Demystifying Mysis

Introductions of Mysis shrimps have a mixed record. In some waters, this new source of food has harmed local fisheries, but in other locations, the trout have thrived. Our author shares his favorite patterns for fishing waters where Mysis are important forage for the fish.

by Al Ritt

THERE IS A DISTINCT MYSTIQUE concerning Mysis shrimps. This should be no surprise, because the introduction of Mysis turned several high-quality tailwater streams filled with 14- to 20-inch-long fish into “hog troughs” that routinely kick out 10-pound-plus fish. But not all the circumstances surrounding Mysis introductions are positive, nor did the expected results always materialize.

Mysis are one of a group of small shrimps known as opossum shrimps; the name comes from the brood pouches in which the females carry their fertilized eggs. The young shrimps fully develop in these pouches.

In you are interested in scientific minutiae, Mysis are of the order Mysida, family Mysidea, genus Mysis. All the members of the Mysis genus were once categorized as one species, Mysis relicta, which had a circumpolar distribution comprising the arctic regions of North America, Northern Europe, Northwest Russia, Finland, Sweden, and Eurasia. In 2005, the one species was divided into four species: Mysis relicta (Northern Europe), Mysis diluviana (United States and Canada), Mysis segerstralei (circumpolar Arctic), and Mysis salemaai (Europe and the Baltic regions). It is believed that glaciers cut these four species off from their saltwater cousins, and they evolved to survive in cold freshwater environments.

Mysis are commonly found in deep, cold water. They tend to stay below the thermocline, where light penetration is minimal. At night, they will make a vertical migration toward the surface to feed. Mysis feed primarily on zooplankton or phytoplankton. As with most shrimps, they are high in calories and commonly used as food for aquariums and commercial fish farms. Due to commercial uses and introductions by various fisheries management agencies, Mysis are now found outside their indigenous ranges.

Early Introductions

In 1949, fisheries managers introduced Mysis shrimp into British Columbia’s Kootenay Lake. This introduction was intended to boost the supply of food to the resident fish, and it initially resulted in a very productive Kokanee salmon fishery. Based on that success, Mysis were introduced into other deep, cold lakes across Canada and the Northern United States. Unexpectedly, to the surprise of management agencies and the dismay of anglers, many of these fisheries exhibited marked declines. It was determined that Kootenay Lake had uncommon upwelling currents that swept the Mysis high in the water column, leaving them available to the marauding salmon; most other lakes lacked those currents, and the shrimps remained in their deepwater hideouts. Worse, the Mysis would migrate toward the surface at night to feed on the same zooplankton and phytoplankton that composed the base of the food pyramid for sport fish; rather than providing another food source, Mysis actually competed for the same food required by juvenile salmon and trout.

When the gates of a dam are opened, everything suspended in the water flows into the river below. As Mysis populations grew, more high-calorie morsels flushed through the gates

The results from the introduction of Mysis are mixed. British Columbia’s Lake Kootenay eventually experienced a decline possibly due to Mysis shrimps, but other fisheries containing deepwater predators, such as lake trout, benefited. Most, however, saw their food chains become unbalanced due to burgeoning Mysis populations; the shrimps were unavailable to most trout and salmon, and the introduced species eroded the base of the food pyramid.

As Mysis populations grew, another unexpected result came to light in Colorado. Mysis were introduced into several reservoirs. Ruedi Reservoir, Taylor Park Reservoir, and Dillon Reservoir are three that burst onto the national fly fishing scene due to their Mysis populations. As was typical after most Mysis introductions, the introduced shrimps were not a boon to the resident fish in these man-made lakes. But each of these reservoirs have bottom-release dams. When the dam gates are opened, the released water flows from the bottom of the reservoir. Those releases of cold water created excellent tailwater fisheries. The relatively constant water temperatures led to high insect densities and were ideal for the trout. The Fryingpan River below Ruedi Reservoir, the Taylor River below Taylor Park Reservoir, and the Blue River below Dillon Reservoir became excellent year-round fisheries, and trout ranging from 12 to 18 inches long prospered.

When the gates of a dam are opened, everything suspended in the water flows into the river below. As Mysis populations grew, more high-calorie morsels flushed through the gates. The trout, formerly measured in inches, gorged on the waves of discharged shrimps and were now measured in pounds. Photos of prodigious trout, many weighing more than 10 pounds apiece, splashed the pages of national fishing magazines. These fish had extraordinarily deep bellies and small heads, and they became known as footballs. The seeds of “Mysis mania” took root, and fly fishers flocked to select tailwaters. Relatively unknown streams like the Fryingpan River were suddenly on the tip of every fly fisherman’s tongue, and they became destination fisheries.

Mysis populations eventually stabilized in those rivers. While a disproportionate number of oversized trout still exist, since Mysis populations stabilized, there is no longer a 15-pound rainbow behind every rock. Even the crowds of fly fishers thinned a bit, although fishing those waters is still referred to as “combat fishing.” The end result is fisheries that have high populations of healthy trout, more than the average number of large fish, and a good deal of angling pressure. Whether because of this pressure actually “educating” the fish, or simply the large amount of natural food in the water, these trout are sometimes difficult to tempt with a fly.

Mysis Tying Tips

A host of Mysis imitations has been developed over the years. A Mysis can grow to nearly an inch long, but most specimens are much smaller. Patterns tied for Colorado typically range from hook sizes 18 to 14, but specs might vary for other regions.

Live Mysis are nearly transparent, and their eyes might be their most noticeable feature as they drift along in the current; because of that contrast, I include prominent black eyes on all my patterns. Mysis don’t live long in streams, which are simply not their natural habitat. As Mysis die, they turn milky and then pure white. Experienced guides and anglers carry patterns that mimic both live and dead Mysis. Fishing a two-fly rig, with one transparent and one white pattern, covers both bases.

Fish a Mysis imitation with a dead drift. Real Mysis often flow near the surface of the water, but fishing deeper in the water column may be effective in eddies and other areas where the current slows. Mysis densities will be higher closest to the dam, but also fish your imitation farther downstream. Trout migrate up- and downriver, so even when they are holding in water where Mysis are not present in high numbers, they still recognize these tasty shrimps.

Although Colorado’s Mysis-rich tailwaters may not be quite the trophy fisheries they were when the first large numbers of shrimps flushed through the dams, they still contain more extra-large trout than most other rivers. Mysis remain a significant source of food for these fish, and you should carry a selection of flies to match.

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Al Ritt is an author and guide. He also works for Peak Engineering, the manufacturer of the Peak vise. Al’s 25 Best Most Versatile Flies: Their Histories, Stories, & Step-by-Step Tying Photos (Stonefly Press) is one of the very best books for new tiers we have ever read. Al lives in Colorado.

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