Henry’s Fork Rainbow Trout migrating to spawn in the Buffalo River have to pass through the fish ladder at the Buffalo River hydroelectric facility to access upstream spawning habitat. At the end of the fish ladder we, HFF, operate a fish trap from early February through the middle of June. Three times a week we check the Buffalo River fish trap and collect data on species, length, sex, and life histories via passive integrated transponders (PIT) tags if one is present, before passing the fish upstream above the hydroelectric facility. This data allows us to quantify run size, run timing, number of spawners, number of return spawners, and other valuable information needed to monitor and understand the Henry’s Fork Rainbow Trout population. Last week (June 11, 2018) we opened the Buffalo River fish trap and concluded our monitoring of the 2018 Rainbow Trout spawning migration. Below is a quick summary of the 2018 data.

Buffalo River and Buffalo River fish ladder looking down stream from the hydroelectric facility.

Notable Numbers

905 fish captured (February 16 through June 11)

Rainbow Trout
- 112 spawning sized fish (greater than 12 inches)
  - 15.8 inches – median size
  - 23 inches – largest fish
- 576 juvenile fish (less than 12 inches)
Brook Trout
- 180 captured
  - 6.0 inches – median size
  - 10.9 inches – largest fish
Other (sculpin, dace, mountain whitefish, etc.)
- 37 captured

Figure 1. Rainbow Trout spawning run for fish migrating from the Henry’s Fork River up the Buffalo River, February 15 through June 1, 2006-2018.

Summary

The 2018 data from the Buffalo River fish ladder verified expected trends for this springs spawning run size, in addition to several surprises relating to juvenile Rainbow Trout abundances. There were 24 fewer spawning sized Rainbow Trout that migrated up the Buffalo River this year than in 2017 and roughly 35 fewer spawning sized Rainbow Trout than the 13 year average (Figure 1). While unfortunate, this comes as no surprise. Winter flows through Box Canyon during critical juvenile life-stages were relatively poor for cohorts that now comprise most of the adult Rainbow Trout in the Box Canyon population (cohorts from 2014, 2015, and 2016). Low winter flow conditions increased mortality for juvenile Rainbow Trout in these cohorts and the low survival rates are being seen in the lower than average abundances of Rainbow Trout in the Box Canyon reach. Since abundances of Rainbow Trout in Box Canyon are lower than last year, and lower than the 30-year average, it is not surprising that the number of Rainbow Trout migrating from the Henry’s Fork to the Buffalo River decreased as well.

A surprising and encouraging note about the 2018 Buffalo River fish ladder monitoring was the number of juvenile Rainbow Trout using the Buffalo River fish ladder. We passed 576 juvenile Rainbow Trout in 2018, compared to 317 in 2017, and XXX for the 13 year average. If you view the number of juvenile Rainbow Trout at the Buffalo River as an indicator for juvenile winter survival in Box Canyon, conditions seemed to be favorable for juveniles Rainbow Trout despite less than ideal flows in 2017 (mean flow in Box Canyon during the coldest 90-day period of 305 cfs). In addition, 2018 estimate of age-2 Rainbow Trout by the Idaho Department of Fish and Game was 2,865 (700 more age-2 Rainbow Trout than 2017). These above expected abundances of juvenile Rainbow Trout, coupled with great winter conditions in 2018 (mean flow in Box Canyon during the coldest 90-day period of 705 cfs!) paint an optimistic picture of increased juvenile survival and increased Rainbow Trout abundances in Box Canyon over the next few years.

Two other notes to mention from the 2018 Buffalo River fish ladder have to do with two of our other studies. First, we didn’t catch any spawning sized Rainbow Trout that had passive integrated transponder (PIT) tags. In 2014 and 2015, juvenile Rainbow Trout migrating up the Buffalo River fish ladder in the fall were tagged with PIT tags. These tags effectively gace each fish a unique identification so we could collect information about their movements between the Henry’s Fork and Buffalo River. From 2014-2016, we collected some great information about juvenile survival and movement (find out more here, here, and here) but these fish are approaching the upper end of their life cycle (5+ years old) and it is unlikely we will be capturing or reporting any new life-history information from PIT tagged fish in 2014 and 2015.

Last, gill lice were found on 24 out of 112 (21.4%) spawning sized Rainbow Trout at the Buffalo River fish ladder. This is very close to the infestation rate of fish caught in Harriman Ranch reported to us via our gill lice study in 2017. This infestation rate has decreased roughly 15% from the inception of the gill lice study in 2016. For more information on gill lice and our gill lice study, check out these links… … …