More than 3,000 fish species have been caught in bottom trawls, with estimates suggesting the true number could be nearly double, according to the world’s first global inventory.

Published in Reviews in Fish Biology and Fisheries by researchers at the University of British Columbia (UBC), the study draws on more than 9,000 records of fish species reported in bottom trawls from 1895 to 2021.

“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 first author Dr. Sarah Foster, senior researcher and program leader at UBC’s Project Seahorse.

Bottom trawling drags heavy nets across the seafloor, sweeping up most marine life and habitats in its path. It is one of the most widespread and destructive fishing practices. Yet very little is known about what species are caught and how catches affect marine biodiversity.

“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,” added Dr. Foster.

The study found that one in seven fish species recorded in bottom trawl catches with assigned conservation status are threatened or near threatened with extinction, based on the International Union for Conservation of Nature (IUCN) Red List, the global standard for assessing extinction risk.

One in four reported fish species are classified as either data deficient, where not enough information is available for a conservation assessment, or have not been evaluated, which means a large portion of bottom trawling is operating in an information vacuum.

Among the species recorded are the critically endangered giant guitarfish, the endangered zebra shark, and at least three vulnerable seahorse species.

Seahorse species designated as vulnerable by the International Union for Conservation of Nature are among the bycatch being captured by bottom trawling, according to a new global inventory published by University of British Columbia researchers (Sarah Foster, University of British Columbia).

The data also shows that bottom trawls catch all or most species in some fish families. These include both the ocean's most nutritious and commercially critical fishes, such as jacks and croakers, and rare, distinct groups such as 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. 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,” said co-author Syd Ascione, a research biologist at Project Seahorse.

The study also found that smaller fish species are often not recorded at all and are likely being grouped under vague categories such as “trash fish” or “mixed fish,” hiding the true number of species being caught. Additionally, where this information was reported, approximately 95 per cent of the species were not being targeted by the fisheries yet 64 per cent were kept anyway.

Together, the researchers say the findings suggest that what is recorded is only a glimpse of the true toll bottom trawling has on marine ecosystems – and highlight the urgent need to rethink how bottom trawl catches are documented, assessed and managed.

Nearly 99 per cent of bottom trawling takes place within countries’ national waters and jurisdictions, meaning governments have both the authority and responsibility to manage impacts on biodiversity and fisheries.

“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. 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.” added senior author Dr. Amanda Vincent, director of Project Seahorse.