DRAFT - Stock assessment blue shark (Prionace glauca) in the Indian Ocean using Stock Synthesis.

This paper presents the second stock assessment of blue shark in the Indian Ocean. The assessment uses the stock assessment model and computer software known as Stock Synthesis (version 3.24f http://nft.nefsc.noaa.gov/Download.html). The blue shark assessment model is an age structured (30 years), spatially aggregated (1 region) and two sex model. The catch, effort, and size composition of catch, are grouped into 8 fisheries covering the time period from 1950 through 2015. Seven indices of abundance, all from longline fisheries, were available as well as three estimates of total catch. The estimates of catch are the reported nominal catch, catch estimates based on a generalized additive model and based ratio estimates, the later being available only from 1971 on. The data collected previous to 1971 were not considered in the previous assessment (WPEB11-28), however are included in in this of the analysis however are considered here for the two catch series extending to 1950.
Blue sharks are most often caught as bycatch in the Indian Ocean tuna fisheries, though some directed mixed species (sharks and tunas/billfish) fisheries do exist. Commercial reporting of landings has been minimal, as has information regarding the targeting and fate of blue sharks encountered in the fisheries. Useful data on catch and effort is mostly limited to recent years, a time series of historical catch has been estimated based on reported effort and observed catch rates.
This analysis was developed as an assessment model that included all seven of the submitted CPUE series, and the nominal catch series, this model named the reference case, as it is referred to in the when presenting the model parametrization and diagnostics. A grid of sensitivity runs using the individual CPUE series and the three catch estimates is used to characterize the major axes of uncertainty. These models vary in their inputs and as such the estimated stock status differs between combinations of the catch datasets and CPUE series. The Indian Ocean Tuna Commission Working Party on Ecosystems and Bycatch is invited to recommend a base case model, for the provision of stock status.
Estimates of stock status from the reference case and sensitivity runs are are SBCURRENT /SBMSY = 1.07-2.06 and FCURRENT/FMSY= 0.35-1.78. Stock status is reported in relation to MSY based reference points however please take note that the IOTC has not yet adopted reference points for sharks. Due to the inherent unreliability of recruitment estimates in the terminal year this study defines ‘current’ as the average of the first four of the last five years (i.e. 2011-2014).
The main conclusions of this assessment are:
1. The stock status is highly dependent on the CPUE series used to fit the model. Among the candidate CPUE models in this assessment no CPUE series runs the through the entire time series.
2. The estimates of catch are highly influential in the model, but mostly in terms of scale, as the current depletion and fishing mortality indicators are approximately equal across all catch estimates for a given CPUE series.
3. The scale of the assessment is influenced by the CPUE series chosen and by the catch estimates used, estimates of B0 range from approximately 700,000 metric tons to over 3 million metric tons.
When considering which model(s) to use for the provision of management advice, it is recommended that advice be based upon multiple model runs that consider the major axes of uncertainty.