This section will give an
overview of the life histories of the Yellowstone Cutthroat trout
and the lake trout, followed by case studies of their interactions,
and finally a brief overview of other exotic species introduced
to the park.
Native Fishes of the Park
The park has 12 fish species native to its waters
(http://www.nps.gov/yell/sotp/three/aquatic.pdf). The park has 3
species of salmonid, the Cutthroat trout, Oncorhynchus clarki,
with three subspecies the Yellowstone,
O.c. bouveri; the westslope,
O.c. lewisi; and the snake
river finespotted, O. c. carmichaeli subspecies, the
grayling, Thymallus arcticus, and the mountain
whitefish, Prosopium williamsoni. The park has three
suckers of the family catastomous, the Utah,
Catostomus ardens; the longnose,
Catostomus catostomus; and the mountain,
Catostomus platyrhynchus. The park also has 5 minnows or
cyprinids, the redside
shiner, Richardsonius balteatus; the lingnose
dace, Rhinichthys cataractae; speckled
dace, Rhinichthys osculus; Utah
chub, Gila atraria; and a redside shiner/speckled dace
hybrid. Only one cottid species inhabits the park, the mottled
sculpin, Cottus bairdi. Of this group only the Yellowstone
cutthroat trout and longnose dace are native to the Yellowstone
Lake. Introduced species that have succeeded in the park are rainbow
trout Oncorhynchus mykiss, brook
trout Salvelinus fontinalis, brown
trout Salmo trutta, lake
trout Salvelinus namaycush, and lake
chub Couesius plumbeus.
Yellowstone Cutthroat trout: on left pursuing
redside shiners and on right building a nesting redd.
Both the Yellowstone cutthroat trout (Oncorhynchus
clarki bouveri) and the westslope cutthroat trout (Oncorhynchus
clarki lewisi) have been recently petitioned for listing as
threatened on the Endangered
Species Act. The Yellowstone cutthroat trout now only occupies
10% of its native range, almost all of the 10% lies within Yellowstone
National Park (see maps). The westslope cutthroat trout has also
had precipitous declines and their only strong holds of survival
in their range, Flathead Basin and Glacier National Park, are being
affected by logging and other activities.
Life History of the Lake Trout
and Yellowstone Cutthroat Trout
The difference in life histories of the cutthroat
trout and the lake trout are important to consider in how they affect
their surrounding ecosystem.
The Lake Trout
Important aspects to take away are:
1). Lake trout live deep in the water column and cutthroats live
their lives at least partly near the surface.
2). Cutthroat spawn in the streams and tributaries of the lake
whereas the lake trout spawns in deep water.
3). That large lake trout eat cutthroat trout.
Since lake trout live deep in the water column they
are inaccessible as prey to terrestrial predators. The inaccessibility
disrupts an important link of energy from the aquatic system to
the terrestrial system of the GYE. The spawning activities of the
lake trout also leave them inaccessible to terrestrial wildlife
since they breed deep in the water. The cutthroat trout on the other
hand has a perilous journey up creeks to spawn while avoiding predators
like otters and osprey and once they have breed they must run the
gauntlet again to return to the lake. The last important aspect
of life history is that lake trout are piscivores (fish eaters).
Once a lake trout reaches 12 inches it starts eating cutthroat trout
that are smaller than it is. A lake trout can eat as many as 3,000
cutthroat trout in its lifetime if not more (Murphy 1999).
US record for sport caught Lake Trout, 71 pounds,
in Lake Superior.
Reasons for Concern About Lake
The reason biologists are so scared that lake
trout were found in Yellowstone Lake is that the conditions of the
lake are perfect for lake trout reproduction. A habitat suitability
index for lake trout, by Marcus et al. in 1984, states that the
top preferred habitat is one that meets the following five pieces
1. 7-16 degrees Centigrade is max temperature for the
hypolimnion or deeper half of water column
2. The lake must have a dissolved oxygen concentration
of greater than 8 mg/l in hypolimnion and deeper.
3. Forage fish must be present for rapid growth.
4. Patches on cobble on lake bottom 0.5 to 50 m deep to
spawn. Best if there is upwelling in area.
5. Annual reservoir draw down of less than or equal to
A criterion that is important to lake trout not listed
above is that they like big, clear lakes. The environment of Yellowstone
Lake closely matches these criteria. Yellowstone Lake has a surface
area of 341 square kilometers and an average depth of 48.5 m (Kaeding
et al. 1995). A thermocline forms in July and may persist to mid-September;
the hypolimnion is well oxygenated during stratification (Kaeding
et al. 1995). The surface temperature rarely exceeds 18 degrees
Celsius (Kaeding et al. 1995), so one can logically assume that
the hypolimnion stays at 16 degrees of less throughout the year.
In the lake the lake trout can choose a variety of forage fishes
to consume. It has been known to eat long nose suckers and cutthroat
trout (Murphy 1999). The lake has miles and miles of shoreline where
cobble, rubble, or boulder substrate occurs (Kaeding et al. 1995).
If the winds were right than all of those miles of shoreline is
potential breeding habitat for lake trout. The last aspect of the
optimal environment for lake trout is that the reservoir is not
dropped more than 15 cm a year. Since Yellowstone Lake is a natural
system with no dams it is unlikely that the lake level would drop
by as much as 15 cm in one year. So, Yellowstone Lake makes a perfect
habitat for lake trout to expand their numbers rapidly.
