Analysis of stock status and related indicators for key shark species fo the Western Central Pacific Fisheries Commission

Rice J, Tremblay-Boyer L, Scott R, et al (2015) Analysis of stock status and related indicators for key shark species fo the Western Central Pacific Fisheries Commission. WCPFC SC11, Pohnpei, Federated States of Micronesia

In this report we present, for seven of the fourteen key shark species, information on the geographic range of catches; temporal trends in catch composition and catch rates; and key biological indicators of fishing pressure such as mean size and sex ratio. Whale sharks are assessed separately due to the unique nature of their interactions with WCPO fisheries. The analysis generally follows the framework first developed and described in the Shark Research Plan (Clarke and Harley, 2010).
This analysis provides indicative trends for silky shark, oceanic whitetip, mako shark, blue shark, whale sharks and porbeagle sharks, but more limited inferences are possible for hammerhead and thresher shark species complexes, largely due to lack of data. These species are not commonly caught in the primary fisheries in the WCPO, and are historically not well reported.
Where possible we have included observer data for 2014, but note that these data are incomplete and certain inferences from these data should be made with caution.
Species occurrence indicators show that five of the seven species are encountered across the breadth of the WCPFC region. The exceptions are hammerheads, which are distributed more patchily, and porbeagle which are restricted to the region south of 20◦S. The proportion-presence and High-CPUE indicators showed relatively steady trends for most species in all regions. However, for blue shark and oceanic whitetip sharks both indicators show declines as great as 80% in the equatorial and south Pacific regions.
Species Composition indicators reveal that shark bycatch differs substantially between longline and purse seine fishing in the WCPO. Blue sharks are the most prevalent longline caught shark, and silky shark are the second most common. There are substantial regional and depth variations. Several species are caught more frequently in deeper sets, while porbeagle form a sizeable component of the shallow sets in the regions where they occur (i.e., Regions 5 and 6). Purse seine shark bycatch has much lower species diversity and is dominated by silky sharks, which generally comprise more than 95% of the shark bycatch. Minor numbers of hammerhead and oceanic whitetip sharks also occur.
CPUE indicators indicate that blue shark CPUE is declining in both the north and south Pacific. Oceanic whitetip continue to decline throughout the tropical waters of the WCPO. Silky shark CPUE exhibits high fluctuations throughout the study period. CPUE for the thresher shark complex, and Mako sharks in the south Pacific, appear to be in decline, though the last years’ data points are based on relatively few data. Mako in the north Pacific is subject to missing data for some years; trends in CPUE appeared relatively stable between 2000 and 2010, but no inference is possible for the last 4 years. Porbeagle shark CPUE experienced a large decrease early on in the study period followed by a fluctuating but increasing CPUE trend.
Biological Indicators show that the sex ratio of sharks in longline catch is approximately equal for all species and regions, with the exception of blue shark in Region 5, which are predominantly female. The majority of the observed hammerhead, silky, thresher, oceanic whitetip and porbeagle sharks were immature. Observed blue shark were mainly immature in Regions 5 and 6, and mature in Region 2. Declines in standardised annual lengths are apparent in most regions for most species, with annual declines of 0.2-0.4 cm/yr. The largest decline was seen for the southern stock of male mako sharks, which have declined by 0.6 cm/yr. By contrast slight increases in annual standardised length were seen for northern mako sharks (both sexes)
Recommendations to the Scientific Committee Based on the indicators examined in this analysis, we invite the SC to consider the following recommendations when considering future research priorities:
• Increased observer monitoring is vital to provide improved information for all shark species to support development of stock assessments as well as gather fishery information necessary to monitor the impact of CMMs.
• Research to assess the discrepancy between shark reporting in logbooks and observer data.
• Silky shark and oceanic whitetip sharks have been declining under recent fishing pressure, and likely maintain their overfished status. While updated assessments may be warranted for both stocks, this would appear most useful for silky shark to understand how its stock status has changed in recent years in conjunction with the new CMMs.
• Stock assessments for blue sharks in the south Pacific, mako sharks, oceanic whitetip sharks (in the WCPO and Pacific wide) and silky sharks should be scheduled within the next five years. Given the marked CPUE declines for blue shark in the south Pacific this should be the highest priority.
• For currently unassessed stocks, the development of catch histories would enable more informative analyses in future. For hammerhead and thresher sharks the analysis of catch species composition would be similarly useful.
• For silky, oceanic whitetip and whale sharks, information on post release mortality rates is needed to monitor the effectiveness of existing non-retention management measures. This work requires an update to the information collected by observers with respect to shark releases; to this end an update to observer forms and data collection procedures will be required.
• A similar indicator analysis be undertaken for relevant key shark species again in 2-3 years, with a stock assessment for blue shark in the south Pacific, and another silky shark assessment conducted in the interim.
• Develop a time series of whale shark interactions and mortalities, taking into account potential changes in reporting and observer coverage.
• Examination of observer data/journals for 2014 to determine whether distinction between compliance/non-compliance with the whale shark CMM is possible.