Buffalo River Fish Ladder: 2018 Summary

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 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 of 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 stopped operating the Buffalo River fish trap, concluding 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 or equal to 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 confimed some expected trends for this springs spawning run, in addition to having a few 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 from 2006-2018 (Figure 1). While unfortunate, this comes as no surprise. Winter flows through Box Canyon during critical juvenile life-stages for Rainbow trout were relatively poor for cohorts that now comprise the majority of adult Rainbow Trout in the Box Canyon population (cohorts from 2014, 2015, and 2016). Low winter flow conditions increased mortality for juvenile Rainbow Trout within these cohorts and the low survival rates are being seen in the lower than average abundances of Rainbow Trout in the Box Canyon reach (IDFG Population Survey 2018). Since abundances of Rainbow Trout in Box Canyon are lower than last year, and lower than the long term average, it is not surprising that the number of Rainbow Trout migrating from the Henry’s Fork to the Buffalo River is below the long term average well.

A surprising and encouraging note about 2018 was the number of juvenile Rainbow Trout that used the Buffalo River fish ladder. We passed 576 juvenile Rainbow Trout in 2018, compared to 317 juvenile Rainbow Trout in 2017, and the average of 512 juvenile Rainbow Trout between 2006-2018 . If the number of juvenile Rainbow Trout using the Buffalo River fish ladder is an indicator of juvenile winter survival in Box Canyon, conditions seemed to be favorable for juvenile Rainbow Trout despite less than ideal flows in 2016 and 2017. In addition, the 2018 estimate of age-2 Rainbow Trout in Box Canyon produced by the Idaho Department of Fish and Game from their population surveys was 2,865 (roughly 700 more age-2 Rainbow Trout than 2017 and 1,000 more age-2 Rainbow Trout than what our winter flow model predicted). The higher than expected abundances of juvenile Rainbow Trout, coupled with great winter flows in 2018 (mean flow in Box Canyon from December 1 through February 28 was 705 cfs! Click here to read why winter flow is imporant), paint an optimistic picture for increased Rainbow Trout abundances in Box Canyon over the next few years.

 

A couple other items to mention about the 2018 Buffalo River fish ladder. First, we didn’t catch any Rainbow Trout that had passive integrated transponder (PIT) tags. In 2014 and 2015, juvenile Rainbow Trout migrating up the Buffalo River fish ladder were tagged with PIT tags. PIT tags provide unique identification for fish so information can be collected on their movements between the Henry’s Fork and Buffalo River. From 2014-2016, we gathered great information about juvenile survival and movement (find out more here in the Buffalo River fish ladder decadel report) but these fish are approaching the end of their life cycle (5+ years old) and it is unlikely we will be reporting any new life-history information from PIT tagged fish.

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 and reported to us via our gill lice study for fish from Harriman Ranch in 2017. This infestation rate has decreased roughly 15% from the beginning of the gill lice study in 2016. For more information on gill lice and our gill lice study, click on this links. 

Additional information

If you have questions, comments, or concerns, feel free to contact me at Bryce@henrysfork.org or 208-652-3567