Improving shark survival after capture as bycatch in longline fisheries
High-tech tags tell researchers whether sharks survive discard from longline fisheries
Pelagic shark populations are declining globally with the majority of worldwide shark deaths due to capture as bycatch in longline fisheries targeting tuna and swordfish. In U.S. longline fisheries most sharks are unwanted and discarded at sea (~98%). Whether sharks survive after capture and release is mostly unknown; however, this information is critical in determining the status of shark populations. In this study, we are working with scientists at the Pacific Islands Fisheries Science Center, the Pacific Islands Regional Observer Program and the American Samoa Observer Program to tag sharks caught incidentally in the Hawaii and American Samoa tuna target longline fisheries. Fisheries observers—who normally work to record data on catch—tagged captured sharks while fishermen cooperatively positioned sharks alongside the vessel for optimal tag placement. This study used high-tech tags that record daily data that can be downloaded after tags pop-off the animal after a programmed deployment period of 30, 60, 180, or 360 days. These tags also transmit data on the fate of individual sharks (whether they are alive, dead, were eaten, or the tag detached prematurely) and their habitat use and movements, which is really hard to study for some shark species.
Researchers tag five shark species to inform best handling practices for fishermen
We are focusing our tagging efforts on five of the sharks named as ‘key shark species’ managed by the regional fisheries management organization, Western Central Pacific Fisheries Commission. These five species: blue (Prionace glauca), bigeye thresher (Alopias superciliosus), shortfin mako (Isurus oxyrhincus), silky (Carcharhinus falciformis), and oceanic whitetip (C. longimanus) sharks are also the species that are most frequently encountered in longline fisheries. It is well known that there are some differences between the survival of these species immediately after capture due to differing resilience to physiological stress of fighting on a fishing line. Our study will expand on this species-specific understanding of shark survival at capture to short-term and long-term survival after capture. Fishermen have also been shown to have a direct impact on the survival of discarded sharks by the handling practices they use to safely remove animals from fishing gear. Our study is working with fishermen to develop best handling practices to improve shark survival based on the fate of sharks from tagging data in combination with shark condition and handling data at release from the vessel collected by fisheries observers.
Recommended best handling practices to improve shark survival after capture
The good news here is that fishermen have the ability to directly impact shark populations and improve their survival by the way they handle and release captured sharks. During this project, researchers, fisheries observers, and fishermen have cooperatively tagged > 200 sharks. Initial results suggest, shark survival is highest when animals are 1) left in the water for removal of fishing gear by either cutting the line or using a long-handled dehooking device, 2) fishers cut away most of the fishing gear leaving less than 2.5 meters or one body length of line attached, and 3) the sharks are released in good condition. This ongoing study will also determine which of these factors are most influential for survival for each species and has opened investigation into other issues surrounding shark bycatch in longline fisheries.