From Wyoming Game & Fish
Since 1991, the year we discovered walleye had been illegally transplanted into Lake DeSmet, we’ve been asked this question. And the answer comes down to biology and habitat.
Young walleye forage on zooplankton and insects until they are about 4-5 inches, but switch to larger forage, such as small minnows, as they grow.
Like all predators, they require more and larger forage as they mature. A 21 inch walleye can eat another fish 1/3 it’s body length (7 inch stocked trout goes down nicely). A trout is the ice crème cone of a walleye diet compared to eating a deep bodied, spiny-rayed perch. If they survive, they become apex predators, meaning that as adults they eat just about every fish around, and aside from a few lucky birds, mammals, and people, not much else preys on them.
Because of their feeding habits, a key to supporting a good walleye fishery is an abundant fish forage base. Spawning is another critical element. Walleye spawn over clean, well-washed cobblesized substrate. A large female walleye is a prolific spawner, capable of producing up to 300,000 eggs.
While Lake DeSmet has an abundance of what appears to be good walleye spawning habitat, we have not seen a lot of small walleye. Annual netting surveys help us monitor these populations. However, we remain concerned that only moderately successful reproduction at Lake DeSmet could result in a burgeoning walleye population that could in turn consume a major portion of the 150,000 5-7 inch trout that are stocked annually.
Lack of a sustainable fish forage base could lead to slow growth, smaller size and, eventually, a population dominated by stunted walleye.
Lake DeSmet is low on the scale of biological productivity with its average depth of 62 feet of cold water, steep drop-offs, scoria substrate, paucity of sediment and vegetation, and few shallow bays.
For trout however, Lake DeSmet is considered relatively productive – to a point.
The open clear water allows good light penetration, essential for producing the microscopic phytoplankton that zooplankton feed on.
In turn, zooplankton supports this trout fishery. But, a zooplankton diet limits a trout’s ability to grow much larger than 18 inches.
While DeSmet appears on the surface to be a potential walleye gold mine --- under the surface the biological story says otherwise.