Buffalo River - Spring Monitoring Summary

Spring monitoring of the Buffalo River Fish Ladder took place from February 18 to June 15, 2016. The trap at the top of the ladder was checked three times a week, typically every Monday, Wednesday, and Friday. All fish trapped in the ladder were identified to species, measured for length, and then released upstream to continue their up-migration. Rainbow trout were also identified to sex when possible and those with missing adipose fins were noted as “recaptures,” trout we had previously tagged in 2014 or 2015, and scanned to obtain their individual tag code to track migration history in our database. The screen at the top of the ladder, installed to trap fish migrating upstream, was removed on the last day of monitoring to allow free passage through the ladder until monitoring efforts convene next spring.

During the 2016 spring spawning season, 202 spawning-sized rainbow trout (defined as trout greater than or equal to 300 millimeters—about 11.8 inches—in length) migrated upstream. In previous blogs, I anticipated this year’s spawning run to be above average compared to the last decade. In fact, this year’s run was the median – at least what you would expect to see in half the years. I was incorrect in my prediction because I considered total spawning run size alone and did not account for differences in run timing from year to year. So although this year’s spawning run wasn’t above average overall, spawning numbers were above average between mid-February and mid-May because the run peaked (meaning half of all spawners had migrated upstream) two and a half weeks earlier than what we’ve seen in the last decade – peaking on April 18th compared with the 10-year average of May 5th. Although we have been unable to find a statistically significant relationship between environmental factors like streamflow and air temperature to explain spawning run size and timing (eleven data points just isn’t enough), it’s possible that the mild winter we had in Island Park triggered an earlier spawning migration between the Henry’s Fork and Buffalo River. However, the start of our sampling season remained the same, beginning in mid-February, so we may have missed trapping a handful of up-migrating trout in early February. Thus, our reporting of 202 spawning-sized rainbow trout migrating upstream is probably a slight underestimate.

Over the course of the spring monitoring season, interns, technicians, volunteers, and field-trip students from Ashton Elementary School and BYU Idaho measured and released 963 individual fish upstream. Most of these fish (78%) were rainbow trout, which is about 12% above average than what we have seen over the last decade.

Of the 989 total fish trapped in the ladder, 26 were recorded as mortalities – fish that had died in the trap or during handling. This 2.5% mortality rate cannot be compared to that of past seasons due to differences in data reporting, but it is relatively low and not cause for concern. Half of the fish that did not survive the trapping/handling processes were rainbow trout less than 170 mm (6 inches) in length, while the other half was mostly made up of brook trout, red-sided shiners, and dace. Three mortalities were rainbow trout around 9 inches in length. The Henry’s Fork Foundation does its best to minimize fish trap and handling mortality by checking the trap and releasing fish regularly (every 48-72 hours) during the 12 week trapping period, using aerators and ice to maintain dissolved oxygen content, cool temperatures, and stress levels in holding containers, and only trapping when we have a specific scientific question we are seeking to answer (in this case – how and why does the spring spawning migration to the Buffalo River from the Henry’s Fork change annually?). As with any fish-handling process, including catch-and-release angling, some of the fish that do not survive the process would die of natural causes in the environment anyway.

As shared on Facebook and in previous blogs, some of the fish that migrated upstream this spring are fish we have encountered migrating upstream inthe past. In total, we had nine rainbow trout that were recorded as “recaptures” – denoted by the absence of their adipose fins. Eight of the nine trout were between 370 and 576 millimeters (14.5 to 22 inches), while one was 255 millimeters (10 inches). Seven of the nine trout had retained their tag since initially being captured, while the remaining two were missing an adipose fin, but did not have a detectable tag code.

Given the size of these trout, that they were previously captured and identified as spawners migrating upstream, and the fact that rainbow trout typically return to their natal streams to spawn, it is probable that these trout were born in the Buffalo River, spent their life in the Henry’s Fork, and are returning to the Buffalo River to spawn for at least the second time in their lifetime (genetic data also support this conclusion). Data from the downstream tag detection antenna indicate that spawning-size trout typically spend around a month in the Buffalo River before migrating back to the Henry’s Fork. Of the seven tag codes we obtained in upstream-migrating fish this season, four were detected by the antenna on or before June 15, the last time the antenna was downloaded. These trout had spent between two and eight weeks in the Buffalo River before migrating back downstream to the Henry’s Fork. Of the remaining three trout yet to be detected two should have already returned downstream and one will likely move downstream sometime in mid-July—assuming they survived spawning.   

Overall, spring 2016 was a successful monitoring season. More spawning-sized rainbow trout were recorded moving upstream than in 2014 and 2015, and the operation of the Buffalo River Fish Trap served as a great outreach tool to a variety of audiences. Local fifth graders and college students learned about fish identification, data collection and analysis, why fish migrate to the Buffalo River, and what the Henry’s Fork Foundation is doing to monitor and facilitate that migration.

Stay tuned for a comprehensive report on the Buffalo River Fish Ladder 2006-2016.

Photo credit: James Chandler