Stock status and reference points for sharks using data-limited methods and life history
Worldwide, many shark populations are classified as data poor, making it difficult to assess their status. However, for many sharks, their longevity, late maturation and low production of pups make them highly vulnerable to exploitation and highlight the need to assess their status. We compared reference points and stock status estimated from full stock assessments for 33 shark populations with those derived analytically, empirically or through simulation. There was excellent agreement between overfished status estimated from an assessment and determined from analytical methods using life history and an index of abundance; in 70% of cases, the analytical estimate of status was robust to assumptions of initial index depletion. We reviewed the ratio between fishing mortality at MSY (FMSY) and natural mortality (M) for chondrichthyans, from published studies and shark stock assessments. We then compared conclusions on overfishing status from the stock assessments to those derived with FMSY proxies and found very good agreement. Finally, we conducted a simulation study across representative life-history parameters and different fishery selectivity patterns to explore the resulting range of FMSY to M ratios. As a rule of thumb, FMSY should not exceed 0.20M for low productivity stocks, 0.50M for stocks of intermediate productivity and 0.80M for the most productive shark stocks when immature individuals are harvested, which is the norm in the vast majority of cases examined. A triage method is proposed that provides a roadmap for using these data-limited methods as an initial step towards assessment of stock status and sustainability of chondrichtyans.