Target changes in the tropical WCPO Japanese longline fishery, and their effects on species composition

Executive Summary This paper represents part of the response to the independent review of the 2011 bigeye tuna (BET) stock assessment, specifically the examination of changes in targeting within the Japanese longline fishery operating to the tropical western and central Pacific Ocean (WCPO). This work was undertaken at the National Institute for Far Seas Fisheries (NIFSF) in Shimizu Japan over a two week period in September 2012, using Japanese-held operational catch and effort data. The interest in targeting changes in the longline fishery is because 1) longline CPUE is the primary abundance index in the bigeye assessment, and 2) there is a stark difference in the trends in CPUE for bigeye and yellowfin tuna (YFT) through time. Changing targeting behaviour (towards bigeye and away from yellowfin) is one explanation proposed for this difference. Changing relative abundance is another. The analyses described in this paper are divided into four parts: 1) a description of spatial temporal patterns in CPUE and catch composition of the longline fishery – the patterns we hope to explain; 2) temporal changes in fishing patterns, in particular the characteristics of the fishing gear used; 3) a series of investigations using regression trees, cluster analyses, and generalized linear models to try and relate the patterns in (1) to either changes in abundance or targeting; and 4) an analysis of how individual vessels react to catch on a set, in terms of where they set next. The key findings under the four areas of investigation are as follows: • Patterns in CPUE and catch composition: o Bigeye CPUE has remained relatively stable since 1960 in the equatorial WCPO, in contrast with yellowfin CPUE which has declined significantly over the same period; and o There are strong spatial temporal patterns in catch composition with yellowfin declining in both time and space as a proportion of both YFT+BET and YFT+ALB • Fishing strategies: o Effort in the fishery has decreased considerably since a peak in the early 1980s, and the areas and seasons (quarters) fished have also declined over this period. Most of the decline has occurred to the south of the main tropical fishery, but effort has also declined in the equatorial region. o There have been large changes in the fishery through time such as an increase in the number of hooks set per day and the number of hooks between floats. Interpreting the latter is complicated by changes …