Fishing gear catches what it encounters, not what the vessel is looking for. Reducing that unintended catch depends on finding a difference between target and non-target species that gear can act on.
Why the problem is structural
A net or longline is a passive filter. It cannot recognise a turtle, a seabird or an undersized fish, so selectivity has to be built into the gear itself.
The economic cost falls on fishers too, since unwanted catch consumes deck time, damages the target catch and in many fisheries must be discarded without payment.
That shared interest is why gear modification has progressed further than most regulatory approaches to the issue, though enforcement still determines uptake.
Sorting by size and body shape
Mesh size sets a lower bound on what a net retains, since smaller fish pass through, and increasing it is the simplest selectivity measure available.
Rigid grids fitted inside a trawl exploit shape instead. Shrimp pass between the bars while larger animals, including turtles and sharks, are deflected toward an escape opening.
Square mesh panels work similarly by staying open under tow, whereas diamond mesh closes and traps fish that would otherwise swim clear.
Using behaviour rather than physics
Species react differently to a net approaching. Some rise, others dive, and separator panels placed at the right height send each group toward a different outcome.
Lights fitted to gillnets reduce turtle entanglement because turtles see the illuminated net and avoid it, while the target fish appear not to respond.
Acoustic deterrents attached to nets produce sounds that porpoises and dolphins avoid, though their effectiveness declines where animals become accustomed to the signal.
Longlines and seabirds
Seabirds are caught when they seize baited hooks during setting, before the line has sunk beyond their diving range.
Streamer lines flown above the setting area deter birds from approaching, weighted lines sink hooks faster, and setting at night reduces encounters for species that feed by sight.
Used together these measures have cut albatross mortality sharply in fisheries that adopted them, which is among the clearest demonstrations that gear change works.
Why adoption is uneven
Modified gear frequently loses some target catch, and a fisher operating on thin margins weighs that loss against a conservation benefit that accrues elsewhere.
Uptake is strongest where the modification is mandatory, where market access depends on certification, or where the excluded species is itself commercially valuable at a larger size.
Monitoring remains the weak point, since much of the world's fleet carries no observer and no camera, leaving compliance largely self-reported.