Effective population size of the critically endangered east Australian grey nurse shark Carcharias taurus

Retaining genetic variation is central to species-level conservation, and knowledge of effective population sizes (N-e) can be used to predict rates at which genetic variation will be lost over generations. Here we used thousands of genome-wide single nucleotide polymorphisms to investigate N-e of the critically endangered grey nurse shark Carcharias taurus across 1400 km of its eastern Australian range. The eastern Australian grey nurse shark population has declined rapidly in number over the last few decades, has relatively low genetic diversity and is extremely susceptible to anthropogenic mortality. We found no evidence for any genetic structure, which is consistent with previous field observations that show widespread movement across the distribution. Estimates of the effective number of breeders (N-b) and N-e were around 400, using an approach based on proportions of siblings and another based on linkage disequilibrium. Forward simulations revealed that even if an N-e of 400 is maintained, the population will experience some loss of genetic diversity over the next 50 generations. The relatively low N-e highlights the importance of maintaining the population size of grey nurse sharks in order to retain genetic variation and therefore resilience to environmental change.