Niche partitioning between river shark species is driven by seasonal fluctuations in environmental salinity

Tropical rivers and estuaries are highly dynamic environments, where environmental conditions change dramatically over spatial and temporal scales. This creates both physiological and ecological challenges for euryhaline elasmobranchs, where fluctuations in salinity can impact not only osmoregulatory function, but also the ability to find and acquire prey.
We investigated how spatial and temporal variation in environmental salinity influences physiological homeostasis, habitat utilisation, and migration timing in two euryhaline carcharhinid sharks within a tropical river in northern Australia. Neonate and juvenile bull sharks Carcharhinus leucas and speartooth sharks Glyphis glyphis were tracked over a four-year period using implanted acoustic tags and an array of hydrophone receivers. Tissue samples were also collected from captured sharks and analysed for interspecific differences in plasma osmolarity, urea and electrolyte (Na+ and K+) concentrations, and the stable isotopes δ13C...