Diagnosing the dangerous demography of manta rays using life history theory

Abstract

Background. The directed harvest and global trade in the gill plates of mantas, and devil rays, has led to increased fishing pressure and steep population declines in some locations. The slow life history, particularly of the manta rays, is cited as a key reason why such species have little capacity to withstand directed fisheries. Here, we place their life history and demography within the context of other sharks and rays.Methods. Despite the limited availability of data, we use life history theory and comparative analysis to estimate the intrinsic risk of extinction (as indexed by the maximum intrinsic rate of population increase rmax) for a typical generic manta ray using a variant of the classic Euler–Lotka demographic model. This model requires only three traits to calculate the maximum intrinsic population growth rate rmax: von Bertalanffy growth rate, annual pup production and age at maturity. To account for the uncertainty in life history parameters, we created plausible parameter ranges and propagate these uncertainties through the model to calculate a distribution of the plausible range of rmax values.Results. The maximum population growth rate rmax of manta ray is most sensitive to the length of the reproductive cycle, and the median rmax of 0.116 year−1 95th percentile [0.089–0.139] is one of the lowest known of the 106 sharks and rays for which we have comparable demographic information.Discussion. In common with other unprotected, unmanaged, high-value large-bodied sharks and rays the combination of very low population growth rates of manta rays, combined with the high value of their gill rakers and the international nature of trade, is highly likely to lead to rapid depletion and potential local extinction unless a rapid conservation management response occurs worldwide. Furthermore, we show that it is possible to derive important insights into the demography extinction risk of data-poor species using well-established life history theory.

Life History, Growth, and Reproductive Biology of Four Mobulid Species in the Bohol Sea, Philippines

“…Managing elasmobranch fisheries is restricted for many species by the lack of available data on catch, effort, landings and trade, as well as limited information on biological parameters. Demographic data are needed to assess the impacts of fisheries (Cortés, 2002;Musick and Bonfil, 2004;Dulvy et al, 2014b;Simpfendorfer and Wetherbee, 2015), but are largely lacking for most devil rays (Chondrichthyes: Mobulidae).…”

Maximum intrinsic rate of population increase in sharks, rays, and chimaeras: the importance of survival to maturity

“…We recalculate r max for 94 chondrichthyan species examined in García et al (2008) and Dulvy et al (2014) …”

“…As most of these species lack stock-recruitment relationships, survival to maturity at low population sizes has been assumed to be very high and hence set to one because they have high investment per offspring (Dulvy et al, 2014;García et al, 2008;Hutchings et al, 2012).…”

“…A commonly used metric for guiding assessments of extinction risk and setting limit reference points is the maximum intrinsic rate of population increase r max ; it reflects the productivity of depleted populations where density-dependent regulation is absent (Myers and Mertz, 1998;Myers et al, 1997). When population trajectories are lacking, r max is useful for evaluating a species' relative risk of overexploitation (Dulvy et al, 2014) as it is equivalent to the fishing mortality that will drive a species to extinction, F ext (Myers and Mertz, 1998).…”

“…This formulation was used for chondrichthyans by García et al (2008), Hutchings et al (2012), and Dulvy et al (2014), and is hereafter referred to as the "previous" equation.…”