Bottom trawling, particularly shrimp trawling, is the commercial fishing technique most strongly associated with excessive bycatch. In tropical shrimp trawl fisheries, bycatch commonly accounts for 80% or more of the total catch, with ratios of 5 to 10 pounds of non-target species hauled in for every pound of shrimp. No other widely used fishing method consistently produces waste on that scale.
Why Shrimp Trawling Tops the List
Bottom trawling works by dragging a large, weighted net along the ocean floor, scooping up nearly everything in its path. The technique has an average bycatch rate of 31 to 55% across all its variations, but the numbers spike dramatically in shrimp fisheries. A study of tropical shrimp trawling in the Eastern Pacific found that bycatch averaged roughly 93% of the total catch per trawl. That means for every haul brought on deck, the shrimp were a small fraction of what the net actually captured.
The reason is simple geometry. Shrimp are small, so the mesh openings on shrimp trawl nets must be small. That fine mesh catches juvenile fish, rays, crabs, and dozens of other species that a larger-mesh net would let escape. In the same Eastern Pacific study, at least 14 bycatch species were listed under international threat categories, and 58 species had moderate to very high vulnerability to fishing pressure. Much of this non-target catch gets discarded, often dead, back into the ocean.
Not all bottom trawling is equally destructive. Scallop and clam dredge fisheries, which also use bottom-contact gear, can have bycatch rates under 5%. The gear design, target species, and local regulations all play a role. But shrimp trawling in tropical and subtropical waters remains the clearest example of a fishery where bycatch dwarfs the intended catch.
Other High-Bycatch Fishing Methods
Pelagic Longlining
Longline fishing uses a main line stretching for miles, hung with hundreds or thousands of baited hooks. It targets large open-ocean species like tuna and swordfish, but the hooks also attract sea turtles, sharks, and seabirds. The bycatch problem here is less about volume and more about which species get caught. Post-release mortality for loggerhead and leatherback turtles hooked by longlines has been estimated at 40% and 32%, respectively, even after fishers cut them free.
How the gear is set matters enormously. Shallow hooks set at night to target swordfish produce turtle interaction rates roughly 25 times higher than deep hooks set during the day for tuna. That distinction has shaped regulations in fisheries like Hawaii’s, where shallow-set longlining faces stricter limits.
Purse Seining With Fish Aggregating Devices
Purse seine nets encircle entire schools of fish near the surface. When fishers target naturally occurring, free-swimming schools of tuna, bycatch tends to be relatively low. But when they deploy Fish Aggregating Devices (floating objects that attract marine life), the bycatch picture changes. FAD-associated purse seine sets produce roughly five times more bycatch by weight than free-school sets. FADs attract a mix of species, so the net closes around juvenile tuna, sharks, and other marine life alongside the target catch.
Bycatch Reduction Technology
The most successful mitigation story belongs to shrimp trawling itself. Turtle Excluder Devices, metal grids installed inside trawl nets that guide large animals out through an escape hatch, are now 97% effective at keeping sea turtles out of shrimp nets. Their widespread adoption in the 1990s directly contributed to significant recoveries in sea turtle populations.
Bycatch Reduction Devices work on a similar principle, using different mesh sizes, escape panels, or sorting grids to let non-target fish exit the net while retaining shrimp. These devices don’t eliminate the problem entirely, but they can cut bycatch substantially when properly designed for the species and conditions in a given fishery.
For longlines, switching from traditional J-shaped hooks to circular hooks reduces turtle hooking rates and makes it easier to release animals alive. Setting hooks deeper and during daylight hours also dramatically reduces interactions with turtles and seabirds. In purse seine fisheries, reducing reliance on FADs in favor of free-school sets is the most straightforward way to cut bycatch, though it requires more skilled spotting of natural tuna aggregations.
Why the Problem Persists
Shrimp is one of the most valuable seafood commodities globally, and trawling remains the most efficient way to harvest it at commercial scale. In many tropical fisheries, enforcement of bycatch regulations is limited, and much of the discarded catch goes unreported. These discards fall under what regulators classify as Illegal, Unreported, and Unregulated fishing activity, making the true scale of the problem difficult to measure.
Even where regulations exist, compliance varies. Turtle excluder devices are mandatory in U.S. shrimp trawl fisheries, but many countries lack equivalent requirements. The Bering Sea bottom trawl fishery operates under strict quota limits on bycatch species like halibut, showing that management can constrain the problem when political will and enforcement capacity exist. The gap between the best-managed trawl fisheries and the worst is enormous, which is why blanket statements about bottom trawling can be misleading. The technique itself is not inherently the most wasteful form of fishing. Shrimp trawling in poorly regulated tropical waters is.