What is bycatch?

In the BMIS, the term ‘bycatch’ mainly refers to the incidental capture of non-target species, including seabirds, sea turtles, sharks and rays, and marine mammals, in oceanic longline, purse-seine and gillnet tuna and billfish fisheries. Sharks are always treated as bycatch for the purposes of the BMIS, although they may be targeted or treated as an associated catch in some tuna fisheries. These four species groups are of special interest as they are characteristically highly migratory, with low reproductive rates (i.e., slow growth rates, late maturation, long gestation, low fecundity and long lives) [1]. As such, they may be may be more heavily impacted by high levels of incidental catch.

‘Bycatch’, however, is a broad term that can describe any of the following:

  • target species that are discarded because they are undersized or otherwise undesirable for economic or regulatory reasons;
  • non-target species that are retained and sold because it is economic to do so (e.g. dolphinfish in swordfish fisheries);
  • non-target species that are discarded, including undersized/damaged marketable species, species of low market value, and protected species; and
  • cryptic or unobserved mortalities, i.e., catch that is depredated or dies and falls from the gear before the gear retrieval, ghost fishing from lost or abandoned gear, and post-release mortality of catch that escapes or is released alive but in poor condition [2].

Whether a captured individual is treated as bycatch can depend on species, size, condition, market demand, season, catch location, on-board freezer capacity, and other criteria [3]. Thus what is defined as bycatch can vary not only between fisheries, but among vessels within the same fishery or among sets from a single vessel [4].

See these recent FAO publications for an overview of bycatch in longline and purse-seine tuna/billfish fisheries:

Clarke, S., et al. 2014. Bycatch in longline fisheries for tuna and tuna-like species: a global review of status and mitigation measures.

Hall, M.; Roman, M. 2013. Bycatch and non-tuna catch in the tropical tuna purse seine fisheries of the world.


  1. Poisson, F., Vernet, A.L., Seret, B., and Dagorn, L. 2012. Good practices to reduce the mortality of sharks and rays caught incidentally by the tropical tuna purse seiners. EU FP7, EBFMtuna-2012 Montpellier, France.
  2. Gilman, E.L. 2011. Bycatch governance and best practice mitigation technology in global tuna fisheries. Marine Policy 35(5): 590–609. doi:10.1016/j.marpol.2011.01.021.
  3. Clarke, S., Sato, M., Small, C., Sullivan, B., Inoue, Y. & Ochi, D. 2014. Bycatch in longline fisheries for tuna and tuna-like species: a global review of status and mitigation measures. FAO Fisheries and Aquaculture Technical Paper No. 588. Rome, FAO. 199 pp.
  4. Clucas, I. 1997. A study of the options for utilization of bycatch and discards from marine capture fisheries FAO Fisheries Circular. No. 928. Rome, FAO. 59p.