Mysis diluviana—opossum shrimp—were introduced upstream of Flathead Lake in Ashley, Swan, and Whitefish Lakes to benefit the kokanee fishery in 1968. Until then native bull trout and westslope cutthroat were the dominant sport fish species in Flathead Lake. There were lake trout in the lake at that time (they were introduced in 1905) but their numbers had stayed down. The non-native mysis began to drift down and enter the food web of Flathead Lake in 1981. Today if you open the stomach of a lake trout or lake whitefish and see little black dots, those are the eyes of the mysis shrimp. Each mysis has two eyes. Most fish stomachs will have hundreds of eyes at a time in their stomach. Sometimes you can see the transparent looking bodies of the shrimp in the stomach of the lake trout. They are very small, about the size of the tip of your little finger.
Since the lake trout have keyed in on these shrimp as a food source, their flesh has changed from white to an orange color. The flesh of lake trout is also not as fatty as it was when they fed principally on kokanee salmon.
Young lake trout in Flathead Lake, which live in the deeper waters of the lake, now have a great food supply with these shrimp because the mysis go to the bottom of the lake where the younger lake trout dwell during the day. They come up in the water column at night to feed. With the introduction of this new food, the juvenile lake trout population exploded. By 1989 the popular kokanee fishery had completely disappeared from the lake, and predation by the increased population of lake trout began to take a huge toll on native fish numbers. Native fish—bull trout and westslope cutthroat trout—started to decline dramatically. Today their populations are a fraction of what they were historically, and it only took ten years for the decline to happen. By 1998 bull trout numbers were so low that they were listed as a Threatened Species under the federal Endangered Species Act. They were once found in about 60 percent of the Columbia River Basin but today occur in less than half of their historic range, with scattered populations in portions of Oregon, Washington, Nevada, Idaho, and Montana. Meanwhile westslope cutthroat trout have been listed as a Species of Special Concern.
Native bull trout and westslope cutthroat trout are part of our fishing heritage in Montana. With their populations so dangerously low, it should be a priority for all of us to protect, enhance, and increase the populations of these native fish.
Mysis diluviana: Some Basic Facts
Mysis diluviana is a mysid crustacean (opossum shrimp) that inhabits
freshwater lakes of northern North America.
Appearance
Mysis is a small, transparent shrimp-like crustacean, of less than 25 millimeters (1 in) length. It has two pairs of relatively long antennae, associated with rounded antennal plates; large, stalked compound eyes; the thorax covered by a coat-like carapace; a muscular, cylindrical abdomen; and a tail fan featuring a telson with a V-shaped terminal cleft.
Reproducing females bear a prominent brood pouch (marsupium) between their thoracal legs. The pleopods (abdominal legs) of Mysis are reduced, except for a specialized pair of mating legs in males.
The natural distribution of Mysis diluviana comprises the Great Lakes of North America, and many other coldwater lakes across Canada and in northern parts of the United States, including Wisconsin (e.g. Green Lake, Trout Lake, and Geneva Lake) and northern New York State (e.g. Finger Lakes, Lake Champlain). As it inhabits the areas covered by ice sheets during the last glacial period, the species has been called a glacial relict.
Mysis diluviana has also been transferred to lakes outside its native range, to provide a new fish-food object, e.g. to Lake Tahoe (Nevada, California) and Kootenay Lake (British Columbia).
Mysis diluviana were known as Mysis relicta until recent years.
Nonindigenous Occurrences: The first nonindigenous introduction was proposed by Clements et al. (1939) for the lakes of the Okanagan basin, British Columbia, to enhance the forage base for the previously introduced lake whitefish (Coregonus clupeaformis). In 1949 Kootenay Lake, British Columbia, was the first lake to be stocked with M. diluviana (Sparrow 1964). Subsequent stockings were performed in 5 Montana lakes upstream of Flathead Lake (Bosworth 2011), and in many other lakes of the western U.S. and Canada.
