Catchability of target and non-target species by circle hook size in the Hawaii and American Samoa tuna longline fisheries

DRAFT Models were used to compare catchability (catch rate, number of fish per 1,000 hooks) as a function of hook size for a number of retained (target and non-target) and bycatch (discarded) species in two longline fisheries. Observer data from tuna longline fisheries in Hawaii and American Samoa were used to investigate catchability for 22 species in the Hawaii fishery and 16 species for the A. Samoa fishery. Generalized linear models (GLMs) were used to estimate catchability based on circle hook sizes, with comparisons for sizes 14/0 vs 15/0, 14/0 vs 16/0 and 15/0 vs 16/0 for the Hawaii fishery, and 13/0 vs 14/0 for the A. Samoa fishery.

The results from the Hawaii fishery are more robust than the A. Samoa fishery as the Hawaii fishery monitored ~8 times (25.8 million) more hooks than the A. Samoa fishery (3.3 million hooks). In Hawaii, there was a significant increase in catchability with larger hook size for 11 of 13 retained species, including bigeye tuna (Thunnus obesus), the primary target species. There were numerous species that were not affected by hook sizes, including two bycatch shark species, oceanic whitetip shark (Carcharhinus longimanus) and silky shark (C. falciformis). Of the eight species of bycatch, catchability was higher on larger hooks only for blue sharks (Prionace glauca). There was a significant decrease in catchability between 14/0 and larger hooks for five bycatch species, including shortfin mako shark (Isurus oxyrinchus), bigeye thresher (Alopias superciliosus), and pelagic stingray (Pteroplatytrygon violacea).

In A. Samoa, there was a significant increase in catchability for the target species, albacore (T. alalunga) as well as for five of eight retained species between hook sizes 13/0 and 14/0. No catchability effects were found for three of the eight retained species. Catchability in a number of retained species had no effect with hook size. With regards to bycatch species, larger (14/0) hooks were associated with higher catchability of pelagic stingray, blue shark, oceanic whitetip shark, silky shark, as well as a lower catchability for snake mackerel (Gempylus serpens). This study provides empirical evidence to suggest that for a tuna fishery in the Pacific Ocean, adoption of a larger hook could provide increased catchability of retained species while simultaneously serving as a conservation tool by decreasing catchability of a majority of bycatch species. With the exception of higher catchability of blue shark, the primary elasmobranch species caught, larger hook size implementation could reduce overall discards.