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.

Percentage contributions of marker fish (fish ≥ 100 lb) to the total number of bigeye tuna caught by the Hawaii-based 
               deep-set longline fishery during 2005-2009, and the associated revenue.  (A) annual percentage of catch number; (B) annual 
               percentage of catch revenue; (C) average monthly percentage of catch number; and (D) average monthly percentage of catch 
               revenue.  Source: Hawaii Marine Dealer data.
Percentage contributions of marker fish (fish ≥ 100 lb) to the total number of bigeye tuna caught by the Hawaii-based deep-set longline fishery during 2005-2009, and the associated revenue. (A) annual percentage of catch number; (B) annual percentage of catch revenue; (C) average monthly percentage of catch number; and (D) average monthly percentage of catch revenue. Source: Hawaii Marine Dealer data.

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: