Resembling a scud or crayfish more than a true shrimp, Mysis (phylum arthropoda and subphylum crustacea) are seldom more than an inch in length. They do, however, superficially resemble tiny shrimp and are commonly called opossum shrimp.

Mysis shrimp

Mysis naturally inhabit subpolar regions around the world such as Russia, northern Canada, and Scandanavian countries and were first used as a fisheries management tool in 1949. At that time, they were stocked in British Columbia's Kootenay Lake to enhance the forage base for the lake's Kokanee salmon. During the next few years, the stocking seemed a success, and biologists all over the West began to plant Mysis as forage for various lake and reservoir species, especially deep-water species such as Kokanee and lake trout.

While the really big Mysis-fed trout (trout over 15 pounds) are all rainbows, large brown trout (above) are common on all three rivers, there are world-record brook trout on the Frying Pan, and 5-pound cutts have been caught on the Taylor.

Decades later, biologists in British Columbia and elsewhere noted that the results of the original stockings were not so positive. Mysis feed on a number of zooplankton, and rather than enhance the food supply for Kokanee and lake trout, the shrimp instead upset the fundamental equilibrium of a lake's environment by devouring the zooplankton that small Kokanee and lake trout depend on for food. Because of the shrimps' aversion to light and warmer water, they migrate daily from deep water to shallow water at night; this is opposite to the behavior of the Kokanee, which were supposed to be able to eat the Mysis. So drastic were the impacts to lakes' ecosystems that the American Society of Limnology and Oceanography in 1981 drafted a resolution calling for a moratorium on further introduction of M. relicta. Still, the stage had been set.

By 1987, according to aquatic experts Pat Martinez and Eric Bergersen (See North American Journal of Fisheries Management, Vol. 9, No. 1, Winter 1989) 134 lakes and reservoirs in nine Western states and British Columbia and Alberta contained Mysis populations. Fifty-three lakes had well-established populations, which became an unexpected gold mine for some Rocky Mountain fly fishers.