Diagnosing the dangerous demography of manta rays using life history theory.

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 r max) 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 r max: 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 r max values. Results. The maximum population growth rate r max of manta ray is most sensitive to the length of the reproductive cycle, and the median r max 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.