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New Study Puts Hard Numbers on Impacts of Bottom Trawling

Mud plumes follow Gulf of Mexico shrimp trawlers like con trails follow airplanes.
NASA image by Jesse Allen, data from Univ. Maryland Global Land Cover Facility.
Public Domain
Mud plumes follow Gulf of Mexico shrimp trawlers like contrails follow airplanes.

Roughly a fifth of all fish eaten globally are caught using nets towed along the bottom of the ocean. There’s long been concern that this method – known as trawling – destroys or severely damages the ecosystems where it’s used. Now, a new meta-analysis of the science available on this topic offers some quantification of the impacts of different type of trawls.  

Previous studies have found that the mud plumes from some trawls can be seen from space, and that “bottom trawling related to commercial fisheries leaves a greater physical footprint on the seafloor than the combined effects of all other human activities, including scientific research, fossil fuel recovery and waste disposal.”

While it is undeniable that dragging metal gear across the seafloor impacts the ecosystems there, a new analysis of seventy different studies finds that not all trawls are equal, and that some do significantly less damage than others.

In particular, otter trawls – the type most commonly used in New England – have the least impact of the four types compared. Otter trawls scrape, on average, just under an inch off the seafloor and remove about six percent of the animals living there. In contrast, hydraulic dredges squirt water into the sediment to release buried animals, reaching about six inches into the sea floor and removing more than forty percent of animals.

The new study also looked at how long it takes for ecosystems to recover from trawling impacts, and found that it ranged between just shy of two years and six and a half years. Of course, there are areas of the Gulf of Maine that get trawled day after day.

While this study puts some hard numbers to what trawls do to New England’s seafloor ecosystems, it still leaves some room for interpretation.

“I think we have to be careful in the words that we use. They’re being impacted. I wouldn’t like to use the word disrupted,” said Steve Eayrs, a research scientist at Gulf of Maine Research Institute who specializes in fishing gear design and innovation. “They’re being impacted regularly, day after day, month after month. But they have for a very long time.”

Eayrs admits that the introduction of trawling to the Gulf of Maine some hundred or so years ago would have been a significant disruption. But, he says, hurricanes also cause significant disturbance of the seafloor. He also notes that many repetitively trawled areas are shallow, high-energy systems where seafloor disturbance is routine and natural.

“Putting it into context, trawling certainly does have an impact,” Eayrs said. “Does it disrupt? I think impact is the appropriate way to describe the impact, particularly in an area that’s trawled over and over again.”

Overall, Eayrs says the study was well-done and provides useful information about an important and productive means of fishing, locally and globally.

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