Study Probes Seasonal Variation in Longline Catch of Marker Bigeye Tuna
A key goal of NOAA Fisheries is to sustain the economic viability of fisheries while ensuring the conservation of protected species that are vital components of the marine ecosystem. To achieve this goal, NOAA researchers are working with the fishing industry and conservationists to find ways to reduce the incidental mortality of protected species accidentally caught in fishing gear. The Hawaii-based deep-set longline fleet that targets valuable bigeye tuna (Thunnus obesus) also hooks false killer whales (FKW, Pseudorca crassidens) on rare occasions; small numbers of FKW are recorded in the fishery's bycatch by scientific observers monitoring the operations on selected longline fishing trips.
Because false killer whales are much larger than bigeye tuna, weak-hook technology can be used to promote the release or escape of FKW while allowing retention of the tuna. In 2010, the FKW Take Reduction Team assembled by NOAA Fisheries recommended measures for reducing the incidental mortality and serious injury of false killer whales in Hawaii-based longline fisheries and advocated research to evaluate the use of weak hooks on FKW bycatch rates and target species catch rates. Accordingly, field trials were planned to compare the catch of bigeye tuna and bycatch of other species using normal hooks ("control" hooks) versus weaker circle hooks (experimental hooks). The experiments were conducted and results reported in 2011. Although no significant differences in the catch of bigeye tuna were found between stronger and weaker hooks, there may have been limitations to these inferences because trials were not conducted during the spring when larger bigeye tuna are available to the fishery and when loss of larger fish would have a greater economic impact on the fleet. Therefore, a study was undertaken by PIFSC scientists to evaluate the effects of season on the catch of large (≥ 100 lb) bigeye tuna (so-called 'marker' tuna) and their value to the fishery.
Records of the number, weight, and ex-vessel sales price of landed tuna reported to the State of Hawaii by Hawaii marine fish dealers (Hawaii Marine Dealer data) during the 2005-2009 period were analyzed. The data were used to describe seasonal variability in the size of bigeye tuna and the ex-vessel revenue ($US) derived from the large 'marker' fish. The study also computed potential economic impacts associated with various assumptions about reduced catches of bigeye tuna by size category of fish.
The study reached several key conclusions:
- The annual revenue of bigeye tuna sold, derived from Hawaii Marine Dealer data, averaged $38.9 million from 2005 to 2009 (range = $32.4-48.2 million).
- On average, 'marker' bigeye tuna comprised 32.8% of the total number of fish sold annually (range = 29.1-36.6%) and 63.2% of the total annual revenue (range = 58.7-66.1%).
- On average, 'marker' bigeye tuna ranged from 22.6% to 40.5% of the total number of fish sold per month and 56.6% to 68.4% of the total monthly revenue. The month of May showed the highest percentage of marker fish.
- Expected losses of annual revenue associated with various assumed reductions in catch rate, brought about by use of weaker circle hooks, were calculated. For example, if adoption of weaker circle hooks led to a 30% reduction in catch rates for large 'marker' bigeye tuna (fish ≥ 45 kg, or about 100 lb), the overall revenue from bigeye tuna would decline by ~ 19%.
- Weight distributions of bigeye caught during the experimental trials in October-December 2010 comparing standard and weaker hooks showed a higher proportion of smaller (≤ 30 kg) bigeye tuna than occurred historically based on average monthly data for 2005-2009. And the proportion of larger (≥ 60 kg) bigeye tuna during the trials with weaker hooks was below the proportion observed historically during most months.
