What is bycatch?
Bycatch is the unintentional capture of non-target marine species, including dolphins, sharks, seals, juvenile fish, marine turtles and seabirds, which may be discarded and thrown back into the ocean, either dead, dying or injured. Discards represent a significant loss of valuable resources in the marine food chain, and an unnecessary waste of sentient marine life.
Marine species with high market or commercial value are targeted for capture in fisheries, however extraction methods used to catch them are indiscriminate, non-selective, cruel and wasteful. Bycatch is the collateral damage of the fishing industry, and a threat to marine species, marine biodiversity and marine environments.
Discard of non-target species in global marine capture fisheries is estimated to be 9.1 million tonnes, or 10.8% of annual average catch (discard rate), based on 1,854 fisheries records logged between 2010 and 2014, and a total annual catch of 84.6 million tonnes (FAO 2019).
Higher discard rates are more common in large-scale, industrial fisheries, although bycatch has been documented in small-scale, artisanal fisheries (see Artisanal & Subsistence Fisheries under Solutions), including the pole and line fishing industry (see Pole & Line Fishing under Solutions), but at lower annual discard rates of 3.7% (Kelleher 2005).
Large whale entanglements in fishing gear (see Entanglements & North Atlantic Right Whales under Impacts) and ghost fishing (see Ghost Nets & Sea Turtles under Impacts) are ‘passive’ bycatch, involving capture in discarded, abandoned or lost fishing gear. ‘Active’ bycatch occurs in real time, when the net is cast or the apparatus is set in motion, when the target species is caught.
Bycatch contributes to increased incidental mortality rates for non-targeted marine species, including those listed as endangered. Incidental capture of endangered, threatened and protected (ETP) marine species in commercial and artisanal fisheries is estimated to be at least 20 million individuals annually, including 650,000 marine mammals (Gray & Kennelly 2018).
In 2006, the Yangtze River Dolphin or Baiji (Lipotes vexillifer) was declared functionally extinct. Human impact was identified as the primary driver of extinction, mainly due to unsustainable fishing practices and the incidental hooking of individuals through the rolling-hook longline method, although pollution and habitat degradation were contributing factors.
Small cetaceans are vulnerable to incidental capture. The rare and ‘critically endangered’ Māui Dolphin (Cephalorhynchus hectori māui), a subspecies of Hector’s Dolphin (Cephalorhynchus hectori hectori) endemic to New Zealand, is facing extinction. In 2012, annual mortality rates from incidental capture in commercial gillnets was estimated to be 4 individuals, but due to conservation measures this has decreased to 2 individuals (Slooten 2020). The abundance estimate is 63 individuals over the age of one (Baker et al. 2016).
Perhaps the most well-known example of incidental capture and increased mortality in cetaceans is the Vaquita (Phocoena sinus), a small porpoise inhabiting the Gulf of California in Mexico, now classified as ‘critically endangered’. Urgent action is needed to save the Vaquita (see Gillnet Fishing & Vaquita Porpoises under Impacts), as current population estimates indicate only 18 mature individuals remain (IUCN 2017).
Marine turtles and seabirds
Bycatch in tuna fisheries is not limited to dolphins (see Purse Seine Fishing, Tuna & Dolphin Mortality under Impacts). Marine turtles may be captured during the setting or hauling-in of purse seine nets, sustain life-threatening injuries to flippers and shells, or be crushed by the weight of the catch.
Seabirds, including albatross and petrels, snag their beaks on barbed hooks used in longlines, dragged underwater and drowned (see Longline Fishing & Pelagic Seabirds under Impacts), and are at increased risk of incidental capture from entanglement in the cables and nets used in trawl fisheries.
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Baker, C.S.; Steel, D.; Hamner, R.M.; Hickman, G.; Boren, L.; Arlidge, W.; Constantine, R. 2016: Estimating the abundance and effective population size of Māui dolphins using microsatellite genotypes in 2015–16, with retrospective matching to 2001–16. Department of Conservation, Auckland. 74 pp. https://www.doc.govt.nz/globalassets/documents/conservation/native-animals/marine-mammals/maui-hectors-dolphins/maui-abundance/maui-dolphin-abundance-2016.pdf
Gray, C.A., Kennelly, S.J. Bycatches of endangered, threatened and protected species in marine fisheries. Rev Fish Biol Fisheries 28, 521–541 (2018). https://doi.org/10.1007/s11160-018-9520-7
Kelleher, K. Discards in the world’s marine fisheries. An update. FAO Fisheries Technical Paper. No. 470. Rome, FAO. 2005. 131 pp. https://www.fao.org/3/y5936e/y5936e00.htm
Pérez Roda, M.A. (ed.), Gilman, E., Huntington, T., Kennelly, S.J., Suuronen, P., Chaloupka, M. and Medley, P. 2019. A third assessment of global marine fisheries discards. FAO Fisheries and Aquaculture Technical Paper No. 633. Rome, FAO. 78 pp. Licence: CC BY-NC-SA 3.0 IGO. https://www.fao.org/documents/card/en/c/CA2905EN/
Rojas-Bracho, L. & Taylor, B.L. 2017. Phocoena sinus. The IUCN Red List of Threatened Species 2017: e.T17028A50370296. https://dx.doi.org/10.2305/IUCN.UK.2017-2.RLTS.T17028A50370296.en. Downloaded on 27 March 2021.
Slooten E. Effectiveness of current protection for Maui dolphin. JCRM [Internet]. 2020Dec.2 [cited 2021Mar.27];21(1):151-5. Available from: https://journal.iwc.int/index.php/jcrm/article/view/135