent monitoring is arguably the most essential part of sustainable fisheries management.
Aside from groundfish, many invertebrates are harvested via bottom trawl or dredge. More data is needed for those stocks; no global summary of abundance trends exists for those species.
Impact on benthic ecosystems
The impact of bottom trawling on the seafloor varies by location and gear type but is mainly determined by three measures:
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% depending on the type of trawl. The authors report recovery rates of 29-68% per year, depending on the substrate. Gravel has a much slower recovery rate than mud, but that is due to the nature of the longer-lived species preferring gravel than the gravel itself. Managing trawling impacts on the seafloor comes down to giving the fished area time to recover.
Last year, a global assessment set out to model the relative benthic status (RBS) of benthic ecosystems in 24 regions worldwide. It found 15 of the 24 regions (most were outside Europe) to have RBS scores above .9, meaning the region was more than 90% intact. The poorest performing region was the Adriatic Sea in the Mediterranean, with an RBS score of .25. However, “across all regions, 66% of the seabed area was not trawled, 1.5% was depleted (status = 0), and 93% had status >0.8” See Figure 2 below.
A similar study modeled benthic invertebrates specifically and found distribution rates ranging from .86 to 1 with an average of .99. The vast majority of species groups had a status over .95, though again, European regions scored the poorest.
Figure 2. 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.
Bottom trawling can have a severe impact on sensitive species like deep-water coral and sponges. Recovery rates for those species are on the order of decades and centuries, so there is a general consensus that most of areas with those species should be closed to trawling.
Bycatch and discards
Bottom trawling generally has the highest bycatch and discard rate of any type of fishing. However, the trend has been improving for several decades—bycatch and discards are less than half of what they were in the 1980s.
The authors note that improved trawl selectivity and a reduction in fishing effort “has contributed to the reduction of discards in many trawl fisheries in Europe, North America, and Australia.“ Further, more species are being utilized instead of discarded, particularly in SE Asia, Africa, and Latin America.
Discards are the truest waste of food and resources. Reducing them is important for the planet, though the species themselves also need to be at healthy population levels.
Carbon Emissions
Food and agriculture production is responsible for a third of all carbon emissions on the planet. Emissions from terrestrial food production mostly come from land-use change—what was once carbon-storing forest is now fields of mono-crop. Ruminants like cows, goats, and sheep are generally the most carbon-intensive food on the planet due to the methane produced by rumination.
Seafood generally has the lowest carbon footprint of animal-based protein. Seafood production requires no land-use change, and no notable methane-producing species exist. Instead, the majority of its carbon footprint comes directly from fuel use.
However, different types of fishing gear have different fuel requirements. Dragging a bottom trawl through the water is one of the more energy-intensive forms of fishing. See the table below.
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.
The authors note, “The most important feature of these data is the high variability within and among different fisheries, indicating that almost any fishing gear type can catch fish with a much lower carbon footprint than the average, and no method is consistently best.”
With newer, more efficient diesel engines and eventually hybrid, hydrogen, and electric vessels, the carbon footprint of seafood will undoubtedly improve over time. The carbon footprint of terrestrial food production goes beyond simple fuel efficiency—decarbonizing will be much more difficult.
You may have seen headlines saying bottom trawling emits as much carbon as airline travel. These claims were based on poor science and are false—here’s our explainer on that.
Management actions to reduce impacts
Like all resource management, fishery management balances environmental, economic, and social tradeoffs.
Managing bottom trawling effectively starts with ensuring target species are not overfished. Excess fishing effort leads to more benthic and bycatch impacts and produces less food per kg of CO2. Stock assessment monitoring coupled with setting and enforcing a total allowable catch (TAC) is the most basic form of effective management.
“Move-on” rules where fishermen must change locations if they pull up too much bycatch or sensitive habitat help preserve benthic ecosystems as well.
A collaborative culture instead of a competitive race-to-fish can also help improve bycatch. In the Bering Sea flatfish fishery, bycatch rates have fallen to 6-8% due to fleet coordination in conjunction with bycatch limits.
Bycatch will never be entirely eliminated, but discarding may be due to regulation. In 2018, the EU banned discarding at sea, requiring fishermen to bring everything caught back to shore to be utilized somehow. Unwanted catch is often reduced into fishmeal, though emerging markets for unusual seafood can also be a solution.
However, the most significant step forward in bycatch reduction and benthic preservation will come with gear technology and improvements. Newer trawl doors can ‘fly’ above the seafloor to reduce contact with the bottom, and changes in net design can reduce unwanted species. Many fishing groups are working on deep-sea cameras mounted to the net so trap doors can open and close to eject unwanted species in the net. Trap doors have been highly effective at reducing turtle bycatch. With better satellite technology, fishermen and managers can better communicate areas with high bycatch to avoid.
Comparing bottom trawling to other forms of food production
Reviewing data from every life cycle assessment (LCA) performed on food, Hilborn et al. 2023 found that the average kg of bottom-trawled seafood produces 4.65kg of CO2. This is double the carbon footprint of chicken (2.28 kg) but one-quarter the footprint of beef (19.2 kg).
Well-managed fisheries often have much lower fuel use, though, as fishing vessels spend less time searching for abundant species. One of the largest fisheries in the world, Alaskan pollock, produces just 0.83 kg of CO2 for every kg of food—easily one of the world’s least-impactful proteins. The high value of the pollock fishery has allowed for capital investments into modern, efficient ships that significantly cut down on fuel use (Alaskan pollock is partially caught using midwater trawls, not a true 100% bottom trawl fishery). Alaska has some of the world’s best fishery management, and Hilborn et al. 2023 estimate that all Alaskan bottom-trawl fisheries produce 1.17 kg of CO2 for every kg of food produced. The Isle of Man scallop fishery (1.73 kg) and New Zealand hoki and ling (2.24 kg) are also individually reported as well-managed bottom trawl fisheries with low carbon footprints.
The roadmap to improving the carbon footprint of bottom trawlers (and indeed nearly all fisheries) is improving fuel efficiency and ending overfishing. Both have clear regulatory roadmaps—encourage vessel modernization and enforce TAC limits. Hilborn et al. 2023 also propose eliminating fuel subsidies that encourage inefficient fisheries.
Carbon emissions are not the only environmental impact of food production, however. Terrestrial food production has been the largest driver of biodiversity loss since the last asteroid. Though fishing and bottom trawling are literal biodiversity removal, it is far less destructive to ecosystems than farming. Once land is cleared for crops or livestock, more than 90% of native biota are eliminated. Meanwhile, most trawled benthic ecosystems retain 90% of their pre-trawled state. Fishing also uses no pesticides, herbicides, antibiotics, or freshwater resources.
Hilborn et al. 2023 conclude that banning bottom trawling would be a net negative for the planet as alternative food sources would be more destructive. Replacing all bottom trawled seafood with chicken, for example, would require land clearing roughly the size of South Dakota. Replacing with beef would require twice the size of Mexico. Replacing with a typical livestock mix of 30% beef, 33% pork, and 37% chicken would need a whole Mongolia.
Bottom line: well-managed bottom trawling is sustainable
After reviewing dozens of papers, Hilborn et al. 2023 concludes that well-managed bottom trawling is sustainable.
Bottom trawling has been in the crosshairs of advocacy groups for years, but arguments for banning bottom trawling are not rooted in science. What would replace the millions of tons of animal protein in the food supply? The answer is not to ban bottom trawling, but to build fishery management capacity, especially in places with poor, unsustainable management.