Mysis: An Overview
“In history, good intentions do not always make good consequences.” - Alija Izetbegovic
In the late 1930s, scientists’ intentions to provide native fish with a food source resulted in the study of Mysis as a desirable addition to various bodies of water. Beginning with scientists in British Columbia, suggestions of mysid “transplants” grew in popularity. Initial efforts resulted in native fish growing larger and more populous, encouraging further Mysis introductions to lakes around the world. However, as time went on, unintended consequences began rising to the surface.
First Introduction
The first introduction of Mysis to a U.S. body of water occurred in 1957, when officials brought the species to Twin Lakes, Colorado from its native habitat of Clearwater Lake, Minnesota. As lake trout were known to feed on Mysis, scientists reasoned that the introduction of the microcrustaceans would bolster populations and help the commercial fishing industry thrive.
After becoming established in their new ecosystems, there were initial signs that the Mysis were benefitting the native trout. In some instances, the trout grew larger, and in others, kokanee (a type of salmon) exploded in size after feeding on Mysis, with the world record for [kokanee] broken in 1975 after the introduction of the species shortly before.
Issues Arise
The early success enjoyed by these fisheries was short-lived, as biologists discovered that Mysis were unavailable for trout and kokanee due to their habit of coming to the surface at night to feed, while the fish fed during the day. As opportunistic omnivores, Mysis strained the water for available food sources, which included the zooplankton that kokanee also relied on, effectively turning the microcrustaceans into competition for the fish they were introduced to feed.
A 1980 moratorium on Mysis transplants soon followed, granting scientists and fisheries managers time to reflect on the far-reaching effects of the somewhat hasty introductions of Mysis to their local lakes. What soon became apparent was that the initial success of the Mysis in Kootenay Lake was inaccurately attributed to a Mysis transplant, when it actually resulted from a “unique upswelling that made mysids available to kokanee”.
In lakes where Mysis populations flourished, native fish populations declined. For example, in 1985, Flathead Lake sportfishers caught 100,000 pounds of kokanee; in 1987, when Mysis peaked, fishers caught zero pounds. A 1993 Federal report on invasive species explicitly listed Mysis and its potentially far-reaching, negative effects:
Recent introduction of the opossum shrimp (Mysis relicta) into the Flathead River-Lake ecosystem of Glacier National Park caused populations of many other animals to drop. Because of feeding by the shrimp, zooplankton became less numerous. This decline, in turn, contributed to a drop in forage fish, ultimately driving away the area’s fish predators — including eagles, otters, coyotes and bears.
What are Mysis?
Mysis, also known as opossum* shrimp, are tiny, translucent microcrustaceans that measure less than an inch long at full size. Though colloquially referred to as such, they aren’t true shrimp**: Mysis have a thoracic pouch and don’t have a free-swimming larval phase like shrimp.
Katie Senft/UC Davis Tahoe Environmental Research CenterFifteen species exist, and the crustaceans are found in both fresh and brackish water, including the:
Baltic Sea
Caspian Sea
Great Lakes of North America
Lake Sommen in Sweden
Mysis have a lifespan of one to two years, and the crustaceans reproduce at a rapid rate. Female Mysis can reproduce once or twice in their short lifespan; they can also carry up to 40 eggs in their marsupium or brood pouch. Mysis hatchlings remain in the marsupium for one to three months.
Mysis are opportunistic omnivores, whose diet varies based on location:
“Mysis is a truly omnivorous filter feeder. Algae, detritus, and other zooplankton are all consumed. Diet may vary from lake to lake, season to season, and year to year depending on availability of food items and abiotic factors. This helps to explain some of the unexpected problems that have resulted from Mysis introductions in British Columbia and elsewhere. For example, in some instances cladocerans such as Bosmina and Daphnia are a major food source. In these instances, Mysis competes directly with planktivorous fish and larval fish for food resources. When cladocerans are not present (or they are not commonly encountered by Mysis) they will sometimes prey upon copepods instead (Lasenby and Furst 1981, Chess and Stanford 1998). In other instances, Mysis is primarily a phytoplankton grazer."
Mysis also typically feed at night, and their eyes are easily damaged by strong or bright light.
*Female Mysis carry their brood pouches at the base of their legs similar to marsupials**Shrimp = Order: DecapodaMysis in Lake Tahoe
In an attempt to provide a “new and abundant food source for the lake’s fish”, California and Nevada’s departments of Fish and Game introduced Mysis to Lake Tahoe in the 1960s . Hindsight, of course, is 20/20, and the “biologists who introduced the shrimp didn’t know what the consequences would be”.
Similar to other lakes, Mysis in Lake Tahoe began feeding on native zooplankton — zooplankton that also served as the main food source of the native fish the Mysis were introduced to support.
Stated plainly, Mysis began competing with fish native to Lake Tahoe, effectively reducing both the size and populations of those fish.
Another unintended consequence of the Mysis was an increase of particles and algae in the lake, most of which were previously removed by Daphnia, zooplankton that are “slow and incredibly efficient at eating up all the algae and sediment in the lake”. As the population of these natural filters declined, so did the visual clarity of Lake Tahoe:
UC Davis Tahoe Environmental Research CenterBiologists at UC Davis’s Tahoe Environmental Research Center (TERC) believe a population of about 5,000 Daphnia per cubic meter would reduce the algae and sediment causing Lake Tahoe’s decline in clarity. In researching Emerald Bay, “a small inlet on the southern shore of Lake Tahoe”, TERC scientists discovered that a temporary (and inexplicable) disappearance of Mysis in 2011 resulted in the return of the Daphnia population. While monitoring Emerald Bay after this disappearance, the team found that the Bay’s clarity exceeded that of Lake Tahoe that year — unusual in part because of its tendency to be “murkier than the rest of the lake”.
“My vision of success is developing year-round techniques for removing shrimp,” — Brant Allen, TERC
TERC and Shrimply Blue are working closely to develop the techniques necessary to safely and consistently remove Mysis from the lake. A commercial-grade Mysis net will allow TERC to answer two critical data points required for ecosystem restoration:
1) How effectively can a commercial-grade net remove Mysis?, and
2) How much by-catch is there, and what’s the best way to minimize by-catch?
Keep following Shrimply Blue as we support TERC in its research. You can help us reach our goal by donating here. We look forward to sharing updates and dog treats along the way!
