Analyses of Japanese Longline Operational Catch and Effort for Bigeye Tuna in the Wcpo

Analyses of operational-level, longline catch and effort data for bigeye tuna in the Western and Central Pacific Ocean were carried out under an agreement between the Secretariat of the Pacific Community and the Japan National Research Institute of Far Seas Fisheries. The objectives of the collaboration were to standardize bigeye CPUE, and to estimate the historical trend of longline catchability using a finer scale of data than had been previously available. The goal was to better understand the role of vessel effects, in particular the role of fleet composition and fishing behaviour, which are otherwise concealed when using aggregated catch and effort data. In this analysis vessel effects were estimated as the average effect over the fleet of factors such as engine type, vessel speed, well capacity, fishing techniques, targeting strategies, technology and crew effects during the modelled period. The data used in this analysis consisted of Japanese longline logsheets from 1976-2009 with location set to the nearest 1 degree of latitude and longitude; depth of set represented by the variable " hooks between floats " (HBF); international call sign used as the vessel identifier; and fishing categories (offshore or distant water), target (swordfish, shark, other), line type and a number of other operational variables included. All sets south of 35˚S and with HBF < 5 were removed to avoid southern bluefin tuna and swordfish targeted effort, respectively. However, sets targeting albacore or yellowfin rather than bigeye tuna could not be easily distinguished. Delta lognormal and offset lognormal models were applied in the standardization with an explicit term for vessel effect. The western and central Pacific was divided into six areas and an indicative regional scaling factor was estimated for each. The results of the analysis revealed many new and interesting perspectives on catch trends. Region 3 (western Pacific between 20˚N and 10˚S) proved difficult to characterize due to the complexities of separating yellowfin targeted from bigeye targeting operations. The analysis suggests that the trends in catch rate in this region may be affected by market factors as well as abundance. Contrary to expectations, bigeye catch rates were higher at shallower depth in the equatorial area. Also it is suspected that differences in trends between region 3 as a whole and its equatorial regions are likely to result from a combination of changes in the fleet, changes in fishing methods by individual vessels, and changes in the concentration of fishing …