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
|Release date||Length (mm)||Weight (g)||Release Location||Summary of movements|
|8/04/2007||395||1363||Nicholson||Moved from Nicholson to Mitchell after tagging. Spent some time around Mitchell and Nicholson entrances, before moving back into Nicholson River in early July. Has been moving up and down Nicholson river since. This fish can be tracked for 2-3 years.|
|8/04/2007||235||268||Nicholson||Moved out of Nicholson briefly. Moved up and down Nicholson often, but spent most time in middle reaches. Departed Mitchell river at 7.30 pm on 19 June. Over next few months, moved between Mitchell entrance and Paynesville, before last detection around Raymond island.|
|8/04/2007||230||240||Nicholson||Moved out of Nicholson in the first few weeks after tagging. Then appeared to move into Mitchell. Turned up at entrance to Nicholson on 21 June. Through July and August detected moving between Mitchell entrance and Paynesville. Last detected moving past Raymond Island.|
|8/04/2007||195||150||Nicholson||Stayed in Nicholson until 8 pm on 19 June. Stayed around entrance and Jones bay until 24 June, when returned up to middle of Nicholson. Back to entrance on 28 June, then on to Mitchell entrance.|
|8/04/2007||220||241||Nicholson||Moved out of Nicholson briefly after tagging, then back to middle/upper reaches. Departed Nicholson 7.45 pm on 19 June. Arrived Metung 20 June at 9 pm. Moved back into Tambo river on 18 July. Stayed mainly around Johnsonville until early August, when returned to the Nicholson.|
|8/04/2007||190||128||Nicholson||Moved out of the Nicholson, probably into the Mitchell. Recorded intermittently around Nicholson entrance.|
|8/04/2007||235||280||Nicholson||Moved to Mitchell after tagging then back to middle reaches of Nicholson by mid May. Departed Nicholson at 6 pm on 19 June and moved to Tambo entrance via Mitchell by 29 June. Moved from Tambo entrance to Metung between 15 and 18 July. Back to Tambo entrance in 8 hours on 19 July. Then from Tambo to Mitchell on 26 July and then to Paynesville in 13 minutes. Last detected south side of Raymond Island.|
|8/04/2007||200||220||Nicholson||Moved straight to Nicholson entrance. Then returned to middle reaches of Nicholson by 8 May. Moved back to Nicholson entrance a few times, always returning to middle reaches. Moved out of the Nicholson at the end of May. Next detected at the Mitchell entrance end of June. Moved to Tambo entrance and then to Metung by early July. Went from Metung to Tambo entrance to Paynesville in 20 hours on 19 July. Then moved through to Raymond island.|
|8/04/2007||220||230||Nicholson||Moved briefly out of Nicholson after being tagged, then returned to middle reaches. Stayed in area until 19 June. Exited system at 6.30 pm on 19 June. Stayed around entrance to Jones bay before moving to Paynesville on 24 July. Then moved to Rotamah Island by 27 July. Back to Jones Bay by 8 August, then back to middle reaches of Nicholson.|
|8/04/2007||205||188||Nicholson||Moved briefly to Mitchell after tagging and then back to middle of Nicholson. Departed middle of Nicholson at 6 pm and exited system at 11pm on 19 June. Moved around entrance of Jones Bay until 13 July, when it returned to middle of Nicholson. Stayed in this region for next 3 months.|
|8/04/2007||185||149||Nicholson||Moved out of Nicholson after tagging. Moved to entrance of Mitchell and was detected in this region for a few weeks. Then moved to Tambo entrance in early July and then to Metung by 6 July. Stayed in this area for the next 3 months.|
|8/04/2007||225||235||Nicholson||Moved to Nicholson entrance after tagging. Stayed here for around 3 weeks before returning to middle reaches of Nicholson. Moved from middle of Nicholson to entrance at 8 pm on 19 June. Then returned to middle reaches of Nicholson by 24 June. Moved up and down Nicholson a couple of times before staying around the middle reaches.|
|8/04/2007||180||120||Nicholson||Moved directly to Tambo river and up to higher estuarine reaches. Moved back to Johnsonville by 25 June, where it stayed until 8 August. Then moved back into Nicholson briefly before returning to Johnsonville on the Tambo.|
|8/04/2007||260||380||Nicholson||Stayed in Nicholson until 1 July. Moved out to Mitchell briefly then back in to middle reaches of Nicholson by 4 July.|
|8/04/2007||270||440||Nicholson||Departed Nicholson and moved to Mitchell. Then moved back to Nicholson and up to middle reaches on 24 June. Moved up and down Nicholson a couple of times before departing for Mitchell. Last detected in Jones Bay 14 August. Detected in middle reaches of Nicholson since.|
|8/04/2007||260||390||Nicholson||Visited Mitchell briefly after being tagged, then stayed in middle Nicholson until 19 June. Departed Nicholson at 8pm. Moved to Tambo and pretty much straight to Johnsonville. Stayed in this area, moving to the Tambo entrance a couple of times until 11 July, when it departed Tambo.|
|8/04/2007||250||340||Nicholson||Stayed in middle of Nicholson until 19 June, then moved straight to entrance in 1.5 hours, then to Mitchell entrance and then to Paynesville - arriving 28 June. Moved back to entrance of Nicholson and then onto Tambo by 21 July. Moved directly to Johnsonville, where it remained until end of August, at which time returned to middle of Nicholson.|
|8/04/2007||275||425||Nicholson||Stayed around middle reach of Nicholson until 20 June. Moved briefly to Mitchell entrance then back to Nicholson entrance. Moved back up to middle of Nicholson by 29 June. Then moved down to Paynesville by 3 July, then back to Mitchell, then back to middle of Nicholson by 10 July|
|8/04/2007||260||390||Nicholson||Last detected moving out of Nicholson 15 May.|
|8/04/2007||235||289||Nicholson||Moved out of the Nicholson and into Mitchell entrance. Returned to Nicholson and mostly detected around middle reach. Between 19 and 27 June used middle reaches, then departed system on 27 June after rapid movement to entrance in 1 hr. Detected around entrance until 10 July, when returned to middle reaches|
|8/04/2007||255||366||Nicholson||Moved up and down Nicholson until 19 June - then moved directly to Tambo and straight to Swan reach - stayed in this area for a while then started to move up and down the Tambo river.|