Bycatch are species caught in a fishery while it is intended to catch another species or reproductively-immature juveniles of the target species.
The OECD (1997) defines bycatch as total fishing mortality excluding that accounted directly by the retained catch of target species.
According to Alverson et al. (1994) there are at least four different uses of the word bycatch in fisheries.
Cetaceans, such as dolphins, porpoises, and whales, can be seriously affected by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets. Cetacean bycatch is increasing in intensity and frequency. In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait. In this fashion, cetaceans can become a target of fisheries.
One example of bycatch is dolphins caught in tuna nets. As dolphins are mammals and do not have gills they may drown while stuck in nets underwater. This bycatch issue has been one of the reasons of the growing ecolabelling industry, where fish producers mark their packagings with something like "Dolphin Friendly" to reassure buyers. Unfortunately for the dolphins, "dolphin friendly" does not mean that dolphins were not killed in the production of a particular tin of tuna, but that the fleet which caught the tuna did not specifically target a feeding pod of dolphins, but relied on other methods to spot tuna schools.
Concerns about bycatch have led fishermen and scientists to develop devices they can put on their nets to reduce unwanted catch. The "bycatch reduction device" (BRD) and the Nordmore grate are net modifications that help fish escape from shrimp nets. All U.S. shrimp trawlers—and all foreign fleets selling shrimp in the U.S—are supposed to outfit their nets with trap-door "Turtle excluder device," or TEDs, to let sea turtles escape. However, not every nation enforces TED use with equal vigor. The size selectivity of trawl nets is often controlled by the size of the openings in the net, especially in the "cod end". The larger the size of the openings, the more easily small fish can escape. The development and testing of modifications to fishing gear to improve selectivity and decrease impact is called "conservation engineering."
Longline fishing is controversial in some areas because of by-catch. Methods to mitigate such incidental mortality have been developed and successfully implemented in some fisheries. These include the use of weights to ensure the lines sink quickly, the deployment of streamer lines to scare birds away from the baited hooks as they are deployed, setting lines only at night with ship lighting kept low (to avoid attracting birds), limiting fishing seasons to the southern winter (when most seabirds are not feeding young), and not discharging offal while setting lines. However, gear modifications do not eliminate by-catch of many species and the controversy continues. In March 2006, the Hawaii longline swordfish fishing season was closed due to excess ive loggerhead sea turtle by-catch after being open only a few months, despite using modified circle hooks which attempt to reduce by-catch.
In addition to efforts to reduce the amount of bycatch caught in nets, some fisheries are starting to implement programs to effectively utilize bycatch species, rather than throwing the fish back into the ocean. One such use of bycatch is the formulation of fish hydrolysate that can be used as a soil amendment in organic agriculture.