Lake trout produce 628 to 1710 eggs per kg of
body weight. On average, a single female lake trout in Jackson Lake,
Wyoming yield 6000 eggs (Marcus et al. 1984). When forage fish are
the primary source of food, a slowing in growth of lake trout is
not apparent (Marcus et al. 1984). These two aspects line up for
a very rapid growth of lake trout populations in Yellowstone Lake.
It is possible that the lake trout in Yellowstone Lake will produce
more than 6000 eggs per female due to their rapid growth potential.
Case Studies of Lake Trout and
Cutthroat Trout Interactions
Introduced lake trout often lead to the demise
of other trout. For instance, introductions into Lake Tahoe led
to the elimination of the native Lahontan cutthroat O. c. henshawi
(McAffee 1966; Moyle 1976). Cordone and Frantz (1966) reported a
drastic decline and eventual extinction of native cutthroat in Lake
Tahoe after the introduction of lake trout. Lahontan cutthroat were
abundant in the lake in 1907, with only an occasional lake trout
reported. By 1938, the cutthroat had virtually disappeared, and
by 1966, lake trout comprised 70% of angler catch in the lake (Cordone
and Frantz 1966). Stocked lake trout have replaced native cutthroats
in deep Rocky Mountain lakes (Benson et al. 1961). This trout has
also virtually eliminated cutthroat and bull trout in Flathead Lake,
Montana, and Pend Orielle Lake, Idaho. Lake trout introduced into
Flaming Gorge Reservoir were found to prey heavily on the Utah chub
Gila atraria (Teuscher and Luecke 1996). So, it is understandable
that fisheries biologists, in the Yellowstone National Park, are
very worried over the introduction of lake trout into fragile Yellowstone
This figure represents an idealized interaction
of cutthroat trout and lake trout in Yellowstone Lake. (www.nps.gov/yell/planvisit/todo/fishing/laketroutweb/)
Other Important Introduced Species
Another introduced species that could possibly
cause the demise of the Yellowstone cutthroat trout is Myxobolus
cerebralis, which is commonly known as whirling disease.
The disease is of European origin. It was first found in the US
in 1956 in Pennsylvania and now occupies 21 states. The disease
may have been transported to Yellowstone Lake by dirty fishing gear
or boats. A fall study in 1998 revealed that of 41 cutthroat trout
sampled from the lake, 27% showed an infection of whirling disease
(Hudson and Mahoney 1998). Whirling disease is caused by microscopic
parasite that attacks the cartilage of some fish, like rainbow and
cutthroat trouts. Ironically this disease does not affect lake trout
so the effects of the disease are additive to the losses caused
by introduced lake trout. A fish affected with whirling disease
is unable to feed normally or swim normally and are therefore more
susceptible to predation and starvation. When a fish dies and decays
due to whirling disease the parasite turns into spores that can
live up to 30 or more years.
Introduced species having an affect on Yellowstone
Lake or a possible effect of Yellowstone Lake is a reoccurring theme.
In 1995 the zebra
mussel was found in Yellowstone Park. This mussel has caused
millions of dollars of damage to the great lakes. These filter feeders
can clean a lake out of its primary production of phytoplankton
and zooplankton. If they get a foothold in Yellowstone Lake they
will affect the ecosystem from the bottom up. Cutthroat trout will
not have food to survive to adulthood, nor would lake trout. The
whole ecosystem of the lake and all linkages to the terrestrial
environment could change. Even the insect structure around the lake
and the rest of the park could be vastly different if zebra mussels
are able to get a foothold in Yellowstone Lake or any other lake
in the park.
Zebra Mussels colonizing on a crawdad.
Lake Trout Life History Versus Yellowstone
Cutthroat Trout Life History
Lake Trout (Salvelinus
Cutthroat trout (Oncorhynchus
In 15 cm to 55 m of depth along the sides
of lakes in areas with rubble 2.5 cm or greater in diameter
Breed in streams. 1/2 of Yellowstone lake's
tributaries used roughly 63 of 126.
Maximum body size
54.4 kg, 119.7 kg
2 lbs. or 1 kg a big fish in Yellowstone Lake,
maximum of 16 lbs. reported in Red Eagle Lake, MT after introduction
of trout species.
<12" eat zooplankton, insects, cyprinids;
12" eat other fish including suckers and trout primarily
Young trout eat diatoms and other zooplankton
in deeper waters; adults concentrate in shallower water on
midge larvae and pupae, freshwater shrimp, and insects blown
628 to 1710 eggs/ kg of body weight; In Jackson
Lake, WY average of 6000 eggs per female
Amount of eggs for females was not reported
in any of my sources. Probably around same range as lake trout
of 628 to 1710 eggs/ kg of body weight
man, maybe a lucky otter
white pelican is 100% dependent, forty other
species also dependent on trout meat
prefer deeper half of water column in lake
can be live life within creeks and rivers
or within the lake, use the top part of water column but also
inhabit deeper waters
dark olive body with light spotting, anal
and paired fins have white leading edge
Little or no spotting on head. Big dark black
spots found dorsally over lateral line. Spots increase in
density posterior to of anal fin. Caudal peduncle heavily
Found in almost all provinces of Canada, NE
USA states, Montana, and Alaska
Found currently only within Yellowstone and
Montana. This is only 10% of their natural range.
Migrate vertically with the thermal cline,
also move nomadically within the lake, and can travel as much
as 306 km in 19 days.
Lake living trout make a migration, each year
that they spawn, up the streams and tributaries of Yellowstone
lake to breed. Fluvial trout often migrate from the rivers
that they live in to the lake in the winter. This migration
is to warmer waters of the larger lake.