Future Stock Projections of Oceanic Whitetip Sharks in the Western and Central Pacific Ocean
The updated stock assessment for oceanic whitetip shark presented to the 15th WCPFC Science Committee (Tremblay-Boyer et al., 2019) showed that the stock was overfished and undergoing overfishing, but also highlighted a small reduction in stock depletion, with increases in recruitment and a reduction in fishing mortality relative to reference points under certain catch scenarios. However, since oceanic whitetip sharks are late-maturing and fishing mortality on juveniles is high, uncertainty remains as to the level of effectiveness of the non-retention measure active for the last 4 years of the assessment (CMM-2011-04 non-retention of the species, and CMM 2014-05 a ban on wiretrace or sharklines) and the impact of the CMM on the timeline for recovery. In parallel, Hutchinson and Bigelow (2019) presented new results quantifying post-release mortality for oceanic whitetip shark that were not available at the time the 2019 stock assessment was completed. The stock assessment characterized the uncertainty in the data and model parameters via a structural uncertainty grid where multiple (648) combinations of data and parameter values were used to show the range of plausible uncertainty to the inputs. This study uses a representative subset of the structural uncertainty in the assessment (108 runs) based on the updated post-release mortality values. Future projections for the 2019 WCPO oceanic whitetip stock assessment are completed to assess the impacts of recent conservation and management measures future fishing mortality on recovery timelines, using updated estimates of post-release mortality. We demonstrate the effect of a range of post assessment (2017 and on) catch trends on the estimates of population growth rate. Population projections are carried forward to estimate the mean time and probability of the population reaching thresholds of 50%, 25%, and 12.5% of current (2016) biomass levels.