27 August 2019

Maintaining water flows in Australia’s northern rivers is essential to conserve the diversity of freshwater fishes and the health of barramundi populations, new Hub research shows.

The research on environmental flows in the NT’s Daly River, led by Associate Professor Alison King from Charles Darwin University, found that dry-season flows are necessary to maintain fish habitat and food webs in this tropical river – flows may be threatened by increasing demands for water extraction by agriculture and mining.

Researchers found that maintaining the variation in water depth and velocity along the river during the dry season is essential to retaining a diversity of fish habitats: from the deep, snag-bottomed pools favoured by larger fish to the rocky riffles favoured by small-bodied species. The river’s shallow, fast-flowing sections – most vulnerable to reductions in water levels – are critical nurseries and refuges for many smaller fish species. Many fish species in the Daly spawn all year round, not just during the wet season when water levels are highest; and some prefer the low water levels of the dry season.

And then big fish eat little fish. The abundance of barramundi in the Daly is strongly influenced by river flows and the availability of prey species. Barramundi that migrate to freshwater sections of the river grow 25% faster than those that remain in estuarine habitats, most likely owing to the greater productivity of the freshwater habitats. Maintaining year-round habitat and spawning opportunities for the small fish species eaten by barramundi is critical in sustaining healthy populations of this culturally and commercially important species.