Catch and CPUE inputs to the South Pacific blue shark stock assessment

Tremblay-Boyer L, Takeuchi Y (2016) Catch and CPUE inputs to the South Pacific blue shark stock assessment. WCPFC, Bali, Indonesia

This paper presents the rationale, methodology and key results for the three catch data scenarios and the three abundance (CPUE) time-series scenarios that were developed to inform the 2016 South Pacific blue shark stock assessment, including a brief summary of length frequency and tagging data. The catch reconstruction approach relied heavily on observer data given general under-reporting of blue shark catches across fleets.
Observer catch rates allowed the delineation of three broad regions in blue shark catches which were subsequently used to refine fleet definitions within the stock assessment: high catch rates within the tropical swordfish targeting region; very high catch rates in high latitudes in the southern WCPFC-CA, but with a strong seasonal component; and lower catch rates in the middle latitudes/eastern area.
The catch scenarios were designed to rely on different assumptions about the information contained in the observer database. The final reconstructed catch series predict blue shark catches of very different magnitudes over the 1994-2014, with the southwestern Pacific wide CPUE surface catch scenario (#3) predicting the highest catches and the blue shark-to-generic shark catch scenario (#1) predicting the lowest catches for most fleets. Only these two scenarios were used in the final stock assessment for blue shark since they bracket the predictions of the fleet-specific observer catch rates scenario (#2) for the fleets that account for the majority of the catches. Of note, a preliminary comparison of reconstructed catches to trade-based estimates highlighted that the catch estimates used in the assessment might be conservative, even for our highest catch scenario (#3).
Three CPUE scenarios were defined to cover the diversity of the data sources available across the basin-wide distribution for this species: The abundance indices produced under the three scenarios show different trends over time, with the South Pacific observer (#1) increasing slightly, the ‘nominal’ Chinese Taipei declining (#2) and the New Zealand operational series (#3) increasing after being stable in early years.
Based on this detailed analysis of the available data to support the southwestern Pacific blue shark assessment, our recommendations for future assessments are:
• To overcome challenges in the analysis of observer records collected independently across observer programs within the WCPO, we recommend the provision of expanded metadata to facilitate the merging of variables from different programs into a single harmonized database. This could be assisted by an active collaboration of member countries with SPC during the initial merging operation;
• Emphasis on improving the consistent collection of accurate observer effort during the set is needed, as this variable is essential to the estimation of accurate by-catch rates;
• Noting the importance of operational covariates like hooks-between-floats and set time to interpret CPUE trends, the provision of these data for all fisheries is encouraged;
• The continuation of ongoing efforts to expand observer coverage for longline fleets operating in the WCPO is encouraged, especially given that the collection of length data for bycatch species relies on this effort and these data can be influential in data-poor stock assessments;
• Noting the significant catch of blue sharks and other species of interest associated with the southern bluefin tuna fishery within the WCPFC-CA, increased collaboration with the concerned parties for the purpose of assessing WCPO stocks should be pursued;
• Noting the increased prevalence of regulations aimed at managing shark mortality across WCPO, future catch reconstruction should prioritize the inclusion of discard mortality scenarios;
• Tagging programs for blue shark are currently restricted to a specific part of the South Pacific blue shark range, limiting their potential to inform movement rates across the assumed range of the stock. Geographical expansion of tagging activities would assist future assessments. In addition, the bulk of the tagging information available to this assessment came from a recreational fishery, but the utility of these data was limited by uncertainty in the length information. The potential to improve these valuable data sources should be examined.
• For the North Pacific, valuable information is available on the location of key mating, parturition and nursery grounds. Gaining comparable information for the South Pacific would be invaluable in the development of targeted data collection programmes, for data analyses and potential management intervention.