Alternative assessment methods for oceanic whitetip shark

Citation

Neubauer, P., richard, Y., & Tremblay-Boyer, L. (2019). Alternative assessment methods for oceanic whitetip shark. WCPFC-SC15-2019/SA-WP-13. Report to the Western and Central Pacific Fisheries Commission Scientific Committee. Fifteenth Regular Session, 12–20 August 2019, Pohnpei, Federated States of Micronesia.

Summary

The present study evaluates potential alternative assessment methods for sharks, using oceanic whitetip shark (Carcharhinus longimanus) as a case study, allowing comparisons with the current age-structured integrated stock assessment of this species. The latter was conducted in parallel to the present study, and used Stock Synthesis 3 (SS3) software.

The most recent previous integrated assessment of oceanic whitetip shark concluded that the stock was overfished and that overfishing was continuing. To minimise ongoing fisheries impacts on this species, a non-retention measure (Conservation and Management Measure CMM 2011-04) was imposed by the Western and Central Pacific Fisheries Commission (WCPFC); however, the non-retention of oceanic whitetip shark also introduced additional uncertainty about the value of indicators such as catch-per-unit-effort (CPUE) for the monitoring of population status. In conjunction with limited data about the efficacy of measure CMM 2011-04 for limiting fishing mortality, the current stock status of this species remains uncertain.

Here, we compared three approaches in conjunction with the current integrated stock assessment of oceanic whitetip shark. These approaches were catch-only simulations, a general spatial risk assessment model, and a Bayesian dynamic surplus production model. We also illustrate the impact of different assumptions on estimates of fishing mortality (F) and risk (F/FcrashcrashlimMSM</sub). The SS3 assessment estimated slightly higher overall fishing mortality and lower productivity and stock status, and therefore provides the most pessimistic view of current fishing mortality and sustainable fishing mortality. All methods suggest that reductions of fishing mortality below likely values in the last year of the assessment (2016; about 45% total fishing mortality including haul-back, handling and post-release mortality) would substantially lower existential risks for this stock.

Based on our findings, we suggest that the Scientific Committee considers the following:

  • Inferences from different models indicate that oceanic whitetip shark continues to be overfished, and overfishing may still be occurring owing to incidental mortality from fishing, despite non-retention measure CMM 2011-04. Estimated fishing mortality rates for the last year in the assessment (2016) lead to substantial risk that the stock will not persist.
  • Spatial risk assessment methods should be employed for species with poor historical records (e.g., poor species identification), but for which recent records are judged reliable. In addition, a standardised methodology based on spatial risk assessment methodology could be employed to prioritise assessment and conservation efforts.
  • Surplus production models can provide a robust cost- and time-effective way to assess shark populations, and provide similar outputs to fully integrated stock assessments such as SS3. Therefore, they may be considered as a rapid assessment tool, either alongside or instead of fully integrated stock assessments, which could be employed for species of high priority.
  • Depletion-based catch-only simulations should be considered for constructing priors for DSPMs and to understand the amount of additional information provided by fitting the DSPM.