The amphidromous river prawn, Macrobrachium spinipes (known locally as cherabin) is a large bodied and abundant tropical prawn species occuring throughout northern Australia. The species supports a significant recreational fishery and is a critical prey species for predators a such as Lates calcarifer. We investigated the upstream migration of Macrobrachium spinipes in the Daly River, Northern Territory, to determine migration phenology, estimate migration biomass and if the migrating shrimps were transporting marine derived carbon upstream.
Monthly observations and sweep net sampling over two years revealed that the migration of juvenile M. spinipes occurred during extended periods of declining discharge during the wet season and specifically, occurred en-mass after the wet season, when stream discharge was returning to base flow. The en-mass migration occurred over a period of 4-6 weeks between March-May. This period was targeted for fine scale sampling using fyke nets. Analysis of environmental descriptors and migration biomass consistently found discharge as the strongest predictor of biomass, while moon illumination and cloud cover were also found to be significant predictors. An estimated 15-20 million shrimp migrated upstream during the period, transporting approximately 100 kg of carbon and 28 kg of nitrogen. Two different methods were trialled to determine if marine carbon was transported upstream; sulphur stable isotopes and strontium isotope ratios. No evidence of marine carbon was found through either method employed.
While we found little evidence the migration of the amphidromous prawns was transporting marine derived energy and nutrients upstream, the migration was providing millions of individuals that likely constitute an important process subsidy to the river system.