How a small change could potentially make a huge difference.
What is the problem?
Breeding, raising and stocking trout are key for managing the Snowy Lakes trout fisheries. However, winter water temperatures at Gaden hatchery, Jindabyne are too cold for optimal development and growth.
Warmer Waters
After observing the growth rate of the rainbow trout in naturally warmer water conditions at another hatchery. Mitch our Gaden hatchery manager had an idea. Why don't we try artificially raising the water temperature at Gaden Trout Hatchery.
It all started with an aquarium heaters, a thermometer and bucket up-wellers to raise the temperature to more optimal levels.
The Process
The warmer water temperatures (~10°C), were around 7°C warmer than the river water (~3°C) used in the hatchery.
The eggs held in warmer water developed on average of 4-6 weeks faster than eggs in the cooler river water.
Fish raised in warmer water have considerably faster growth rates than those raised in regular Thredbo River water temperatures.
Additionally, delaying fish stocking until April/ May allows them to be released at a greater size.
Pictured: Normal fish top and heated fish below
How do we monitor fish stocking success?
Coded Wire Tagging
Coded wire tagging is a technology developed by Northwest Marine, USA. It involves a small piece of wire (~1mm) being implanted in the nose of juvenile trout. This wire has a number series laser engraved on it for future identification.
The majority of rainbow trout are tagged and stocked in January/February, with heated fish plus a control non-heated group of fish tagged and released in April.
Fish tagging has been undertaken in both Lake Jindabyne and Lake Eucumbene since 2013 to understand stocking effectiveness. Typically ~10% of rainbow trout in the Thredbo River spawn run are stocked fish.
The small number of heated trout stocked in April contributed nearly equal numbers of fish to the spawn run in 2020 as the main stocking event in January.
Similarly angler catches during the Snowy Mountains trout festival in 2019 also showed proportionally high contribution of heat treated fish.
Looking Ahead
These results suggest that fish from heated water trials provide good returns and supports further trials and increasing capacity of DPI hatcheries to produce trout in suitable water temperatures.
These fish will continue to be tagged so we are able to more accurately assess their contribution to the fishery and the overall population within the Snowy Lakes.
Heated water is also being used to improve production of brown and tiger trout.
The image shows brown trout development in warm water (top) and normal cooler river water (bottom).
Larger rainbow trout are also being stocked in locations to overcome predation from redfin. Keep an eye on the DPI website and NSW trout strategy for further information on these stocking programs.