Scientists fathom mysteries of the river’s fish

SHOALHAVEN River’s surface belies the activity that is happening beneath.

Apart from the usual estuarine goings on, 50 electronic listening stations positioned along the length of the river are tracking the movements of 110 very special fish.

The project began in September last year, when the fish were fitted with transmitters that give a unique code to the receivers as they swim past.

The species in question were estuary perch and Australian bass.

The study hopes to shed light on the migration cues, including timing and location of spawning of both species in the Shoalhaven.

Before taking part in the study each fish is taken to the Shoalhaven Marine and Freshwater Centre at West Nowra, weighed and measured before undergoing a small operation to insert the tiny transmitter.

They are then given an external tag so if they are caught fishermen will know the fish is carrying something inside it.

Estuarine biologist with the Department of Primary Industries Chris Walsh is heading up the two-year project as part of his PhD.

This part of the study has received funding from the Recreational Fishing Trust, the pool of funds created by the purchase of fishing licences.

He said this phase of the study has reached the halfway mark and already a lot of information has been downloaded from more than one million detections by the receivers.

“We had a flood in February and we recorded significant movements down the river and then back up again after that event,” he said.

Mr Walsh said apart from learning about the fish movements and their preferred habitat, by monitoring salinity and water temperature the impact that the state of the river’s flow has on the fish could also be examined.

“That kind of data gives us an idea as to why the fish are moving to where they are.

“Already we’re seeing considerable movement as a result of the discharging from the dam as salinity levels rise and fall in different parts of the river,” he said.

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