Shrinking rivers threaten Australia’s super shrimps

A town of 454 people, Daly River is a remote, sparsely populated corner of Australia’s remote, sparsely populated Northern Territory. The town and its namesake river are known for one thing – the plentiful and large barramundi. But the river is also home to Macrobrachium spinipes, a giant freshwater prawn that can reach a quarter of a metre in length, which could be under threat from agriculture, mining and climate change.  

Locally known as cherabin, the prawn is the favourite bait for barramundi. But it was only in 2011 that science recognised that it is not the same species as the cherabin of Malaysia, Indonesia, and India (Macrobrachium rosenbergii).

Peter Novak, a PhD candidate at Charles Darwin University, has devoted years to studying the prawns and is rapidly filling in many of the blanks in their lifecycle. At the Joint Aquatic Sciences meeting in Portland, Oregon in late May, he revealed that they are at particular risk from climate change and human activities.

Like salmon, the river prawn lifecycle requires that the creatures spend part of their lives in the sea. But in this case it’s not the adults that migrate but the larvae. They are washed out to sea where they metamorphose into juveniles, and then migrate back upriver to mature and later reproduce. The larvae reach the sea – up to 400 km away – by floating with the river current. In lab tests, Novak found that the prawn’s larvae have to reach saltwater within five to seven days of hatching. “By the ninth day virtually all of those in freshwater had died,” he says.

Dams could have a disastrous effect on the giant prawn.
Climate change could also reduce stream flows.

How sensitive, he wondered, would the prawns be to changes in river flows?

Using data from gauging stations along the river, Novak and his colleagues calculated how long the river remained high enough to whisk the larvae out to sea. At a station 178 km from the sea, for example, he calculated that larvae hatching at random intervals would encounter strong enough flows to make it to the sea about 60% of the time. But at the uppermost station, 405 km from the sea, flows were strong enough for successful trips only 13% of the time. In other words, for the upper reaches of the river, “any changes that slow the drift of larvae downstream are going to have a large impact”, Novak says.

And that’s not just a theoretical concern. Until now, the Daly has flowed unrestricted. But with a government review under way to increase agriculture in the Northern Territory, Novak warns that resultant dams could have a disastrous effect on the giant prawn. Climate change could also reduce stream flows.

The collapse of the giant prawn population would mean more than the loss of barramundi bait. By feeding on leaf litter and algae, the prawns recycle carbon, nitrogen and phosphorous for other animals, says Todd Crowl, a conservation biologist at Utah State University.

Crowl predicts widespread ramifications if the giants no longer roam the upper reaches of the Daly.

“These creatures play a central role in the river ecosystems, releasing nutrients for fish and birds.”