Categorising use patterns of non-marine environments by elasmobranchs and a review of their extinction risk

As the state of non-marine aquatic environments (freshwater and estuarine environments with salinities ≤ 30 ppt) continues to decline globally, there is increasing concern for elasmobranchs (sharks and rays) that use them at critical stages of their life history. Due to a range of impediments including unresolved taxonomy, lack of fisheries data, and poor public perception, our knowledge of elasmobranchs in non-marine environments has lagged behind marine species. Here, we refine previous categorisations of elasmobranchs that occur in non-marine environments by reviewing the timing and duration of freshwater (≤ 5ppt) and/or estuarine (> 5 to ≤ 30 ppt) habitat use throughout each species’ life history. We identified five categories describing elasmobranchs in non-marine environments: (1) freshwater obligates (43 spp.); (2) euryhaline generalists (10 spp.); (3) estuarine generalists (19 spp.); (4) non-marine transients; (5) non-marine vagrants. Criteria for species inclusion is provided for all categories, and species lists are presented for categories 1–3. Euryhaline and estuarine generalists had the highest number of species that are threatened with extinction on the IUCN Red List of Threatened Species (50% and 65%, respectively), and freshwater obligate species have a very high portion of Data Deficient and Not Evaluated species (77%). The refinement of non-marine elasmobranch categories will aid in our understanding of elasmobranchs that occur in non-marine environments, helping facilitate more strategic conservation and management initiatives. Research on the biology of elasmobranchs and their human interactions in non-marine environments are suggested, as this will lead to better availability of information for conservation and management.