Lake Trout – Kokanee Interactions and Their Implications for Fisheries Management of New Fork Lakes, WY.
Kokanee in New Fork Lakes
Wyoming Game and Fish Department (WGFD) uses New Fork Lakes as a source of Kokanee Salmon broodstock for their hatchery system. Every fall, Kokanee leave the lakes and run up the New Fork River to spawn. There, they are captured by the WGFD and their eggs are harvested and fertilized. The WGFD then brings the developing embryos to a hatchery where they are reared throughout the winter, and then distributed throughout the state the following summer.
Why are we conducting this research?
WGFD has had concern about the Kokanee population in recent years because the number of spawning fish returning to the river from the lake has declined, making it difficult to reach hatchery targets. My master’s degree project aims to determine why the Kokanee population is in decline. We started with two hypotheses of what might be occurring. The first is that Lake Trout are heavily preying on Kokanee, thus reducing Kokanee populations. The second is that interest in angling for Kokanee has increased across Wyoming in recent years, and current stocking rates on New Fork Lakes might not be keeping up with angler demand.
By determining the cause of Kokanee declines, WGFD will be able to better manage the population in New Fork Lakes. This will help ensure the future of Kokanee stocking throughout the state, as well as the ability to offer positive angler experiences at New Fork Lakes.
WGFD is conducting creel surveys—interviews designed to gather scientific data about fisheries—to evaluate the second hypothesis. Anglers fishing on New Fork Lakes may be interviewed about their fishing experience. Interviews will include questions like: What fish species are you targeting today? How many fish have you harvested? How many days do you expect to fish at New Fork Lakes this season? These questions will allow us to estimate how many Kokanee are harvested throughout the year. We can then compare these estimates to current stocking rates for the lakes to investigate whether current stocking rates meet angler demand for Kokanee fishing.
My research primarily focuses on investigating the first hypothesis—are Lake Trout heavily preying on Kokanee and driving population declines? I am looking at the diets of Lake Trout and their growth rates to determine how many Kokanee they are likely consuming. To do this, we first capture Lake Trout using gill nets. We have had very good luck with gill nets in New Fork Lakes so far, with minimal bycatch of other fish species and just enough Lake Trout to collect the samples we need. We occasionally catch Mountain Whitefish, Rainbow Trout, and Kokanee, and these fish are used to help complete our understanding of the food web in New Fork Lakes.
We bring the fish captured in our nets back to the University of Wyoming for analysis. We dissect out their stomachs, muscle tissue, liver tissue, and otoliths (ear stones). These samples and what we do with them are explained below.
Stomachs: We remove the whole stomach from Lake Trout. We then cut open the stomach and identify all the prey present inside. Small Lake Trout tend to have lots of zooplankton, specifically Daphnia species, as well as Chironomids, small invertebrates that live on the lake bottom until they emerge from the water as midges. Large Lake Trout transition to eating fish, and we see Mottled Sculpin, Rainbow Trout, and Kokanee in the stomachs.
Muscle and Liver Tissue: We take a small piece of muscle and liver tissue to run a chemical analysis called stable isotope analysis. Fish follow the rule that “they are what they eat.” Each prey type that a Lake Trout eats has a unique chemical “fingerprint.” By running this chemical analysis, we can see what prey the Lake Trout has been eating in the last 3—6 months. The stomachs only tell us what the fish has been eating in the last day or so, so these chemical fingerprints give us a better understanding of what the fish has been eating over a longer time frame.
Otoliths: The otoliths, or ear stones, of a fish are bony structures in the head of the fish. As the fish grows, it lays down layers on the otolith in a manner very similar to tree rings. When the fish is growing very fast it lays down a wide layer, and when it is growing slowly it lays down a narrow layer. This growth pattern results in annual rings on the otolith. By counting these rings, we can determine the age of the fish, and by measuring the distance between rings we can determine the relative amount of growth that fish underwent during a particular year (for example, if a ring takes up 25% of the otolith and the fish is 20 inches long, the fish grew 5 inches during that year).
Once we have data from all these sources, we incorporate them all into a model. The model uses the short- and long-term diet data to figure out what a fish’s most likely prey has been. It then correlates that prey, and the energy content (calories) it contains, and figures out how many of that prey item would have to be eaten to achieve the amount of growth noted in the otolith. By doing this, we can expand the relatively short window of data we have about a Lake Trout’s diet to a much longer time-period. Modeling allows us to estimate the total impact Lake Trout have on the Kokanee population through predation.