AbstractAlthough the vast majority of fishes spend their entire lives in either fresh water or the sea, some are capable of moving across salinity gradients. “Diadromous” species migrate between fresh and salt water on a regular, well-defined basis, whereas “euryhaline” species move across salinity gradients freely and not necessarily at particular life history stages. These species are particularly vulnerable to anthropogenic developments that regulate flow and diminish connectivity between freshwater and marine biomes, such as the construction of dams and extraction of water for agricultural use. As a result, diadromous and euryhaline species are considered among the most threatened vertebrate groups in the world. Studying fish movement behaviour provides vital information on the importance of different aquatic habitats and species’ ecological roles, and improves environmental management outcomes for diadromous species.
This study aimed to determine the timing and frequency of movement across salinity gradients by three species of forktail catfish; blue salmon catfish (Neoarius graeffei), shovelnose catfish (Neoarius midgleyi) and lessor-salmon catfish (Neoarius leptaspis) in the Daly River, Northern Territory, using otolith strontium isotope ratio (87Sr/86Sr) transect analysis.
By comparing otolith 87Sr/86Sr transects with water 87Sr/86Sr mixing models, this study found that the species displayed distinct life history strategies. N. leptaspis exhibited residency in estuarine or marine water during the early life stages, with a subsequent transition into freshwater providing evidence for “marginal diadromy”. N. midgleyi exhibited residency in freshwater only and did not migrate into estuarine or marine water at any point throughout the life history. Although N. graeffei are known to commonly occur in both estuarine and freshwater, they did not appear to undertake consistent, well defined migrations between fresh and saline water during their life history, providing evidence of euryhalinity rather than diadromy for this species.
The study also revealed strong variation in water and otolith 87Sr/86Sr corresponding to wetdry seasonality in water chemistry in the Daly River. This finding emphasises the importance of undertaking extensive water sampling in the wet-dry tropics to minimise the potential to confound temporal and spatial movements in otolith 87Sr/86Sr transect analysis.
The results of this study provide important insights into the movement behaviours of forktail catfishes and highlight the importance of maintaining connectivity between estuarine and freshwater habitats, as well as between freshwater habitats, for maintaining fish populations in tropical rivers.
|Date of Award||Jun 2014|
|Supervisor||David Crook (Supervisor) & Thor Saunders (Supervisor)|