Trout spawning and rearing habitats in the Goulburn River
Paul Brown, Marine and Freshwater Sytems, Primary industries Research Victoria, DPI
The depth and speed of water flows during the spawning and rearing periods in the lifecycle of brown and rainbow trout are crucial to determining the size of their populations in the Goulburn River.
Fisheries scientists from PIRVic Snobs Creek studied how brown and rainbow trout spawning and rearing habitats were affected by different flow rates. Funded by Fisheries Victoria, this study was designed to contribute to the review of environmental flow allocations for the Goulburn River.
The mid-Goulburn trout fishery is one of Victoria’s premier recreational fishing and tourism assets; and at the same time the Goulburn River is the main carrier of vital water supplies to the farming communities of central and northern Victoria.
Irrigation water flows in the mid-Goulburn (between Eildon and the Rubicon River) typically run from 6,000 to 10,000 megalitres (ML) per day throughout the summer months, while river flows during the non-irrigation period over winter are typically around 120-250 ML/day.
There are two crucial stages in the lifecycle of both brown and rainbow trout:
* spawning, when the adult fish bury their eggs in a gravel nest called a redd. This occurs during winter months peaking in June and July for brown trout and from July to mid-Sept for rainbow trout, and
* fry rearing, after the eggs hatch and the small larvae (alevins) emerge from the gravel and they have to establish territories and begin to feed and grow as juvenile trout (fry). This phase lasts until at least mid-summer. Changes to water flow during these periods can affect the hatching and survival rate of the eggs and of alevins and fry.
To investigate the effects of changing water flows on these life stages, Mr Paul Brown and his team of researchers, characterised the habitat available to trout in the mid-Goulburn River. These data were then fed into computer models, which allowed the effects of different water flow levels on the amount and quality of habitat available to the trout to be determined.
“The timing of water flows in the mid-Goulburn River is mainly governed by the irrigation cycles. We wanted to know what effect this water flow regime was having on the critical spawning and rearing stages.” Mr Paul Brown explains. “Once we had a handle on this, we could manipulate the models to tell us the flow conditions that would maximise trout production in the mid-Goulburn.”
The results of this modelling study suggest that both brown and rainbow trout like moderate water flows during the spawning seasons, but require periods of low flow during the fry rearing stage.
The model suggested that maximum spawning habitat for brown trout is produced with flows of 860 to 1200 ML/day repeated. River flows of 2160 ML/day produced the maximum amount of spawning habitat for rainbow trout.
Maximum fry rearing habitat for brown trout could be produced with river flows of 260 ML/day, while flows of 90 ML/day were needed to maximise fry rearing habitat for rainbow trout.
While flow rates of these magnitudes are produced in the Goulburn River – the timing of these flows does not coincide with the breeding cycles of either trout species. Peak flows occur over summer, the period when the small fry prefer a low flow environment. Winter is a time where river flow decreases, yet this is a period of spawning where the trout require higher water flows.
To measure the effect the current water flow patterns have on available trout habitat and production levels, mean Goulburn River water flow data collected from 1990 to 1999 was used. This information was then fed into the computer model.
The model suggested that the mean water flows experienced during spawning from 1990 to 1999 provided access to 75-85% of the potential maximum brown trout spawning habitat available and 88% of the potential maximum rainbow trout spawning habitat.
“These results show that there is sufficient spawning habitat in most years to allow for successful egg production for both brown and rainbow trout,” Mr Brown said.
However, the modelling also showed that the mean flow rates experienced over the past 10 years at the time of fry rearing restricted both trout species from accessing the majority of the potential rearing habitat available.
The mean water flow experienced from 1990 to 1999 during the brown trout fry-rearing season (September to February) supplied only about 18 to 25% of the maximum potential fry habitat available. While for rainbow trout only about 4% of the maximum potential fry habitat was available.
Within a few days of emerging out of the gravel, young trout fry are very aggressive and competitive and fight to hold and maintain a territory in which they will feed on small drifting animals. Fry that do not establish a territory die of starvation.
“Fry rearing habitat appears to be particularly rare when it is needed at the beginning of the irrigation season,” Paul says. “Our results indicate that fry have less – usually much less, than 14 cm2 in which to live. This does not appear to be adequate when you consider fry territories elsewhere usually range from 15 cm2 to 1 m2.”
“Given average autumn and winter flows, fry production in the mid-Goulburn is sufficient but the survival of these fry may be limited by a lack of suitable fry-habitat,” Paul says. “The results of this study suggest that the best way to increase the recruitment of juvenile brown and rainbow trout to the fishery would be to create extra fry habitat.”
“This habitat would have to function effectively at the water flows of >4000 ML/day that are experienced during September to February,” Paul says. “It is important for this habitat to be shallow (0.3 to 0.9 m) and have low velocity water.”
Consistent with the research priorities identified in the Goulburn Eildon Fisheries Management Plan, this information will now be considered by the partnership committee, which includes representatives from Fisheries Victoria, VRFish and Goulburn Murray Water.
Some experimental fry-habitat will be constructed this year by the Goulburn-Broken Catchment Management Authority, funded by a Recreational Fishing Licence grant. An assessment of the use of this artificially-enhanced fry habitat should enable us to better understand if this is a potentially useful management method for the fishery.
For more information about this project please contact Paul Brown at Marine and Freshwater Systems, PIRVic, Snobs Creek on 03 5770 8000.