Coeur d'Alene Tribe monitors trout population
PLUMMER, Idaho - The Coeur d'Alene Tribe is working at bolstering populations of westslope cutthroat trout in streams flowing into the south end of Coeur d'Alene Lake. That portion of the lake was returned to tribal jurisdiction following a decision by the Supreme Court in 2001. Lake Creek on the lake's west side is one of the more important tributaries that trout use for spawning, and current efforts are to estimate the productivity of the stream.
Dale Chess, research monitoring and environmental specialist, designs the experiments, monitoring and evaluating strategies, and analyzing the data that are produced. ''We're trying to determine how many juveniles are produced per adult fish,'' he said. ''This is a measure of the productivity of the stream. We're using standard measurements used throughout the Columbia Basin for migratory fish, especially anadromous fish.'' These westslope cutthroat in Lake Creek are migratory adfluvial trout, meaning they leave the lake to reproduce in the streams and then return to the lake. That makes them a perfect fit for the same techniques used in studying salmon and steelhead.
A passive integrated transponder tag system is being used in conjunction with both upstream and downstream traps. As young fish move downstream from the headwaters where they hatched, they are funneled into a trap, where a certain percentage is tagged then released to continue downstream to the lake. The tags are about the size of a grain of rice and are injected into the stomach cavity of the fish using a syringe. Each tag has its own unique code of letters and numbers to identify that particular fish. ''We know, when we tagged that fish, the size and age of the fish when we tagged it,'' Chess explained. ''When it returns as an adult we measure the fish, take weight and scale samples to age it so we essentially know the entire life cycle of that individual fish. This also allows us to estimate survival when they are in the lake.''
Fish are recaptured when they return to the stream as adults two to six years later. As they pass under the bridge with the trap, they also pass through an electronic monitoring device, essentially a PIT tag antenna. Scanning at 100 milliseconds, the device instantly identifies each fish and eliminates the need to handle each fish. Roughly 2,000 fish have been tagged since 2005, providing a large sample size.
''These are almost pure westslope cutthroat. Genetic analysis revealed only a small fraction of the population having any introgressed rainbow trout genes. That's good. They are very unique fish, not only from the genetic standpoint but the adfluvial life history is becoming rare any more,'' Chess remarked.
''A key part of the management of this species is to manage both the tributary habitat and lake habitat because the fish require both. This is important to the tribe because it's a watershed management tool, or indicator.'' Eighteen people are employed in the tribe's fisheries division.
The tribe closed the stream to fishing in 1993, so no fish have been harvested since that time. ''We know there's been an increase in numbers but we need to keep trapping yearly to more accurately estimate the increase in numbers. We also see fish coming back that have spawned multiple times. An important part of the PIT tag system is that it allows us to know what percentage of fish return a second, third and even fourth time. That all increases the productivity of the system,'' Chess said.
Electroshocking gear is also being used to sample fish numbers at 30 sites located on Lake Creek. This provides a better estimate of total population and trends than other methods. That research began in 1996, so 11 years of data have been gathered. This data also shows an increasing trend, a good sign in light of declining westslope cutthroat populations in many other waters.
Lake Creek was selected because of its importance for spawning with still reasonable numbers of fish. More recently, a somewhat similar study has begun on Benewah Creek on Coeur d'Alene Lake's south end.