Pacific halibut are captured in large numbers by vessels fishing for other species, primarily using trawl, pot, and longline gear that are targeting groundfish. Not all Pacific halibut caught will die from the injuries if the fish are returned to the sea (discarded) in a careful and timely manner. In many areas, observers work onboard groundfish vessels and gather information regarding the amount of Pacific halibut incidentally caught and the condition of those Pacific halibut at release. From these data, the IPHC is able to estimate both the total amount of Pacific halibut caught and discarded in each fishery, and the discard mortality rate, or percentage that subsequently die. Many Pacific halibut captured as bycatch are below the commercial minimum size limit of 32 inches (81.3 cm) fork length, especially in the Bering Sea. Because Pacific halibut are migratory, incidental catches of juveniles in one area will have a potential effect on the future abundance in other areas.
The IPHC regulates which gear types can legally retain Pacific halibut. The IPHC regularly makes policy recommendations to its member governments and assists in designing and analyzing bycatch reduction measures.
Prior to the 1960s, bycatch was relatively minor due to the minimal amount of fishing for those species with overlapping distribution with Pacific halibut. However, in the early 1960s, Japanese vessels began fishing for groundfish in the Bering Sea and bycatch increased dramatically, peaking in 1965 at approximately 21 million pounds (12,701 t) from all areas of the north Pacific. Bycatch levels have risen and fallen several times since then in response to fishery development, the introduction of bycatch management measures, and the level of Pacific halibut abundance. Since the early 1990s, total bycatch mortality has decreased steadily to about 6 million pounds (2,720 t) in 2017. The history of Pacific halibut bycatch off the United States of America versus Canada is quite different, as described in each IPHC Regulatory Area’s section on bycatch.