Regional Offices > Laramie Region > Laramie Region News > Hydro-acoustics study on kokanee salmon in Rob Roy Reservoir

Hydro-acoustics study on kokanee salmon in Rob Roy Reservoir

April 23, 2019
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Using sound to monitor kokanee populations

Laramie - The Wyoming Game and Fish Department introduced kokanee salmon into Rob Roy Reservoir in 2015 to establish a brood population from which to collect eggs to enhance our kokanee stocking program across the state.

Kokanee are a pelagic species, which means they live in the open-water areas of lakes and reservoirs. This makes them difficult to sample with standard methods and gear. Instead, biologists use a combination of hydro-acoustics and mid-water curtain nets to sample these fish.
Hydro-acoustics involves using an echo sounder to transmit energy into the water and listening for the “echo” or return of energy. Depending on the amount of energy that returns in the “echo,” different types of objects and the depth they are located can be determined. For example, the bottom of the lake will send a different echo sound than will a fish. Mid-water curtain nets are similar to gill nets but they sample a larger area of the water column. By suspending the nets at different depths, biologists can determine what species of fish are living at what depths. The information from the hydro-acoustics and netting can then be paired together to estimate the abundance of different fish species in the open-water areas of the lake.
The Aquatic Assessment Crew sampled Rob Roy Reservoir between July 30 and August 1, 2018.  During two nights of sampling, 232 Kokanee were captured in the nets. Most kokanee were collected at depths between 20 and 40 feet, and they ranged in size between 6.5 and 15 inches. Based on the hydro-acoustics survey, the population of pelagic fish in Rob Roy Reservoir is around 10,250 with over 8,000 of those fish being kokanee. Rainbow trout, cutthroat trout, brown trout and splake were also collected in the open water areas at lower abundances. These results are very exciting since kokanee are surviving to sexual maturity and growing to decent size. All in all, kokanee are now available to anglers and it may not be long before a brood source is established at Rob Roy.

- WGFD -

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