Underwater video identifies individual sharks to aid population management.

Abstract

Sharks and rays (elasmobranchs) are among the most threatened taxa on the planet and play vital roles in marine ecosystems. The IUCN red list suggests that just 25% of species are considered safe. The key threat is overfishing. Multiple fish and shark stocks around the globe have collapsed due to poor management, often due to a lack of data (eg spiny dogfish in Europe). In order to effectively manage species, it is imperative that we gather basic information on demographic data such as longevity and growth rates as well as movement patterns and population size. Many fisheries management schemes have relied on tag-recapture techniques to get a handle on these metrics or are based solely on fisheries catch data. However, this is inherently difficult for many sharks simply because tagging and recapturing them is technically challenging not to mention stressful for the animals. Recent advances in technology such as baited remote underwater video (BRUV) and pattern recognition software allow scientists to repeatedly identify individual sharks. This approach relies on natural individual variation in external patterns or markings. In this issue of JFB, Lewis et al. (2020) deployed BRUVs to monitor the population size of seven gill sharks around Stewart Island. Their data suggests that 149 individuals were present in a shallow embayment and that the sharks use of this habitat varied over the seasons with higher abundance in spring and summer. 17 individuals repeatedly showed up on the cameras indicating partial residency. BRUV photo-ID capture-recapture approaches offer a non-invasive method to monitor shark populations over extended periods of time and can provide vital information about poorly known shark species. Such data will be vital for managing populations into the future where they are threatened with a variety of anthropogenic impacts such as fishing and climate change.