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YNA 2000 WInners Hero

"Looking Back, Looking Ahead"

Can Tropical Fish Survive in a Lake in Southwestern Montana?


Last summer I heard about a pond of tropical fish near Ruby Reservoir in southwestern Montana. It is called Trudau Lake. We found it on a map at 45° 14' north latitude, 112° 8' west longitude, and it has an elevation of 5,620 feet. It is about a half-mile west of Ruby Reservoir. To the best of their knowledge, the fish biologists that we contacted said no studies have been done on the lake.

Tropical fish normally live in conditions of 66° to 76° Fahrenheit and pH of 7.2 to 8.0. Montana has severe winters, and I wondered how tropical fish could live here. How did they get in that lake? I decided to try to find out.

I visited the lake three times with different objectives each time.


OBJECTIVE 1: Confirm That There Were Tropical Fish and Observe Their Environment


I first visited Trudau Lake on October 10, 1999. The nearby hills were rocky and uneven, thinly covered with dry grass and scattered juniper trees. The whole area looked arid.

I arrived at 1:30 P.M. There was a light breeze, and it was sunny with a few wispy clouds and a temperature of 59°F. There was a slight sulfur smell in the air.

Trudau Lake was a small, round pond about 47 yards in diameter. Around the edges, the water was shallow and amazingly clear, but toward the middle it was a deeper, dark blue. I could see fish close to the surface. I immediately recognized guppies, but there were many kinds I didn't know. There were water plants growing under the water and reeds and grasses on northern and eastern edges of the lake.

Water flows into the lake from a spring about a hundred yards west. Some of the water flows overland into the lake, but most of it goes into the ground 75 feet above and comes out of the limestone rocks on the west side of the lake. The outlet on the east side of the lake is about the same size as the stream above.


Trudeau Lake sits at the bottom of limestone cliffs on the west side of Ruby Reservoir.

From a raft, I took readings of depth and temperature at various spots in the lake. Near the western edge it was 65°F and had a depth of 4'9". I paddled to the middle where the temperature was 69º and the depth 10'9". Toward the southern shore it was 65°F and 12'10" deep, and at the edge of the rock shelf on the south side it was 64°F and 14'11" deep. There seemed to be no indication of the water level changing. Ruby Reservoir, about a half mile away, had dropped approximately 30 feet from its high point in the spring.The water felt warm to the touch. The spring above the lake was 75°F; in the stream before it runs into the ground above the inlet it was 70°F; and at the inlet it was 62°F. At the outlet it was 63°F.

I spotted a large goldfish in water too deep for my net, but I was able to net many of the other fish. We put them into a large glass container, then took photographs and made sketches to help us identify the fish later.

After we returned to the lake, I took water samples for testing. The tests showed the following:

Source PH TDS Soluble Iron Nitrate Nitrate
inlet 8.6 370 slight none none
pond 8.3 420 slight none none
off rocks 8.9 360 slight none none
outlet 8.5 450 slight none none

Platys and Mollies swimming around the aquatic plants in very clear water.

From my first trip to Trudeau Lake, I concluded that tropical fish are indeed living wild there. Many of them require warm water, the water must stay warm year-round.

The water samples were examined under a microscope. A few diatoms, filaments of algae, and a colony of green algae were present. Where were the bugs that fish eat?

To learn more about the history of the lake, I spoke with Mr. Edwards, a local resident. He said a pet store owner introduced the fish into the lake about 30 years ago in order to supply his store.


OBJECTIVE 2: Identify the Fish and Find What They Eat


On October 23, 1999, I returned to the lake at about 11:00 A.M. The air temperature was 64°F, sunny with almost no wind, and the grasses and soil by the lake were dry. I found many of the smaller fish close to where the water came out of the rocks into the lake.

I dropped some corn into the lake to attract the fish. Five minutes later, the two most beautiful fish I have ever seen slowly swam up. The first was about four inches long and one inch tall, yet it was skinny; he was a sort of clearish white. It looked like an albino, but its eyes were brown. The second one was bright orange with a forked tail and long and clear fins. He was about six inches long and two inches tall and he looked skinny, too.

I netted some smaller fish to photograph and measure.

I rafted across the lake and took more depth measurements. The depth ranged from 4'1" to 17'3". The temperatures were about the same as recorded on October 10.

I screened for macro-invertebrates near the outlet, trying to figure out what the fish were eating. Most of these fish were omnivorous, and there was plenty of vegetation too. I found scuds and leaches. Along with Mrs. Weigand, my science teacher, I identified swordtails, guppies, mollies, and platys. Dragonflies and many tiny gnat-like flies were observed. I found at least five kinds of tropical fish, and even at this time of year there seems to be plenty for them to eat.


