Feb 142014
 

Original story by Sharon Kennedy and Ron Tait,  ABC south west WA

The death of thousands of fish has prompted Murdoch University researchers map the movement of fish in one of the most important waterways in the South West.
Dead fish in the estuary in April last year. Photo: Roxanne Taylor - ABC News

Dead fish in the estuary in April last year. Photo: Roxanne Taylor – ABC News

The research was initiated following fish deaths in the Vasse Wonnerup estuary in April last year.

Over the years, there have been several large fish kills, with the most recent happening just weeks ago.

Dr James Tweedley is part of the Murdoch Fish Health Unit. Community concerns following the deaths focussed on the use of the system’s flood gates, he says.

Understanding the fish movement could then lead to better use of the gates.

“Just because you open the door, doesn’t mean the fish are going to walk through it.”

The researchers will focus on two species, sea mullet and black bream.

“The Vasse is an interesting system because traditionally estuaries are fresh at one end and salty at the other.

“The Vasse switches between the two. The upper becomes very salty at some point of the year and the bit down the bottom by the bar is actually the fresher, even though it’s at full strength sea water.

“We’re really interested to see how fish naturally cope with this variation.

“That’s why we’re tagging them for a year so we get the seasonal changes in salinity.

“If a low oxygen event occurs, we’ll be able to see how fish move into and away from areas.

The species under study use the estuary in very different ways, says Dr Tweedley.

The black bream are born and die in the estuary while the sea mullet spawn in the ocean before swimming into the sheltered, food rich environment as juveniles.

“Once mature, they move back into the ocean to spawn.”

There are several reasons why mass fish kills can happen, says Dr Tweedley.

“In the Swan (River) we have phytotoxic blooms that break down the gills so the fish can’t breathe.”

“Fish will try to escape differently in high salinity waters or low oxygen waters.”

Community consultation has highlighted concerns about the use of the flood gates in the Vasse.

Dr Tweedley argues that understanding fish movement may underpin changes in their use.

The gates, he says, are manmade flood prevention and stop salt water moving up into the system.

“We don’t get that (flooding) in summer. Providing we can still maintain the value of the ecosystem for birds, we could use them (the flood mitigation) to better protect fish.

“Our research is the first step…we don’t know how fish interact with the gate.”

The researchers will use two types of tagging to follow the fish in the estuary, explains Dr Tweedley.

“The first is an acoustic tag. We catch, anaesthetise and then we perform surgery.”

A very small lozenge is inserted into the body cavity and the fish are released.

Acoustic listening stations throughout the estuary will log a tagged fish each time it swims past.

The buoys will be deployed for 380 days of monitoring and will operate 24 hours a day.

As well, the Murdoch team hope to involve recreational fishers in other aspects of the project.

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