Bycatch Rate Calculator

Bycatch rate is a compositional ratio: the non-target fraction of a haul, computed as (total catch minus target catch) divided by total catch. Because it is bounded between 0 and 100 percent and independent of absolute volume, it allows a small inshore trap boat and an industrial trawler to be compared directly. The effort-normalised figures serve a different purpose. Catch per unit effort, here kilograms per fishing hour, is the classic relative abundance index in stock assessment, resting on the assumption that catch is proportional to stock size times effort; bycatch per unit effort applies the same logic to unwanted catch and reveals fisheries that look clean on a ratio basis only because target catches are large. The endangered-individuals-per-tonne figure exists because rate metrics are weight-based and a few kilograms of turtle or vaquita matter far more than their mass implies. Typical values span an enormous range: trap and pot fisheries often sit below 10 percent, pelagic longline in the 20 to 40 percent band, and tropical shrimp trawls historically above 60 percent. Three caveats matter. The gear selectivity multiplier applied here is a user-supplied heuristic, not a measured selectivity ogive from a covered-codend or paired-tow experiment. The ratio says nothing about discard survival, which ranges from near total for pot-caught invertebrates to near zero for deepwater fish suffering barotrauma, so two fisheries with identical rates can differ tenfold in actual mortality. Finally, the assumption that CPUE tracks abundance fails under hyperstability, where fleets targeting aggregating species maintain catch rates while the underlying stock collapses, a pattern implicated in the Newfoundland northern cod failure of 1992.

Systematic concern with incidental catch grew out of the eastern tropical Pacific tuna fishery, where purse seiners setting nets on dolphin schools killed hundreds of thousands of animals a year through the 1960s. That toll motivated the US Marine Mammal Protection Act of 1972, and consumer pressure produced dolphin-safe labelling under the Dolphin Protection Consumer Information Act of 1990. In parallel, sea turtle mortality in Gulf of Mexico shrimp trawls led the National Marine Fisheries Service to develop the turtle excluder device in the late 1970s, a grid and escape hatch that regulators phased into US shrimp fisheries from 1987 and which trials showed excluded the great majority of captured turtles. Quantification followed. Dayton Alverson and colleagues produced the first global discard estimate for the FAO in 1994, putting annual discards near 27 million tonnes; Kieran Kelleher's 2005 FAO reassessment, using better observer data and reflecting genuine reductions, lowered it to about 7.3 million tonnes. The FAO Code of Conduct for Responsible Fisheries in 1995 and the UN driftnet moratorium that took effect in 1992 embedded bycatch reduction in international practice, while the US Sustainable Fisheries Act of 1996 required federal plans to minimise bycatch and its mortality. The European Union's landing obligation, adopted in the 2013 Common Fisheries Policy reform and phased in between 2015 and 2019, took the opposite approach of banning discarding outright to force gear and behaviour change.