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

This paper presents the first 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 1971 through 2014. Data collected previous to 1971 are not considered in this analysis.
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.
Multiple data gaps relating to the true state of nature with respect to catch and abundance trends were overcome through the use of integrated stock assessment techniques and the inclusion of alternative data. Multiple models with different combinations of the input datasets and structural model hypotheses were run to assess the plausible range of stock status for blue sharks. The reference case presented here was selected based on the reliability of the data and parameterization and is characterized by the bold values in the table below; (NOTE TABLE FORMAT IS NOT CORRECT)
GROUP Variable Options Run
CPUE
1. Japan Late
2. Portugal
3. Spain
Length Composition
Sample Size weighting 1. Iteratively Re-weighted
2. 0.2
3. 1.0
Stock Recruitment
Steepness 1. 0.2
2. 0.3
3. 0.4
Sigma R (SD on the recruitment deviations) 1. 0.1
2. 0.3
3. 0.5
Catch series 1. IOTC database derived catch
2. Trade based catch
This reference case model is used as an example for presenting model diagnostics and representative trends, but does not necessarily represent the most appropriate model run to base management advice.
The sensitivity of the reference case model to key assumptions (i.e. regarding the stock recruitment relationship, the catch per unit effort time series, data weighting) were explored via sensitivity analyses.
The results of these analyses should also be considered when developing management advice. The axes of uncertainty considered are provided in the table below, reference case options are shown in bold. A full factorial grid of all options was run (this gave a total of 162 model runs), one change sensitivities to the CPUE series and catch estimates are presented in the results, with full results for any run are available on request.
Results for the assessment are compared across different assumptions with reference case parameterization. Estimates of stock status from the reference case and one change CPUE series are SBcurrent /SBMSY =1.2-1.6 and and Fcurrent /FMSY = 1.3-2.5_ though the range of uncertainty is extensive. Stock status is reported in relation to MSY based reference points however the authors 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. 2010-2013).
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 both sets of catch estimates.
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 600,000 metric tons to over 5 million metric tons.
4. Among the one change sensitivities and estimates of current stock status with respect for SBcurrent /SBMSY ranged from approximately 0.96-1.63 across the CPUE trends and the catch series which are the main axes of uncertainty considered. This implies a stock that is not likely currently overfished.
5. The stock status implied by the estimates of and Fcurrent /FMSY for the reference case and one change sensitivities across the CPUE and catch estimates series was highly divergent with the Spanish series showing almost no impact of fishing on the stock (Fcurrent /FMSY = 1.7) and while the Japanese and Portuguese show a estimates of and Fcurrent /FMSY = 3.07 and 4.63, respectively.
When considering which model(s) to use for the provision of management advice, we recommend that advice be based upon multiple model runs that consider the major axes of uncertainty.