There are some major differences in fish populations and habitat conditions in the Sun River above and below Gibson Reservoir. Two forks of the river, each about 20 miles long, feed into the reservoir. The forks are inaccessible by road and are located within the Bob Marshall and Scapegoat Wilderness areas. They are quite natural and undisturbed and therefore provide good reference areas to give an idea of how many fish the river can support with its natural habitat and all of its natural flow intact. However, since there are no roads and motorized equipment is not allowed in the wilderness area, it is difficult to do standard fish surveys up there.
On most rivers, FWP uses electrofishing equipment to census fish populations. This usually involves a boat equipped with a generator and special equipment to send electricity into the water. This special form of electricity attracts and stuns the fish just long enough for the crew to catch them with a net and put them in a holding tub in the boat. After the fish revive, they are measured, weighed and tagged or marked, and then released unharmed back into the river. These boats do not catch all the fish in the river, because the electricity is relatively weak and most fish avoid it.
Typically, a single pass down the river catches less than 10% of the fish, but that is enough to get an idea how many actually live there. The marked fish are given a week or so to settle back into the river and mingle with the unmarked fish that were not caught. Then, a second pass is made through the section, and the crew carefully notes if the fish caught on the second pass are new ones (not marked) or ones that were caught and marked on the first pass. By comparing the number of marked and unmarked fish, biologists can estimate the actual number of fish living in the river. Fish population surveys are often done several years in a row to see how the number of fish changes year-to-year. This information can be used to determine how drought, severe weather, or changing fishing use affects the fish population.
Because we cannot use motorized equipment in the wilderness areas on the forks of the Sun River above Gibson Reservoir, we developed a different method for censusing these fish populations. In a one-mile section of each fork, standard fishing tackle is used to catch and tag (with bright-colored plastic tags) approximately 100 trout over 8-inches long. Then, a team of two FWP biologists float through the sections with scuba suits, masks and snorkels and count all the trout they see. They are careful to record whether each fish has a tag on it, or not. Then this information is used just like the electrofishing method to estimate the total number of trout in the section. On average, the angling team tags about 18% and the snorkeling team sees about 52% of the “catchable-sized” trout (over 8-inches long) living in the river sections.
