Development and testing of a Bayesian population model for the bigeye thresher shark, Alopias superciliosus, in an area subset of the western North Pacific

Abstract

A Bayesian population modelling tool integrating separable virtual population analysis, per‐recruit models and age‐structured demographic analysis was developed for the bigeye thresher Alopias superciliosus (Lowe) population in an area subset of the western North Pacific. The mortality rates for years 1989–2016 were estimated, various biological reference points and associated risks of decline were also estimated, and alternative harvest strategies for the stock were evaluated. Estimates of the posterior mean of fishing mortality for bigeye thresher shark suggest fishing pressure has been high in recent years (2011–2016). The estimated population growth rate (λ) (without fishing) obtained from age‐structured demographic model was relatively low (λ = 1.01 per year; 95% confidence intervals of 1.00 and 1.03 per year). Risk analyses revealed that only low levels of fishing pressure (10% of the current fishing pressure) over a wide range of ages could maintain a relatively low risk of population decline for bigeye threshers. Sensitivity testing indicated that the model is robust to prior specification. The developed framework could be used as an assessment tool to evaluate the risk of decline for other widely distributed pelagic shark species where insufficient catch and effort data are available.

Data‐limited approach to the management and conservation of the pelagic thresher shark in the Northwest Pacific

“…F cur was estimated to be lower than F max in all simulations. However, most of the SPR values based on length‐frequency data were estimated to be below 60% which is believed to be appropriate level for low‐productivity shark species (Clarke & Hoyle, 2014; Tsai et al, 2019). In addition, the probability of the current SPR% being lower than SPR40% and SPR60% was 0.42 and 1.00, respectively, indicating that the overexploitation risk is higher for female pelagic thresher sharks in the Northwest Pacific Ocean.…”

“…However, empirical evidence for density‐dependent responses to population reduction are unavailable for most sharks (Cortés, 2004, 2007) or difficult to measure (Brewster‐Geisz & Miller, 2000). Although it has been proposed that the density‐dependence effect may result in compensatory increases in fecundity and growth rate for marine fishes (Carlson & Baremore, 2003; Romine et al, 2009), these responses are likely to be smaller in shark species because of their life history characteristics (Brewster‐Geisz & Miller, 2000; Tsai et al, 2014, 2019). In regards to the life history traits of pelagic thresher sharks (slow growth, low fecundity, and late maturity), density‐dependent effects are unlikely to affect this species (Tsai et al, 2010).…”

“…Clarke and Hoyle (2014) recommended that an SPR > 40% should meet management needs, but noted that for long-lived and low-productivity shark stocks, the appropriate SPR level could be 60% or even 70%. In this study, in addition to the YPR-based BRPs, SPR60% was adopted as the precautionary management target and SPR40% as the limit reference point (Tsai et al, 2019(Tsai et al, , 2020, defining unsustainable levels of fishing. The estimations of BRPs without considering the parameter uncertainty are set as the base case.…”

Vulnerability Assessment of Pelagic Sharks in the Western North Pacific by Using an Integrated Ecological Risk Assessment

“…However, for the late-maturing species such as dusky, sandbar, shortfin mako, oceanic whitetip, bigeye thresher, and pelagic thresher shark, a reduce of catch or TAC management measure has been suggested [ 57 ]. Tsai et al [ 30 , 31 ] demonstrated that the bigeye thresher stock in the Northwest Pacific was declining in population size under current fishery condition and overfishing was likely occurring. Similar conclusion was also made for the shortfin mako shark [ 32 , 33 , 34 , 43 ] using various approaches.…”

“…Species composition in terms of weight indicated that the blue shark was the dominant species, followed by the shortfin mako and the scalloped hammerhead. Although stock assessments of some of these species such as pelagic thresher [ 28 , 29 ], bigeye thresher [ 30 , 31 ], shortfin mako [ 32 , 33 , 34 ], and smooth hammerhead [ 35 ] in the region have been conducted using demographic or per recruit models. The relative risk on the pelagic sharks from fisheries in the region has not been evaluated, thus, this study attempts to assess the vulnerabilities of 11 pelagic shark species commonly caught by the Taiwanese coastal and offshore longline fisheries in the western North Pacific.…”