Summary of movements by black bream in the Gippsland Lakes

Fish use of re-established large woody debris (LWD)

Rates of visitation by black bream was found to vary between estuarine regions with- and without LWD in the Tambo and Mitchell Rivers. Fish were more likely to visit middle and upper estuarine reaches during the day, regardless of whether LWD was present. Conversely, fish were found to use the lower reaches of the estuaries more at night.

Fish were more likely to visit areas with LWD than areas without this habitat in the Mitchell River than the Tambo. Fish were also more likely to use the LWD in both rivers during the day than at night.

Fish use of the Rivers versus the Lakes

Fish have been found to move throughout the Gippsland Lakes, with some fish moving distances of up to 2600 km, at average speeds of 8.7 km.d-1 over 12 months. Fish frequently move among the major estuarine rivers (Tambo, Mitchell and Nicholson Rivers), sometimes moving up to 30 km in a day.

Fish use of the rivers, river entrances and lakes varies strongly with the time of year. Fish appear to spend more time in the lakes than rivers in late summer and early autumn, but began to use the rivers more than the lakes at the end of autumn. River use was greatest in early to mid winter then gradually decreased through spring. Fish also spent more time in some rivers than others, with use of their respective entrances peaking during transition phases when fish were moving from the rivers to the lakes and vice versa.

Time of the day was a weakly linked to movement patterns, although during the period March through May, when fish were moving from the lakes back to the rivers, fish were spending more time in the lakes at night and more time up the rivers during the day.

Movements of bream during flooding of the Gippsland Lakes

As in the previous 2 years of drought, bream continued to move widely throughout the Gippsland Lakes during the period of high rainfall and river flow.

Over the previous two years of low rainfall, black bream generally spent more time in the lakes than the rivers in late summer/early autumn. At the start of winter, bream started to move into the upper reaches of rivers such as the Tambo, Nicholson and Mitchell. Prior to the floods, bream were doing as they had in previous years – with many fish moving into the upper estuarine reaches of rivers in early June. Once the flood occurred, however, fish moved out of the rivers and started to move more widely around the Lakes. This continued for up to 8 weeks before fish started to spend most time back in the upper reaches of the estuaries.

One particularly interesting finding was the similarity in timing of fish moving downstream and exiting the Nicholson River estuary during the floods. Prior to the flood, several fish were using the middle to upper reaches of the Nicholson River. On the 19th June, fish suddenly moved to the entrance of the Nicholson River, with all fish exiting between 6 and 8 pm. The fish did not appear to be moving in a school – rather, we suspect that stimuli caused by increasing flows of freshwater were affecting all fish in a similar way, causing the mass exit within a narrow period of time.

Summary of the movements by black bream during flooding of the Gippsland Lakes