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Every day, thousands of fishing boats around the world drag huge weighted nets across the seafloor, ensnaring everything in their wake and destroying marine habitats.
Almost one quarter of the world’s annual catch is from this bottom trawling — a process that has been described by scientists as “bulldozing” the ocean floor. The method can be traced back to the 14th century but technological advances in the latter half of the 20th century have allowed it to extend its reach from shallow waters to the deep sea.
Amanda Vincent, a professor at the Institute of Oceans and Fisheries at the University of British Columbia, in Canada, has witnessed the destruction of bottom trawling over three decades researching seahorses across the world.
“Consider your favorite forest or hillside and imagine helicopters dropping heavily weighted wires and clear cutting everything in their path, plowing into the soil, but also taking out every bee, butterfly, bird, bush and bear,” she says. “We wouldn’t allow that on land, not for a minute, but this is what’s happening in the ocean all day, every day.”
“It’s just devastating and it wreaks ecological havoc,” Vincent says. “It’s annihilation fishing, pure and simple, and it has to stop.”
Project Seahorse, a non-profit founded by Vincent in 1996, contributed to getting marine fish recognized by the International Union for Conservation of Nature — a major milestone in protecting seahorses and other marine life. Vincent was also instrumental in uncovering a huge global trade in seahorses — a popular ingredient in traditional Chinese medicine — and her campaigning led to global restrictions on the seahorse trade. Her next crusade is bottom trawling.
More than 70 million seahorses are caught every year by bottom trawling or other fishing methods, according to Project Seahorse, making fishing the biggest threat to the species.
“Most trawl boats only catch one or two seahorses per boat per night (as bycatch). It sounds like nothing,” she says, but continues “countries like Thailand or India export five million seahorses a year, (so) it tells you something about the scale of those bottom trawl operations, because that’s the main way they catch seahorses.”
Impact on the oceans
A 2018 study estimated that around 19 million tons of fish and invertebrates end up in the nets of tens of thousands of bottom trawlers worldwide every year. Other research has found that bottom trawling accounts for nearly 60% of fisheries discards, with unwanted catch thrown back into the ocean.
For the past seven decades, bottom trawling has wasted 437 million tons of fish, leading to an estimated loss of revenue of $560 billion, according to another study.
“It has made a tremendous impact on the world’s oceans,” says Juan Mayorga, a marine scientist at the University of California, Santa Barbara. “For each kilogram of shrimp, you could get up to 25 kilograms of incidental catch … There’s no such thing as a selective bottom trawl.”
As well as threatening fish stocks, a recent paper Mayorga co-authored studied the movements of over 20,000 bottom trawling vessels worldwide and found that by dredging the carbon-rich sediment on the ocean floor, they had a carbon footprint comparable to that of global aviation.
The ocean absorbs a third of the carbon emissions in the atmosphere, making the seafloor the largest carbon repository on the planet.
“The first meter of the sea floor stores twice as much carbon as all of the terrestrial soils combined.” says Mayorga. “So it’s a huge, huge, huge reservoir of carbon.”
Reducing the damage
Mayorga says the number of bottom trawling vessels has plateaued in recent years, partially due to fuel costs, and that some changes have been made the industry less destructive, such as modifying the gear to reduce bycatch and allow species like sea turtles to escape, but he adds that these measures have not been adopted widely.
Vincent says one of the most effective ways to reduce the damage is to end government subsidies, which researchers say play a significant role in supporting the industry. They say that deep-sea bottom trawling would not be globally profitable without them.
Vincent has found “great hope” in global commitments to protect 30% of the ocean by 2030 by implementing Marine Protected Areas (MPAs), where fishing is tightly regulated or banned. Project Seahorse has helped establish 35 MPAs across the Philippines, whose island archipelago is home to 10 of the 46 known species of seahorse.
But for Vincent there is always more work to do.
Although bottom trawling is currently the biggest threat to seahorses, Vincent says they are also increasingly at risk from habitat destruction and climate change. “They really are a harbinger of things to come when you look at the fate of seahorses right now.”
“The ocean is everything to us — 99% of the space on Earth where life is possible is in the ocean,” Vincent adds. “Essentially we’re using seahorses to help save the seas. If we get it right for these funky little fishes, we will have done a lot for the ocean.”