2015 Missouri River trout population summary
Montana Fish, Wildlife, and Parks
November 2015
FISH SURVEY RESULTS
Trout Numbers Remain High in Missouri River
Fish surveys this year indicate rainbow and brown trout numbers remain above the long-term average in the Missouri River between Holter Dam and the town of Cascade, says a state fisheries biologist.
State fisheries crews this fall estimated 4,073 rainbow trout greater than 10 inches long per mile near the town of Craig on the Missouri. Not only is that above the long-term average of 3,174 rainbows per mile, but continues a trend of above average numbers over the past three years: 6,034 per mile in 2011 and 7,312 in 2012.
This year’s population was bigger in size and slightly lower in abundance than the past two years, says Fish, Wildlife and Parks fisheries biologist Grant Grisak, which is typical as the current population reaches its maximum size.
“This year,” Grisak says, “87 percent of the rainbow trout in the Craig section were 15 inches long or greater, and 35 percent of the population was 18 inches long or longer.”
Next year, the population should return to normal levels, unless an unusually high water event occurs in the spawning tributaries, Grisak says. High water in the Missouri River tributaries typically results in high rainbow trout production.
Brown trout in the Craig section at 10 inches long and greater were estimated at 433 per mile. The long-term average is 578.
In the Cascade section, near the town of Cascade, the estimate for rainbow trout 10 inches long and greater was 2.260. The long term average is 1,551 per mile.
Brown trout in the Cascade section 10 inches long and greater were estimated at 447 per mile. The long term average is 387.
Brown trout populations are sampled in the spring and rainbow populations are sampled in the fall.
2012 Missouri River trout population summary
Rainbow
Craig – estimate of rainbow trout > 10 inches long is 7,312 (+149) per mile. The 30 year mean is 3,036. This is the highest estimate on record and surpasses the previous record of 6,034 per mile from last year. There continues to be a substantially higher than normal number of hatchery fish in this section and for the second time ever we were able to calculate an estimate of 700 (+184) per mile. Wild fish = 6,612 per mile, Hatchery fish = 700 per mile. We handled 6,922 fish, which is the highest number we’ve ever handled during this estimate. We sampled 68 walleye and 73 burbot. The largest walleye was 28.0 inches long and weighed 9.6 pounds and the largest burbot was 21.8 inches long and 3.45 pounds. After looking at the length frequency distribution of hatchery fish from 2011 and 2012, there was a positive shift in growth and abundance, suggesting the hatchery fish from this year are likely those that entered the river from Holter Lake last year.
The estimate of brown trout > 10 inches long was 850 per mile (+ 25). The 28 year mean is 570 per mile. In the spring we sampled 13 burbot and 54 walleye. The largest burbot was 20.8 inches long and 1.62 pounds and the largest walleye was 30 inches long and weighed 11.2 pounds.
Cascade – estimate of rainbow trout > 10 inches long was 2,182 (+66) per mile. The 28 year mean is 1,506. We sampled 12 walleye and 5 burbot. The largest burbot was 22.5 inches long and weighed 2.51 pounds. The largest walleye was 21.6 inches long and weighed 4.28 pounds.
The estimate of brown trout > 10 inches long was 545 per mile (+ 26). The 27 year mean is 384 per mile. In the spring we sampled 21 burbot and 3 walleye. The largest burbot was 20.2 inches long and weighed 2.24 pounds. The largest walleye was 22.5 inches long and weighed 4.65 pounds.
Something that is a bit uncommon but consistent in both sections this year was crews sampled more rainbow trout in the recapture runs than in the marking runs. For Craig the number was 14% more and for Cascade, the number was 30% more.
