Pacific-wide Silky Shark (Carcharhinus falciformis) Stock Status Assessment

Citation
Clarke S, Langley A, Lennert-Cody C, et al (2018) Pacific-wide Silky Shark (Carcharhinus falciformis) Stock Status Assessment. In: WCPFC Scientific Committee 14th Regular Session. WCPFC-SC14-2018/SA-WP-08, Busan, Republic of Korea, p 137
Abstract

This study to update the stock status of the silky shark is one of four Common Ocean (ABNJ) Tuna Project-supported assessments, all of which are required to be conducted on a Pacific-wide scale. Through collaboration with IATTC Secretariat scientists, this study was not only able to update the time series of data included in the previous assessment with new data (i.e. from 2010-2016), it also incorporated a long catch rate time series and large size composition datasets from the Eastern Pacific Ocean (EPO). Using the same modelling software (Stock Synthesis) and the same life history parameters, this new assessment has confirmed that a number of the CPUE indices used in both the 2013 Western and Central Pacific Ocean (WCPO) assessment, as well as in the current assessment, are closely correlated with prevailing oceanographic conditions and thus may not represent reliable indices of abundance. Previous studies of EPO catch rate indices also show correlation with oceanographic conditions, and a previous attempt to assess silky sharks in the EPO was unsuccessful in fitting the sharp decline in catch rates in the late 1990s. The present study shows that WCPO and EPO regional catch rate indices are in conflict and linking these indices in a Pacific-wide model is not able to resolve that conflict given the current level of understanding of regional stock structure and movement dynamics. These estimation issues undermine confidence in any conclusions drawn from the currently available data and preclude definitive findings on the acceptability of current stock biomass levels and fishing mortality rates. This assessment has improved our understanding of the complexity of the regional structure and the influence of oceanographic conditions on Pacific silky sharks. However, these insights also caveat previous findings which did not take these factors into account. Precautionary management actions were taken on the basis of the prior stock assessment and these appear justified: there are several indications that silky shark biomass has substantially declined and fishing mortality has considerably increased over the last two decades. This new expanded and more informed assessment model now better acknowledges the uncertainties, and also provides a basis for clear recommendations for data and methodological improvements. SC14 is invited to consider whether to: • Recognise that previous management decisions for WCPO silky sharks were based on the best available science at the time; • Endorse the results of this assessment as the currently best available science concerning the stock status of silky shark at the Pacific basin scale and at the regional scale for the WCPO; • Accept that due to various uncertainties in this Pacific-wide assessment, estimates of management quantities such as SB/SB0 and F/FMSY are unreliable and should not be used as the basis for management advice; • Acknowledge that the assessment model, though not sufficiently robust to estimate management quantities, suggests that WCPO and EPO silky shark biomass has substantially declined and fishing mortality has considerably increased over the last two decades; • Maintain the no-retention measure for WCPO silky sharks (CMM2013-08) as a precautionary approach until such time as reliable stock status advice is available; • Call for data improvement initiatives as outlined in the assessment as follows: • Ensure that observers are able to see and accurately record which sharks are caught, and to better code their condition at release; • Implement tagging programmes to improve understanding of silky shark movement dynamics and population structure, as well as post-release mortality; • Direct future shark assessments to explicitly incorporate oceanographic indices and explore their effects; • Increase longline observer coverage to avoid continuing problems with unrepresentative sampling and highly variable catch rate indices; • Collaborate with IATTC, NOAA and other partners toward a more robust Pacific-wide assessment model for silky sharks. • Recommend that this silky shark assessment be revisited under the WCPFC Shark Research Plan no later than 2021.