Standardization of blue shark catch per unit effort in the north Pacific Ocean based on longline observer data for use as an index of abundance

This report presents a Catch Per Unit Effort (CPUE) series of blue shark (Prionace glauca) taken in longline fisheries in the North Pacific, based on observer data held by the Secretariat of the Pacific Community ‐ Oceanic Fisheries Program (SPC‐OFP). This is used to develop a candidate time series of standardised CPUE for use as an index of abundance in an updated stock assessment. This specific analysis was motivated by: 1) the Scientific Committee concern that the primary index of abundance in the 2013 assessment was biased due to inadequate accounting of targeting practices documented in recent studies, and 2) the recent provision by the US of updated observer data for the Hawaiian fleet that filled data gaps in SPC holdings. Previous studies have analyzed observer data for blue shark CPUE in the North Pacific (Clarke et al. 2011a, Walsh and Teo 2012), however this is the first study to analyze the combination of the regional observer program dataset. The sections of this report include a) a summary of the exploratory data analysis of blue shark (BSH) CPUE in the north Pacific ocean, b) a brief presentation of the method used to standardize the CPUE trend for blue sharks, c) model diagnostics, and d) a discussion of the quality of the available data and the relative strengths and weaknesses of the standardization procedures. Initial data analysis based on boxplots and linear models indicated that SST, 5° latitudinal band and month were important factors in explaining the variation in observed CPUE. The standardized CPUE series for blue sharks in the north Pacific based on SPC held observer data covering the years 1993 ‐ 2009. The negative binomial approach was used to standardize longline observer data. The step plot shows that the inclusion of SST had the most impact on the trajectory.