1. 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. 2. We investigated how spatial and temporal variation in environmental salinity influences physiological homoeostasis, habitat utilization 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 4-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 δ 13 C and δ 15 N. 3. We discovered that immature C. leucas and G. glyphis segregate spatially along a salinity gradient between September and December, where the mean volume of intersection between species dropped as low as 0.01. While G. glyphis used higher salinity environments (mean salinity = 19.22) located between 30 and 70 km from the mouth of the estuary, C. leucas occupied freshwater reaches (mean salinity = 1.98) between 60 and 110 km upstream. Plasma osmolarities of both species were always hyperosmotic to the environment, with electrolyte concentrations maintained within a narrow range independent of environmental osmolarities. At the onset of the wet season, both C. leucas and G. glyphis undertook a coordinated downstream migration towards the lower estuary before returning upstream. 4. Carcharhinus leucas blood tissues were more enriched in δ 15 N and δ 13 C than samples collected from similar size G. glyphis, suggesting that these species not only spatially segregate within low salinity habitats but also prey on different dietary resources. Furthermore, a loss of enriched isotopic values in fin tissue with increasing body length provides further support for marine use by adult sharks in conjunction with dependence on rivers as important nursery habitat. Our results | 2171 Functional Ecology DWYER Et al.
Understanding movement and habitat requirements of endangered species is critical to conservation management. We evaluate fine-scale acoustic telemetry to study breeding-related movement and habitat use of the endangered Mary River cod Maccullochella mariensis in a riverine environment and, in relation to key environmental variables, to inform management. Movement activity varied significantly in relation to water temperature and diel period, and spatial occupancy and habitat selection varied substantially in relation to the nesting behaviour of Mary River cod. Important nesting habitat included a large hollow log mid-channel and well-shaded logs and log jams adjacent to the river bank. Extrapolating this information to the general population was inhibited by relatively small sample sizes, due in part to the restricted spatial scale of the hydrophone arrays and longitudinal movements of tagged fish beyond the acoustic range of our array. Notwithstanding this, our results demonstrate that (1) fine-scale acoustic telemetry can quantify complex biological behaviours in riverine environments; (2) Mary River cod require specific environmental stimuli and habitat to support the reproductive cycle; and (3) changing environmental conditions may influence Mary River cod behaviour, and understanding this response is necessary for sustainable management. Findings from this study can inform future applications of this methodology in riverine environments and contribute to the development of management strategies and habitat restoration activities supporting the recovery of Mary River cod populations.
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