As mentioned previously, there are some big differences between trout populations in the river above and below Gibson Reservoir. The river itself changes a lot, some of which is natural, and some caused by man's activities. The river has not been stocked with trout since the early 1970's, when the State of Montana adopted its “wild trout policy”. These fish populations maintain themselves by natural reproduction.
The chart below shows the average number of trout larger than 8-inches per mile of river for five different areas of the river. From the chart, we can see there are far more trout living above the reservoir than below. There are an average of around 550-650 trout per mile in the two forks of the river above Gibson. The trout population above the reservoir is primarily rainbow trout, with some rainbow/cutthroat hybrids and a few brook trout. The trout up here are generally smaller (average length approximately 11 inches) than further downriver. Only about 2% of the trout populations in the forks above Gibson Reservoir are made up of fish 15-inches or larger. Fish are smaller here probably because this area is high elevation and has a shorter growing season and colder water than further downriver.
Graph of Sun River fish populations
We monitor trout populations in three sections of the 97-mile stretch of the mainstem Sun River between Diversion Dam and Great Falls. The uppermost section is near Augusta, the middle section is just upstream from Simms, and the lower section is just downstream from the town of Sun River. Fish populations are dramatically lower in these sections, ranging from 50-120 trout per mile. The trout population in the mainstem is about 35% rainbow and 65% brown trout. However, the fish in the mainstem Sun are much larger than in the forks upstream from Gibson Reservoir. This indicates growing conditions are good for those fish that can survive the harsh conditions they are exposed to. About 54% of the trout in the mainstem are over 15 inches, compared to only 2% in the forks above Gibson.
