BILLINGS — Trout numbers in the Bighorn River hit record lows this spring, which state biologists say corresponds with high river flows below Yellowtail Dam the previous two spring hatch seasons.
This spring’s survey showed 370 brown trout and 870 rainbow trout, all over eight inches long, per mile of river, the lowest since the survey launched in 1992, Montana Fish, Wildlife and Parks reported Wednesday.
Mike Ruggles, the agency’s regional manager, said brown trout eggs typically hatch in March and April, which is when high spring flows from the Yellowtail Dam likely moved enough riverbed gravel to crush unhatched eggs and kill small fish.
Rainbow trout eggs typically hatch in the early summer and may have also been harmed by heavy water flow, Ruggles said in a news release.
Fish numbers this spring were similar to the early 2000s, when drought conditions forced dam managers to limit water flow below the threshold required for trout to thrive in the river, according to the agency.
Biologists cited a bright spot from the survey: Many fish counted were longer than 15 inches, likely from earlier hatches in 2014 to 2016.
However, they added that those fish are starting to die off, leading to a significant decline over the next two years.
Biologists also noted that high water flows this spring did not start until after most brown trout had hatched, leaving hope that this year’s schools will thrive with less competition.