Rainbow trout population in Box Canyon remains strong

John Heckel Rainbow Trout Box Canyon
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Sage Unsworth Idaho Fish and Game

Despite low flows in mid-May, Fish and Game biologists were still able to conduct their 25th population estimate in the Box Canyon reach of the Henrys Fork Snake River. This estimate is conducted in May on an annual basis and extends from the confluence with the Buffalo River downstream about 2.5 miles. Based on our long-term dataset extending back to 1994, the population estimate in Box Canyon this year (4,209 RBT/mile) is among the top 5 highest on record. The dashed line in the figure below represents the average number of Rainbow Trout per mile for the previous 24 surveys, which is 3,103.

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John Heckel Idaho Fish and Game

Winter discharge out of Island Park Reservoir continues to be an important factor for juvenile Rainbow Trout survival and abundance; it is imperative for juvenile Rainbow Trout to have adequate overwintering habitat in Box Canyon by maintaining overwinter flows around 300 cfs or greater. In the figure below, this year’s estimate of age-2 Rainbow trout abundance in relation to their first winter (i.e., 2019-2020 winter) discharge is represented by the red dot. The estimate falls right on the regression line of this relationship, indicating that the model continues to be a good predictor of age-2 abundance based on first overwinter discharge. Using the model to estimate age-2 abundance, we estimate 4,073 age-2 RBT, which is very close to our observed value of 3,948 age-2 RBT.

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John Heckel Idaho Fish and Game

The average total length of Rainbow Trout in Box Canyon was 13 inches with fish up to 21 inches sampled in the survey this year. We observed the highest frequency of Rainbow Trout between 12 and 14 inches, which are predominately age-2 fish. At this level of abundance, anglers should expect some fantastic fishing in the Box Canyon reach of the Henrys Fork this year. Furthermore, catch rates should also be very high if anglers can correctly match the hatch!

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John Heckel Idaho Fish and Game