OBJECTIVE 3: Test Water Quality and See if the Fish Go Dormant in the Winter


On December 26, 1999, I again returned to Trudau Lake. Although this December had been far warmer than normal, Ruby Reservoir was frozen over and the road to Trudau Lake had about two inches of snow. I arrived at 12:30 P.M. The temperature was 36°F, the sky was clear and bright, there was no wind, and a mist rose from the water.

Dr. Bauder, a water-quality specialist at Montana State University, said that the geologic formation was probably about 100,000 years old. The lake was in a sinkhole in the limestone and approximately 1,000 years old. He showed us how to do water tests, and we found the water to be highly mineralized, mostly with calcium carbonate from the limestone. There was a trace of iron but no nitrates or nitrites. He also looked for insects in the water because the species and number of insects are a good indicator of environmental quality.

I tested for pH (8.4), which was high, and oxygen level (6), which was too low to support trout but apparently enough to support tropical fish. The water temperatures were again measured to compare them to our earlier visits.

Temperature Comparison 10/10 12/26
Trudeau Lake inlet 62°F 62°F
Trudeau Lake shallows 65°F 58°F
Trudeau Lake center 69°F n/a
Stream above lake 70°F 75°F
Spring above lake 72°F 80°F
Ruby River 52°F 33°F
Ruby Reservoir 40°F 34°F

The inlet water and the outlet were warmer in late December than during our October visit.

Fish netted by the inlet were identified as guppies, mollies, swordtails, and platys. All were active despite the winter season.

Fish 4

There were two beautiful blue-tailed swordtails, two pregnant female swordtails, two male mollies with yellow-and-black stripes, and three female mud-colored molly females. Three fish were netted that we could not positively identify. They were about three inches long and looked like miniature trout. They were probably daces, a freshwater fish of the minnow family.

Fish 3

Dr. Bauder showed us how to do several more water tests. He had expected to find lots of larvae and many kinds of small animals but he didn't. He said that the water in Trudau Lake is class 4 water--meaning that it has the least number of potential uses and is the poorest-quality water.

A quick search for aquatic animals turned up three kinds of snails: the orb, the pouch, and the gill; and two kinds of beetles, a giant water beetle and a backswimmer. Both types of beetles are carnivorous and eat small fish.




I identified several types of goldfish: red comet, red and white comet, and red common goldfish. Goldfish (Carassius auratus) can adjust to a wide range of water temperatures. They are egg layers. With the help of my science teacher, we identified various versions of swordtails (Xiphophorus helleri), mollies (M. sphenops) , platys (Xiphophorus maculatus), and guppies (Poecilioides reticulatus). These fish are all native to tropical waters. They give birth to their offspring. They ordinarily like to live at temperatures between 66°F and 76°F with a pH of 7.2 to 8.0. The temperature of Lake Trudau is somewhat lower and the pH somewhat higher than they like best. I also found some minnow-like fish that may be a type of dace. The waters of Trudau Lake are so highly mineralized and low in oxygen that relatively few larvae and water animals live there to provide food. Aquatic plants must provide an adequate, additional food source.

Fish 2

These fish have several natural enemies in addition to the beetles mentioned above. In October we saw dragonflies. Dragonfly larvae catch and eat small fish. Other predators that live in the area are great blue herons and garter snakes. Still, there appears to be a thriving population of fish.

In midwinter the water was nearly as warm as in October and the fish remained very active.




I'm very optimistic about the future of this site. It is on private land and part of a grazing association. A developer is trying to get hold of the land because it is so close to the reservoir and recreation. If this happens I still don't think that the lake will be in danger because it is unique. I think that many people will try to preserve it.

If there was an earthquake, it might shut off the warm-water spring, but if the water temperature remains constant, the fish should survive. I don't think silt will settle in during the next 100 years if the stream flow remains the same.


I think the fish will remain well adapted and the population well regulated if people don't interfere. I hope Lake Trudau will remain like this for the next century so more people can enjoy the beautiful fish and lake as much as I do.




"Cyprinidae, Minnow Family."

Encyclopaedia Britannica.

Goldfish photos.

"Guppy Facts Page."

"Goldfish INFO."


Innes, William T.  Exotic Aquarium Fishes.  MetaFrame Corporation, 1966. 


Telephone Interviews

Jim Deboer, fish biologist, Montana Department of Fish, Wildlife, and Parks, Sheridan, MT.

Dick Oswald, Montana Department of Fish, Wildlife, and Parks, Dillon, MT.

Jim Edwards, long-time resident, Sheridan, MT.

Chris Hunter, Fishery Division, Montana Department of Fish, Wildlife, and Parks, Helena, MT. 


Personal Interviews

Mrs. Jenny Wiegand, Science teacher, Reed Point High School, Reed Point, MT.

Dr. Jim Bauder, Department of Plant, Soil, and Environmental Sciences, Montana State University, Bozeman, MT.

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