Tracking the movement of Black Bream in the Yarra River

Black bream were found to move up and down the Yarra River estuary regularly, with fish detected up to 18.3 km from the Yarra mouth. During the study 7 fish moved over 10 km in less than 2 days, and 13 fish moved at least 6 km in a single day. Fish were also commonly detected in a single region of the estuary for many days. Characteristics of the Yarra River estuary that may be influencing this movement include the positioning of the salt wedge and the location of suitable habitat.

Bream in the Yarra generally appeared to follow the sea-ward (or downstream) boundary of the salt wedge, with depth-integrated measures of salinity at the times of recording fish suggesting that bream were most often associated with salinities of 27 ppt (seawater is 36 ppt, and freshwater is generally below 5 ppt). When a rainfall event flushed higher salinity waters downstream, fish responded by moving to the entrance of the Yarra River estuary and into Port Phillip Bay. As the saltwater moved back upstream after the rainfall event, fish quickly moved back upstream as well.

Fish were most commonly detected in a region around 6 km upstream of the Yarra River mouth. This region of the Yarra contains many artificial structures, including pier pylons, over-hanging wharves and pontoons, which are thought to function as valuable habitat for black bream by providing shelter and food sources (e.g. mussels and barnacles living on the structures).

Only three fish were detected at the listening station placed at the mouth of the Maribyrnong River, which suggested that despite fish regularly moving along the lower reaches of the Yarra, tagged fish from the Yarra rarely moved into this alternative waterway.

One fish was recorded to have left the Yarra River immediately after tagging and to have moved to the Werribee River in December 2007. This fish was then detected three days later back at the mouth of the Yarra River. It was then not detected for a further 6 weeks, during which time it was assumed to be in Port Phillip Bay, before again being detected moving upstream from the Yarra mouth, past Pier 35, through Victoria Harbour and on to North Wharf. For 3 days the fish was detected in the Docklands area, before being sequentially recorded by listening stations at North Wharf, Pier 35 and the Yarra River mouth, suggesting that it had again left the Yarra again. These out-of-estuary movements by black bream are consistent with tagging studies in the Gippsland Lakes, where black bream move between the major rivers and Lakes throughout the year.