Protecting Ocean Ecosystems

Destructive Trawling and the Myth of “Farming the Sea”

Trawlers trail massive plumes of sediment in the Gulf of Mexico. Photo credit: SkyTruth

In the wake of significant but highly warranted cuts to catch limits for cod, the New England Fishery Management Council spent the last day of their most recent meeting in January discussing the development of a suite of habitat protection measures known as the Omnibus Habitat Amendment. Despite the obvious need for new habitat protections to help restore Atlantic cod populations, the Council had already taken action to potentially open over 5000 square miles of previously protected areas to destructive bottom trawling. By doing so, the Council has continued to demonstrate a lack of regard for the immeasurable documented benefits of habitat protection to the health and productivity of our fisheries.

Even more concerning were the misperceptions of the effects of bottom trawling on display at the January meeting—even by members of the Council itself. Laura Ramsden, a relatively new member of the Council and an owner of the Boston-based seafood distributor Foley Fish Company, suggested that the scientists tasked with evaluating habitat protection priorities might be missing the benefits of bottom trawling. She asked the members of the Closed Area Technical Team: “As you’re evaluating the different areas, are you also taking a look at potential damage of closing them in terms of invasive species and the potential risk of not ‘tilling the soil’, if you will?”

The inexplicable myth that bottom trawling might “farm the seabed” is all too common, but it has no foundation in scientific reality. There are few serious studies that suggest that trawling may increase any kind of food production, and they are very limited in scope. A single study suggests that plaice in the North Sea may benefit from the reduced competition and increased production of some invertebrate species on which they prey. But the trawling is very limited—1 or 2 trawl passes a year—and the study does not examine the effects on other species of tearing apart complex bottom structures, removing higher trophic level predators, and reducing natural competition and biodiversity. A second, empirical study found that higher levels of trawling reduced productivity of even small invertebrates and that variability in productivity was far more closely linked to climate change than bottom trawling.

There is similarly scant scientific evidence for Ms. Ramsden’s assertion that trawling has any beneficial role in limiting the spread of invasive species. On the contrary, multiple studies suggest that human disturbance makes habitat more vulnerable to the spread of invasive species.

Meanwhile, the scientific consensus on the destruction caused by bottom trawling is nearly unanimous. It’s hardly surprising that dragging massive trawls along the seafloor destroys habitat—scallop dredges can weigh up to a metric ton (2205 lbs), and furrows up to 20 centimeters (8 inches) deep are common in trawled areas. A recent study in European waters even showed bottom trawling was changing the bathymetry of the seafloor on a massive scale.

Numerous scientific studies have shown that trawling  lowers overall productivity and can completely change the composition of local fish populations. Trawling tears apart biological structures like kelp forests and sponges and flattens out the seafloor structures that protect juvenile fish, leading to increased predation and reduced recruitment. It reduces biodiversity and species richness, which have been repeatedly shown to build resilience to invasive species. And areas with complex bottom structure, like the rocky ridges and horse mussel beds of current protected area Cashes Ledge, are the most vulnerable. In some areas trawling can stir up so much sediment—which then settles to smother eggs, larvae and other ocean creatures—that it leaves a trailing plume visible from outer space.

Protecting valuable habitat areas from trawling provides more spawning adults and juvenile fish, harbors older females with higher rates of reproductive success, and protects complex habitat like kelp forests. The current protected areas have proven themselves beneficial to struggling fish populations—they have helped scallop populations recover, and some species, like haddock, are larger and more abundant inside these closed areas. Fishermen target the edge of the protected areas because they know that more and larger fish can be found there.

Opening protected areas to bottom trawling threatens to instantaneously reverse these benefits. The best scientific evidence is that bottom trawling does not “till the soil”, but that opening protected areas will destroy vital habitat and keep cod populations from recovering. NOAA and the Council should heed the scientific record and make the right decision—to keep bottom trawling out of the groundfish closed areas.



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