DOI: 10.5061/dryad.prr4xgxjd
title: Data from: Niche partitioning between river shark species is driven by seasonal fluctuations in environmental salinity

description: 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 and δ15N. 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. C. leucas
 blood tissues were more enriched in δ15N and δ13C 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 illustrate the importance of
 long-term integrative studies into resource competition and the
 significance of discrete brackish habitats for threatened euryhaline
 elasmobranchs. 
description: Tracking dataset contains raw detection records for 22
 <em>Carcharhinus leucas</em> and 22
 <em>Glyphis glyphis</em> collected on VEMCO VR2-W and VR2-AR
 units deployed in the Wenlock River, the Ducie River, and Port Musgrave.
 Data was collated using the VEMCO VUE software, and merged with associated
 longitude and latitude coordinates and adopted middle thread distance from
 Cullen Point (AMTD).  Stable isotope dataset includes
 tissue  δ<sup>13</sup>C and δ<sup>15</sup>N
 measurements, with associated tissue type, shark body length measurement
 and capture month. Blood osmolarity, urea, and
 electrolyte (Na<sup>+ </sup>and K<sup>+</sup>)
 concentrations from 18 captured <i>C. leucas</i> and 10
 <i>G. glyphis.</i>