Using dolphins to catch tuna: Assessment of associations between pantropical spotted dolphins and yellowfin tuna hook and line fisheries in Hawai‘i Fisheries Research

In Hawaiian waters fishermen use the association between pantropical spotted dolphins (Stenella attenuata) and yellowfin tuna (Thunnus albacares) to catch tuna. Targeting fishing effort around or in spotted dolphin groups has the potential to lead to bycatch, and anecdotal reports of hooking dolphins exist. We recorded information on fishing vessels associated with spotted dolphin groups from 2008 through 2018 to inform discussions about potential bycatch. Associations occurred from Oʻahu to Hawaiʻi Island, but were most prevalent off Hawaiʻi Island, where 29.7 % of spotted dolphin groups had fishing vessels present. When fishing vessels were present, trolling through the dolphin group envelope was recorded in 91.7 % of encounters, and re-positioning through the dolphin group and dropping hook and line fishing gear at the leading edge of the group was recorded in 54.2 % of encounters (most of which also had vessels trolling through). Associations occurred over all four oceanographic seasons, with no obvious seasonal trend. Off Hawaiʻi Island, fishing vessels with spotted dolphin groups were concentrated in a narrower depth range than dolphin groups without fishing vessels present. Groups with fishing vessels were also concentrated in a smaller geographic area that corresponded to proximity to harbors and boat launches. The number of fishing vessels that associated with spotted dolphin groups off Hawaiʻi Island was estimated in the low hundreds (159, (SD = 12) for 2012; 330 (SD = 17) for 2013). Overall, our results suggest that fishing vessel associations with pantropical spotted dolphins in Hawaiian waters are widespread, occur frequently, and involve many participants, suggesting the risk of accidental hooking may be greater than perceived.