How Bycatch Reduction Devices Actually Work

A fishing net is an indiscriminate instrument. Towed through the water or hung across a current, it catches whatever swims into it, and the ocean does not sort itself neatly into commercial species and everything else. The result is bycatch, the accidental capture of animals the fisher never wanted: sea turtles that drown because they cannot surface to breathe, dolphins and porpoises tangled in mesh, albatrosses hooked as they dive for bait, sharks, rays, and vast quantities of juvenile fish too small to sell. Global estimates of the scale run from roughly seven million to as much as twenty million tonnes a year depending on how it is counted, and more than 250,000 sea turtles alone are thought to die this way annually. The response has been one of the quiet success stories of fisheries science: instead of banning fishing, engineer the gear so it catches what it is meant to and lets the rest go. Here is how those devices work, and how well they actually perform.

 

1. The Problem Is Identified

 

The starting point is recognising that bycatch is not an occasional accident but a structural feature of how most fishing gear operates. Trawls, gillnets, and longlines are designed to exploit the behaviour of a target species, and any animal that shares that habitat or is drawn to the same bait is vulnerable. Sea turtles, dolphins, sharks, seabirds, and undersized juvenile fish are all caught alongside the intended catch, and much of it is thrown back dead or dying.

Two things make this more than an environmental concern. First, bycatch falls hardest on exactly the species least able to absorb it, since turtles, seabirds, sharks, and marine mammals are long-lived and slow-breeding, so losses that a fast-reproducing fish stock could shrug off can push these populations toward collapse. Second, bycatch is a loss to the fisher too, filling the net with unsellable weight, damaging the target catch, and consuming fuel, sorting time, and deck space. That overlap of interests is precisely what has made gear solutions politically achievable where outright restrictions often are not.

 

2. Special Devices Are Added

 

The core insight of bycatch reduction is beautifully simple: exploit the differences between the target species and everything else. Target species and bycatch species usually differ in size, shape, strength, swimming behaviour, or sensory ability, and a well-designed device turns one of those differences into a sorting mechanism built into the gear itself.

The best-known example is the Turtle Excluder Device, or TED, a grid of metal bars fitted into the neck of a trawl net. Shrimp, being small, pass straight through the bars into the collecting bag at the end. A turtle, being large, cannot, and is instead deflected along the grid and out through an escape opening. Alongside TEDs sits a wider family of Bycatch Reduction Devices, or BRDs, which use mesh panels, sorting grids, and escape windows to release finfish and other unwanted animals. The devices were developed collaboratively rather than imposed, with NOAA gear specialists and shrimp fishermen working together through the 1980s to refine designs that would exclude turtles without losing shrimp, and that collaborative origin explains much of their eventual success.

 

3. Target Fish Enter the Net

 

Crucially, the gear must keep doing its job. A device that let the turtles out but also lost half the shrimp would never be adopted, no matter how well-intentioned the regulation. So a properly designed and installed BRD allows the commercial catch to pass through unimpeded while guiding larger or unwanted animals toward the exit.

This is the entire commercial test, and it is where most failed designs fall down. Fishers judge these devices on catch loss, and the history of bycatch reduction technology is largely a history of engineering iterations aimed at driving that loss toward zero. Flaps were added over TED escape openings to stop shrimp slipping out, bar spacing was adjusted, and materials were lightened. Recent NOAA work on TEDs with narrower bar spacing, at two and a half inches rather than the standard four, found they could exclude up to 100 percent of small juvenile turtles, which the wider bars had allowed to slip through, with no significant reduction in shrimp catch. When shrimpers were shown the results, around 90 percent said they would be willing to use them. Prove there is no catch penalty and adoption follows.

 

4. Escape Openings Do the Work

 

The physical escape route is where the sorting actually happens. Grids deflect large animals upward or downward toward an opening in the net; panels of large mesh let fish swim out sideways; escape hatches in traps and pots release undersized animals. All of it must happen while the gear is still fishing, because a turtle that reaches the surface inside a net has already drowned.

The performance figures are genuinely impressive. NOAA assesses current TED designs as 97 percent effective at excluding sea turtles from shrimp trawls when properly installed and maintained, and in Australia's Northern Prawn Fishery, the combination of TEDs and other bycatch reduction devices has cut turtle bycatch by more than 99 percent. There are useful side effects too: TEDs eject debris and large trash from the net, reduce the capture of stingrays and sharks, improve the quality of the shrimp that arrive on deck, and can even lower fuel consumption by reducing drag. Two caveats matter, though. The phrase "properly installed and maintained" is doing real work in that 97 percent figure, since a TED sewn in at the wrong angle or with a blocked opening does little. And the devices are far better at excluding large animals than small ones, which is why so much recent effort has gone into juvenile turtles and small finfish.

