Stock assessment of Blue Shark in the North Pacific Ocean using Stock Synthesis

Citation
Rice J, Harley S, Kai M (2014) Stock assessment of Blue Shark in the North Pacific Ocean using Stock Synthesis. In: WCPFC Scientific Committee 10th Regular Session. WCPFC‐SC10‐2014/ SA‐WP‐08, Majuro, Republic of the Marshall Islands, p 83
Abstract

This paper presents an updated age‐based statistical catch‐at‐length stock assessment of blue shark in the North Pacific Ocean (NPO). The assessment uses the stock assessment model and computer software known as Stock Synthesis (version 3.24F http://nft.nefsc.noaa.gov/Download.html).
This is one of the two stock assessment approaches being applied to blue sharks in the NPO. The ISC Shark Working Group (WG) has agreed to use both a Bayesian Surplus Production (BSP) model and the age‐based statistical catch‐at‐length stock assessment, presented here, to examine the status of this stock. This paper should be read with the full assessment report of the WG which provides greater details of the data sources and how they were derived as well as pertinent summaries of biological knowledge and the papers describing each CPUE series. The updated assessment represents the efforts of the WG to address concerns raised by the Western and Central Pacific Fisheries Commission (WCPFC) Scientific Committee (SC). Generally speaking the major concerns were:
• Both assessments: the key Japanese late CPUE series could be biased due to inadequate accounting of targeting practices;
• Stock Synthesis model: 1) the basis for the weighting applied to the length frequency data; and 2) why the model had the greater level of initial depletion compared to the BSP.
The main differences between the present assessment and that presented to SC9 are 1) the inclusion of revised CPUE series; 2) changing the time period of the model to 1971‐2012 to utilize earlier catch estimates and more recent indices of abundance; 3) more structured examination of exploitation levels prior to the start time of the model; and 4) sex‐specific estimates of natural mortality‐at‐age based on growth studies from the north Pacific.
The primary reasons to use Stock Synthesis were to a) explicitly model the different sizes of blue sharks taken in each fishery; b) utilize the Low Fecundity Spawner Recruitment relationship (LFSR) functionality; c) incorporate the strong sex‐specific patterns that are seen in many of the biological and fishery data sets; and d) provide an alternative approach that could be compared to the production modelling.
This is an integrated stock assessment using estimated catch, several standardized catch per unit effort time series, catch‐at‐length, and published life history information. The model is age (30 years) structured; spatially aggregated (1 region); and sex specific. The catch, effort and size composition of catch are grouped into 18 fisheries from 1971 to 2012. The fisheries within in the assessment cover a range of fleets, bycatch and target fisheries and both longline and gillnet gears…