Ecology: M. diluviana stays at the bottom of the water column during the day and migrates toward the surface during the night to feed. This behavior means that they can have a direct impact on all other organisms living in the water body.
Means of Introduction: Stocked to provide additional forage for sport fish (Clements 1939, Bosworth 2011).
Status: Established
Impact of Introduction: Mysis diluviana is an opportunistic feeder with both predatory and filter feeding habits. When zooplankton is abundant they serve as the primary food source; when scarce M. diluviana will feed on suspended organic detritus or from the surface of benthic organic deposits (Pennak 1989). Within its native range it has been shown to be an important prey item for freshwater fishes (Nesler and Bergersen 1991). However, when introduced into what was considered to be an "empty" niche, its impact on the aquatic community was significant. Dramatic changes and species extinctions of native zooplankton communities have been attributed to this opportunistic lifestyle. This change in the primary consumer composition has lead to drastic ecosystem changes in Flathead Lake, Montana. These smaller shrimp replaced larger native species but were unable to keep up with the growth of algae in the lake. The benthic tendencies of this species provided a massive new food source for bottom dwelling lake trout, allowing the trout to increase their population and overtake kokanee as the top predator in Flathead Lake (Bosworth 2011). Declines in the number and size of game fish have been documented since the introduction of M. diluviana , provoking doubt regarding their utility as a forage base for game fishes.
Remarks: Found in deep, cold oligotrophic lakes; has been reported from brackish and estuarine waters. Can live for 2 years; breed in the winter; adults carry young in a brood pouch until fully developed in the spring; sexually mature at 12-14 mm.
In 1981 a resolution calling for a moratorium on introductions of M. diluviana was drafted by participants of a Mysid symposium held during a conference of the American Society of Limnology and Oceanography in Milwaukee, Wisconsin.
References: Bosworth, B. 2011. How non-native shrimp transformed the ecosystem at Montana's Flathead Lake. New West Travel and Outdoors. http://www.newwest.net/topic/article/how_non_native_shrimp_transformed_the_ecosystem_at_montanas_flathead_lake/C41/L41/
Clements, W.A., D.S. Rawson, and J.L. McHugh. 1939. A biological survey of Okanagan Lake, British Columbia. Fisheries Research Board of Canada Bulletin 56.
Nesler, T.P. and E.P. Bergersen. 1991. Mysids and their impacts on fisheries: an introduction to the 1988 Mysid-Fisheries symposium. Pages 1-4 In: T.P. Nesler and E.P. Bergersen, editors. Mysids in fisheries: hard lessons from headlong introductions. American Fisheries Society Symposium 9, Bethesda, Maryland.
Pennak, R.W. 1989. Fresh-water invertebrates of the United States, protozoa to mollusca. John Wiley & Sons, New York, 3rd ed.
Sparrow, R.A.H., P.A. Larkin, and R.A. Rutherglen. 1964. Successful introduction of Mysis diluviana Lovén into Kootenay Lake, British Columbia. Journal of the Fisheries Research Board of Canada 21:1325-1327.
Mysis diluviana is a mysid crustacean (opossum shrimp) that inhabits
Appearance
Mysis is a small, transparent shrimp-like crustacean, of less than 25 millimeters (1 in) length. It has two pairs of relatively long antennae, associated with rounded antennal plates; large, stalked compound eyes; the thorax covered by a coat-like carapace; a muscular, cylindrical abdomen; and a tail fan featuring a telson with a V-shaped terminal cleft.
Reproducing females bear a prominent brood pouch (marsupium) between their thoracal legs. The pleopods (abdominal legs) of Mysis are reduced, except for a specialized pair of mating legs in males.
The natural distribution of Mysis diluviana comprises the Great Lakes of North America, and many other coldwater lakes across Canada and in northern parts of the United States, including Wisconsin (e.g. Green Lake, Trout Lake, and Geneva Lake) and northern New York State (e.g. Finger Lakes, Lake Champlain). As it inhabits the areas covered by ice sheets during the last glacial period, the species has been called a glacial relict.