 

5. Fishing Practices Improve

 

Gear inserts are only one part of the toolkit. Fishers also change how, when, where, and with what they fish, and some of these adjustments are as effective as any hardware. Switching from J-shaped hooks to circle hooks, which are harder for a turtle to swallow and tend to catch in the jaw rather than the gut, substantially reduces both turtle capture and mortality on longlines. Changing bait, adjusting the depth at which gear is set, shifting fishing seasons away from migration periods, and closing specific areas at specific times all help. Bird-scaring streamer lines, trailing brightly coloured ribbons behind a vessel, keep albatrosses and petrels away from baited hooks as they sink, and weighting lines so they sink faster achieves the same end.

Some of the most promising recent innovations work on the senses rather than the body. Attaching green LED lights to gillnets appears to warn animals off without deterring the target fish: trials in Baja California cut green turtle bycatch by roughly 40 to 60 percent, a 2025 study reported a 63 percent reduction in expected turtle bycatch while maintaining target catch, and work in Peru found illuminated nets cut small cetacean bycatch by around 70 percent and seabird bycatch by more than 80 percent. Acoustic pingers, which emit a sound that alerts echolocating animals, have shown reductions in harbour porpoise bycatch as high as 94 percent in Norwegian trials. These technologies are cheap, simple, and increasingly popular precisely because they add no catch penalty, though evidence from UK waters suggests pingers can shift the problem, reducing porpoise catches while being associated with increases in the bycatch of seals, a reminder that solutions need testing fishery by fishery.

 

6. Fewer Non-Target Species Are Caught

 

Combine better gear with better practice and the effect on accidental capture and mortality is substantial. The critical measure is not only how many animals avoid the net but how many survive the encounter, which is why devices that release animals quickly and unharmed, and hooks that cause less severe injury, matter as much as raw exclusion rates.

It is worth being clear-eyed about the pattern in the results. Bycatch reduction works spectacularly well for large, distinctive animals, since a turtle or a shark differs so obviously from a shrimp that gear can sort between them almost perfectly. It works far less well where the bycatch closely resembles the target, which is the case for juvenile fish of the same or similar species, where the same Australian study that recorded a 99 percent cut in turtle bycatch found much more modest reductions in small bycatch overall. This is the honest boundary of the technology: it excels at protecting charismatic megafauna and struggles with the far larger tonnage of small fish discarded worldwide.

 

7. Fisheries Become More Sustainable

 

The cumulative benefit is real. Reducing bycatch helps endangered species recover, protects the juvenile fish that will become tomorrow's spawning stock, and cuts the sheer waste of hauling up and discarding millions of tonnes of dead marine life. TED implementation in the southeastern United States shrimp fishery is regarded as one of the most significant sea turtle conservation achievements of recent decades, and it was accomplished without shutting down the fishery.

That last point is the strategic lesson. Bycatch reduction devices demonstrate that conservation and commercial fishing need not be zero-sum, because a well-designed device delivers a cleaner catch, less sorting labour, reduced drag, and access to markets and certifications that increasingly demand demonstrable bycatch performance. When the incentives line up this way, compliance stops depending purely on enforcement and starts becoming something fishers have their own reasons to want.

 

8. Regulations Encourage Adoption

 

Voluntary uptake alone was never going to be enough, so adoption has been driven by law and by market access. Many countries mandate TEDs and BRDs in their trawl fisheries, and Regional Fisheries Management Organizations require or promote specific mitigation measures for the stocks and areas they manage. The United States has gone further by tying market access to performance, requiring that nations exporting shrimp to the American market demonstrate turtle protection comparable to its own, a lever that has spread TED use across tropical shrimp fisheries worldwide.

Regulation only works if it is followed, however, and compliance is the system's persistent weak point, since a TED can be disabled, sewn shut, or simply installed incorrectly. This is why sustained investment in outreach, training, and enforcement matters so much. NOAA's gear monitoring team, working across the Gulf coast on education and enforcement training alongside fishing communities, has helped push TED compliance rates in the Gulf shrimp fishery above 90 percent. That figure captures the essential truth about bycatch reduction: the engineering problem was largely solved decades ago, and what remains is the slower human work of getting the devices properly fitted, correctly maintained, and consistently used on every vessel, in every fishery, everywhere.

 

Did You Know?

 

According to the FAO, bycatch and discards account for millions of tonnes of marine life every year, with global estimates ranging from around seven million tonnes to as high as twenty million depending on the methodology, making it one of the biggest challenges facing world fisheries. Set against that, the humble turtle excluder device stands out as one of the most effective conservation technologies ever devised: a simple grid of metal bars, costing a few hundred dollars, that is 97 percent effective at keeping sea turtles out of shrimp trawls without meaningfully reducing the shrimp catch. It is a rare case in ocean conservation where the answer turned out to be neither a ban nor a treaty, but a piece of engineering that fishers and scientists designed together.

 

Note: This article reflects the state of bycatch mitigation as of mid-2026, drawing on sources including NOAA Fisheries, the FAO, the Marine Stewardship Council, and peer-reviewed fisheries research. Estimates of global bycatch volume vary widely with methodology, and device effectiveness figures are specific to the fisheries and conditions in which they were measured.