Life history of rays (Batoidea), intrinsic sensitivity to fishing, and implications for conservation and management

Abstract

Rays (Superorder Batoidea) are overfished through targeted and incidental catch in industrial and small-scale fisheries. Rays are important components of healthy marine ecosystems and as sources of income and protein for global fishing communities, particularly in small-scale fisheries. The interlinked need to conserve these species and support the livelihoods dependent on fisheries is a major global challenge. This thesis aims to investigate the intrinsic sensitivity of rays to fishing mortality to inform species status and fisheries stock assessments. The thesis first investigates the biological traits and indicators of fishing exposure that best predict species extinction risk in pelagic rays (Families Myliobatidae, Aetobatidae, Rhinopteridae, and Mobulidae, and Pteroplatytrygon violacea) using ordinal regression models. The analyses revealed that species with larger geographic ranges and greater exposure to small-scale fishing pressure in tropical, coastal waters were more likely to be threatened. Thereby, highlighting the need for coordinated, transnational management action, with focus on small-scale fisheries. Next, the thesis investigates global patterns in the intrinsic sensitivity of 85 ray species using a multi-model, information-theoretic approach. It was found that tropical rays (Orders Torpediniformes, Rhinopristiformes, and Myliobatiformes) were more intrinsically sensitive (lower maximum intrinsic rate of population increase, rmax) to overfishing compared to temperate skates (Order Rajiformes). This result contrasts the expectation from metabolic theory that species in warmer waters have faster metabolism and life histories (grow faster, mature earlier, have shorter generation times, and higher rmax) and therefore are more resilient to fishing. It was found that the larger absolute offspring size of live-bearing, tropical rays likely explained the lower rmax compared to egg-laying skates with relatively small but more numerous offspring. For many ray species, the life history data needed to inform demographic and stock assessment models are lacking. To fill this data gap for two Endangered devil rays, Mobula mobular and M. thurstoni, age estimates were generated using caudal vertebrae of individuals caught in small-scale fisheries in Indonesia and Pakistan. A Bayesian approach was used to calculate key life history parameters using the resulting age-at-length dataset. The results indicated that both species have relatively low somatic and population growth rates and that calculated fishing mortalities are likely unsustainable. In summary, the thesis provides an approach for assessment of data-poor species and presents new information highlighting the nuanced complexities of species vulnerability to fishing. The results inform much needed conservation and management actions to prevent further ray species extirpation and extinction.