Bottom trawling is catching thousands of fish species, including many at risk

Bottom trawling is widely seen as one of the ocean’s most heavy-handed methods: a weighty net hauled over the seabed, gathering up whatever lives in its path and frequently leaving damaged habitat behind.

Despite the scrutiny it attracts, one very basic detail has been missing from the wider debate: the precise number of fish species being captured.

A new study led by researchers at the University of British Columbia (UBC) sets out to pin that down - and the total is higher than most would expect.

Bottom trawling catches thousands of species

Drawing on more than 9,000 records of fish species reported in bottom trawls between 1895 to 2021, the researchers found that over 3,000 fish species have been documented in bottom trawl catches globally.

Even that figure is probably low: their estimates indicate the real number could be nearly double.

The study’s first author, Sarah Foster, is a senior researcher and programme leader with UBC’s Project Seahorse.

“This is the clearest picture we’ve had of the breadth of bottom trawling. It reveals just how many species are being caught, and how much we have been missing,” said Foster.

Bottom trawling is hard on biodiversity

Bottom trawling is nothing like taking a few intended species on a hook and line. It is more akin to mowing a lawn - except the “lawn” is a richly interconnected seabed ecosystem, and the mower does not pause for anything in its way.

This is why it is regarded as among the most damaging fishing methods: it can remove creatures from across the food web, and it can also harm seabed habitats that may take years or even decades to recover.

The new inventory adds to that worry: the catch list is not merely extensive - it also contains species already facing intense conservation pressure.

Extinction risk in the net

Looking at fish species recorded in bottom trawl catches that have an assigned conservation status, the study reports that one in seven are threatened or near threatened, according to the IUCN Red List.

At the same time, one in four of the listed species were either data deficient (insufficient information to judge risk) or not evaluated at all.

Put simply, a substantial portion of trawling occurs amid an information fog: animals are being removed from the sea without knowing whether their populations can withstand the loss.

The trawl records also include species many people assume industrial fishing gear does not reach.

The study highlights the critically endangered giant guitarfish, the endangered zebra shark, and at least three vulnerable seahorse species among those appearing in the data.

“We can’t manage what we don’t know. When we remove thousands of species without understanding the impacts on their wild populations, we risk destabilizing the very systems that fisheries depend on,” Foster said.

It’s not just a few unlucky species

One of the clearest patterns in the data set is that trawling pressure can fall across whole evolutionary lineages. The inventory suggests bottom trawls capture all or most species in certain fish families.

These include commercially valuable and commonly eaten groups - such as jacks and croakers - alongside rarer and more unusual lineages, including giant guitarfish and plough-nosed chimera.

“Bottom trawling sweeps up entire branches from the marine tree of life. It does not discriminate between common species and those already on the brink of extinction,” said study co-author Syd Ascione, a research biologist at Project Seahorse.

“From critically endangered giant guitarfishes to vulnerable plough-nosed chimeras and seahorses, we put pressure on evolutionarily unique species, including many we still know too little about.”

This “tree of life” issue matters because biodiversity is not only a question of how many species exist. It also involves safeguarding distinct evolutionary history.

When a truly unique species is lost, an entire branch of traits and ecological functions may disappear with it - and no other fish can simply take its place.

The “trash fish” problem

The researchers also contend that the recorded figures likely represent only the tip of the iceberg.

Smaller species are often not identified or reported in detail and may instead be grouped under labels such as “trash fish” or “mixed fish.”

That approach may be practical for a busy commercial fishery, but it is scientifically disastrous, obscuring the true diversity being hauled up.

Even where species-level information was available, the results still offered little comfort: about 95% of recorded species were not being targeted, yet 64% were kept anyway. This underlines that “bycatch” does not necessarily mean “thrown back”.

Bottom trawling needs to be treated as high-risk

A crucial point here is oversight: bottom trawling is not taking place mainly in the unregulated open ocean.

The study notes that nearly 99% of bottom trawling happens within national waters, which means individual governments have the ability to tighten or change the rules.

“We allow at least 100,000 trawlers to scrape the ocean floor, without even knowing what they are catching, and what damage they are doing to those species,” explained senior author Amanda Vincent, director of Project Seahorse.

“It is important that governments take a precautionary approach and exclude bottom trawling from large swathes of the ocean, and particularly from so-called marine protected areas.”

The inventory’s message is straightforward: bottom trawling is not just catching “fish”. It is capturing thousands of species, including threatened ones - and the harm is still not being properly tracked.

If decision-makers want to protect ocean biodiversity while keeping fisheries viable over the long term, the first step is uncomfortably clear: measure what is being taken, and treat the unknowns as a risk - not a justification.