An Indicator-based Analysis of Key Shark Species based on Data Held by SPC-OFP
Shelley Clarke, Shelton Harley, Simon Hoyle and Joel Rice.
ABSTRACT:
Longline and purse seine logsheet and observer datasets held by SPC-OFP were examined to assess the stock status of eight WCPFC key shark species. Both longline and purse seine logsheet datasets suffer from missing shark catch records and a lack of species-specific recording, therefore the indicator analysis was based on observer data only. Shark data from the observer data sets are, however, also constrained by a lack of representativeness, particularly for the North Pacific, and for the purse seine fishery by the physical practicalities of onboard sampling.
Shark status indicators in four main classes were assessed: range based on fishery interactions, catch composition, catch rates and biological indicators of fishing pressure (e.g. median size, sex ratio). For blue sharks, which dominate longline catches in most regions, declines in catch rates were observed in nominal and standardized analyses for the northern hemisphere. In the southern hemisphere catch rates declined in the nominal analysis but increased in the standardized analysis in recent years. Both significant increases and decreases in blue shark size were identified. Data for makos in the northern hemisphere were comparatively sparse, although this species is known to be commonly found there. Catch rate analysis showed different trends in different regions and no significant size trends. Oceanic whitetip sharks were once commonly caught in both longline and purse seine fisheries in tropical waters but their presence in observer samples has become increasingly rare over time. Catch rate analyses of data from both longline and purse seine fisheries showed clear, steep declines in abundance. Declining median size trends for oceanic whitetip sharks were observed in all regions and sexes in both fisheries until samples became too scarce for analysis; these trends were significant in the core habitat areas in tropical waters. Silky sharks comprise the largest proportion of the shark catch in both longline and purse seine fisheries in the western tropical WCPO. Silky shark catch rates follow an upward then downward trajectory for both longline and purse seine fisheries. Most catches in both fisheries were juveniles and within the core habitat of the western tropical WCPO significant declines in median sizes were identified for both sexes in both fisheries. The three thresher species have divergent, but not necessarily distinct distributions which, in combination with low sample sizes, produced no clear catch trends for the group. A significant decrease in median size was identified for threshers in tropical areas, most of which are expected to be bigeye threshers.
WCPFC 7th Regular Session of the Scientific Committee, 9-17 August 2011.