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

Abstract

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.

Papua New Guinea: A Potential Refuge for Threatened Indo–Pacific River Sharks and Sawfishes

“…Despite extensive surveys in upstream freshwater environments during this study, neither species occurred far from the estuary. Similar habitat use patterns have been observed for G. glyphis (Dwyer et al, 2020) and G. garricki (Pillans et al, 2009) in northern Australia, suggesting that neither species penetrates far into freshwater reaches of rivers like other euryhaline species do (e.g., C. leucas, Dwyer et al, 2020). Meanwhile, the absence of larger size classes is likely because fishers in the Gulf of Papua remain within rivers and delta environments.…”

Environmental Drivers of Fine-Scale Predator and Prey Spatial Dynamics in Sydney Harbour, Australia, and Adjacent Coastal Waters

“…Conceptual models that describe spatial segregation in elasmobranch species are evolving rapidly. In tropical Australian estuaries, differences in nursery habitat use between juvenile bull and speartooth (Glyphis glyphys) sharks have been related to distinct diets, leading to the two species occupying separate core areas within the same estuary (Dwyer et al 2020).…”

Spatial and Ontogenetic Patterns in the Trophic Ecology of Juvenile Bull Sharks (Carcharhinus leucas) From the Northwest Gulf of Mexico

“…Given the similarity in trophic positions implied by similar δ 15 N values, slightly diminished values of δ 13 C in bull sharks may indicate this species exploits 13 C depleted freshwater/brackish food webs to a greater extent than blacktips. This is supported by direct comparisons of bull and blacktip shark habitat use (Matich et al, 2020), and may exist as a form of niche partitioning in which bull sharks exploit their tolerance for low salinities to mitigate potential competition with blacktips or other stenohaline piscivores (Dwyer et al, 2020).…”