Trophic ecology of euryhaline and coastal elasmobranchs in northern Australia

Abstract

Knowledge of an organism’s trophic ecology is important so that the structure, function and predications of their ecosystems can be determined. Temporal and spatial variations in the trophic niches of organisms can be quantitatively examined to determine the essential resources for survival of species and how changes in the availability of resources affect ecosystem processes.

In this study I utilised biochemical tracers within muscle and fin tissue to examine the trophic ecology of coastal and euryhaline elasmobranchs (sharks and rays) within the South Alligator River, Northern Territory, Australia. Firstly, I examined the suitability of different tissue types for profiling fatty acids (FA). Both fin and muscle tissue contained important dietary FAs, although some discrepancies between FAs in tissue types suggested muscle tissue as the preferred option for use in this study.

I then explored elasmobranch dietary connections using FAs and stable isotopes (SI) and through a comparison of putative prey species. Coastal and euryhaline elasmobranchs were distinguished by their trophic sources – river/estuary and marine – with varying degrees of trophic niche overlap among species. Putative prey species were generally found to exhibit omnivory and showed seasonal differences in their biochemical tracers. While most euryhaline elasmobranchs were generalist consumers, the different species tended to consume prey from either marine or freshwater sources.

Collectively, the analysis of biomarkers suggests that coastal and euryhaline elasmobranchs support large-scale cross-biome connectivity in tropical aquatic ecosystems. Consequently, changes in the movement and abundance of euryhaline elasmobranchs could have major implications for tropical ecosystem function and connectivity. Whilst this assemblage of elasmobranchs is relatively well protected in the World Heritage Kakadu National Park, similar assemblages elsewhere face multiple threats. It is therefore important that integrated management approaches, which work across marine-freshwater boundaries, are used to protect these high mobile and ecologically important components of tropical aquatic food webs. Show more