- Objectives
- Develop new methods to estimate trends in relative abundance of silky sharks from purse-seine observer data that are less influenced by inter-annual variability in oceanographic conditions
- Background
- Fluctuations in the index of relative abundance for juvenile silky sharks correlate with inter-annual variability in oceanographic conditions in the offshore area of the northern EPO.
- Recent fluctuations in the index are not biologically realistic, compromising the reliability of the index as a stock status indicator.
- The index based on purse-seine observer data is the only index available for management because of data deficiencies in other fisheries.
- New methods are necessary to estimate more reliable trends in relative abundance for the silky shark using purse-seine observer data.
- Relevance for management
- Improving the reliability of the purse-seine index will improve management advice for the silky shark in the EPO.
- Duration
- 9 months
- Workplan and status
- Months 1-6: develop new methods for catch-per-set standardization.
- Months 7-9: apply new methods to estimate a revised index.
- Deliverables
- Presentation for SAC-10, May 2019
- Updated date: 01 May 2019
- Progress summary for the reporting period
- Pacific-wide analysis of environmental effects on silky shark indices finalized.
- Current options for revising the silky shark indices evaluated.
- Challenges and key lessons learnt
- A recent Pacific-wide silky shark assessment was unable to fit to EPO and western Pacific indices simultaneously, even though movement was considered, which may indicate a lack of movement or assessment model mis-specification; tagging data were not available to be included in the assessment model.
- Absent better information on specific environmental processes affecting silky shark distribution and movement, current options for revising the indices are problematic and may lead to additional bias.
- A Pacific-wide tagging project, in collaboration with EPO coastal nations and the WCPFC, would produce invaluable information on movement and stock structure with which to improve indices.
- SAC-10-17 Purse-seine indicators for silky sharks in the EPO
- Lennert-Cody, et al. Fisheries Oceanography 28.
- Clarke, S.C., et al., WCPFC Scientific Committee, August 2018.
- Invited presentation, Institute of Statistical Mathematics, Tokyo, Japan, March 2019.