Mysis diluviana has also been transferred to lakes outside its native range, to provide a new fish-food object, e.g. to Lake Tahoe (Nevada, California) and Kootenay Lake (British Columbia).
Mysis diluviana were known as Mysis relicta until recent years.
Nonindigenous Occurrences: The first nonindigenous introduction was proposed by Clements et al. (1939) for the lakes of the Okanagan basin, British Columbia, to enhance the forage base for the previously introduced lake whitefish (Coregonus clupeaformis). In 1949 Kootenay Lake, British Columbia, was the first lake to be stocked with M. diluviana (Sparrow 1964). Subsequent stockings were performed in 5 Montana lakes upstream of Flathead Lake (Bosworth 2011), and in many other lakes of the western U.S. and Canada.
Ecology: M. diluviana stays at the bottom of the water column during the day and migrates toward the surface during the night to feed. This behavior means that they can have a direct impact on all other organisms living in the water body.
Means of Introduction: Stocked to provide additional forage for sport fish (Clements 1939, Bosworth 2011).
Status: Established
Impact of Introduction: Mysis diluviana is an opportunistic feeder with both predatory and filter feeding habits. When zooplankton is abundant they serve as the primary food source; when scarce M. diluviana will feed on suspended organic detritus or from the surface of benthic organic deposits (Pennak 1989). Within its native range it has been shown to be an important prey item for freshwater fishes (Nesler and Bergersen 1991). However, when introduced into what was considered to be an "empty" niche, its impact on the aquatic community was significant. Dramatic changes and species extinctions of native zooplankton communities have been attributed to this opportunistic lifestyle. This change in the primary consumer composition has lead to drastic ecosystem changes in Flathead Lake, Montana. These smaller shrimp replaced larger native species but were unable to keep up with the growth of algae in the lake. The benthic tendencies of this species provided a massive new food source for bottom dwelling lake trout, allowing the trout to increase their population and overtake kokanee as the top predator in Flathead Lake (Bosworth 2011). Declines in the number and size of game fish have been documented since the introduction of M. diluviana , provoking doubt regarding their utility as a forage base for game fishes.
Remarks: Found in deep, cold oligotrophic lakes; has been reported from brackish and estuarine waters. Can live for 2 years; breed in the winter; adults carry young in a brood pouch until fully developed in the spring; sexually mature at 12-14 mm.
In 1981 a resolution calling for a moratorium on introductions of M. diluviana was drafted by participants of a Mysid symposium held during a conference of the American Society of Limnology and Oceanography in Milwaukee, Wisconsin.
References: Bosworth, B. 2011. How non-native shrimp transformed the ecosystem at Montana's Flathead Lake. New West Travel and Outdoors. http://www.newwest.net/topic/article/how_non_native_shrimp_transformed_the_ecosystem_at_montanas_flathead_lake/C41/L41/
Clements, W.A., D.S. Rawson, and J.L. McHugh. 1939. A biological survey of Okanagan Lake, British Columbia. Fisheries Research Board of Canada Bulletin 56.
Nesler, T.P. and E.P. Bergersen. 1991. Mysids and their impacts on fisheries: an introduction to the 1988 Mysid-Fisheries symposium. Pages 1-4 In: T.P. Nesler and E.P. Bergersen, editors. Mysids in fisheries: hard lessons from headlong introductions. American Fisheries Society Symposium 9, Bethesda, Maryland.
Pennak, R.W. 1989. Fresh-water invertebrates of the United States, protozoa to mollusca. John Wiley & Sons, New York, 3rd ed.
Sparrow, R.A.H., P.A. Larkin, and R.A. Rutherglen. 1964. Successful introduction of Mysis diluviana Lovén into Kootenay Lake, British Columbia. Journal of the Fisheries Research Board of Canada 21:1325-1327.