Bycatch is non-target catch within any fishery. Bycatch can be either different species from the target of the fishery or individuals from the target species, generally juveniles, which are not of a marketable size. Most bycatch is tossed back because it is illegal to possess or it has no commercial value. In certain fisheries, however, a certain amount of marketable bycatch can be kept to sell.
Because bycatch is difficult to quantify, it makes it very difficult to assess the population status of bycaught species. Overfishing for these species is sometimes not recognized until the population is severely overfished. Including bycatch in stock assessment techniques is helping to improve sustainability of these species.