• Bryce Oldemeyer

Yellowstone Cutthroat Trout Management on the South Fork Snake River (Update 2021)

Updated: Jul 14

In February 2019, I wrote a blog highlighting native Yellowstone Cutthroat Trout management on the South Fork Snake River that provided background on Yellowstone Cutthroat Trout and Rainbow/Hybrid Trout dynamics in the system, management strategies aimed to protect Yellowstone Cutthroat Trout, and how reducing non-native Rainbow/Hybrid Trout abundances can help protect and promote native Yellowstone Cutthroat Trout in the system. This is a continuation of that blog.

Highlights

  • In 2020, the combined abundance of all trout in the South Fork Snake River at the Conant reach was the highest on record, and the abundance of Rainbow Trout and Rainbow x Yellowstone Cutthroat Trout hybrids was the second highest on record.

  • Hybridization and interspecies competition from non-native Rainbow Trout and Hybrid Trout pose a serious threat to native Yellowstone Cutthroat Trout in the South Fork Snake River.

  • Public opinion supports protecting and conserving native Yellowstone Cutthroat Trout in the South Fork Snake River.

  • Prior management practices have slowed the increasing trend of Rainbow and Hybrid Trout in the system but efforts have not been enough to reverse or stop the trend.

  • In an effort to conserve and protect Yellowstone Cutthroat Trout in the South Fork Snake River by decreasing Rainbow/Hybrid Trout abundances, the Idaho Department of Fish and Game has set a goal to remove approximately 12,000 Rainbow and Hybrid Trout from the South Fork Snake River in mid-April through the end of May.

Introduction

Yellowstone Cutthroat Trout, Rainbow Trout, and Rainbow x Yellowstone Cutthroat hybrids (hereafter referred to as, “Hybrid” Trout) all contribute to the world class South Fork Snake River fishery. Ideally, Yellowstone Cutthroat Trout and Rainbow/Hybrid Trout could cohabitate with minimal interspecies competition and hybridization. Unfortunately, Yellowstone Cutthroat Trout and Rainbow/Hybrid Trout share similar life history strategies, utilize the same habitat, and compete for the same resources, with Rainbow/Hybrid Trout exhibiting a competitive advantage in the system. Rainbow/Hybrid Trout would likely hybridize and outcompete native Yellowstone Cutthroat Trout if left unmanaged in the South Fork Snake River. Since 2004, at the direction of the public and the Idaho Department of Fish and Game Commission, the Idaho Department of Fish and Game (IDFG) has been managing the South Fork Snake River to protect and promote native Yellowstone Cutthroat Trout. Specifically, their overarching goals are to protect the genetic integrity and population viability of Yellowstone Cutthroat Trout and decrease the threats Rainbow/Hybrid Trout pose by reducing their abundance to less then 10% in the upper South Fork reach. These have been IDFG's goals since 2004 and IDFG has been working towards these goals through a three-prong approach of excluding Rainbow/Hybrid Trout from critical Yellowstone Cutthroat Trout spawning habitat, reducing Rainbow/Hybrid Trout spawning success through a springtime freshet flow, and reducing abundance of adult Rainbow/Hybrid Trout through angler harvest. The weir program has been effective at minimizing hybridization during Yellowstone Cutthroat Trout spawning at major tributaries but angler harvest and the spring freshet have been marginally effective at reducing Rainbow/Hybrid Trout abundances. Rainbow/Hybrid Trout abundances have continued to increase in proportion to Yellowstone Cutthroat Trout abundances and pose a serious threat to the Yellowstone Cutthroat Trout population (Figure 1). To prevent future declines in the Yellowstone Cutthroat Trout population, IDFG intends to boat electrofish Rainbow/Hybrid Trout when they congregate at redds during the spawning season in 2021, remove them from the South Fork Snake River, and translocate them to other public waterbodies in the Upper Snake River region. The Henry’s Fork Foundation - South Fork Initiative (HFF-SFI) recognizes this is not an ideal situation but we fully support the management decision in order to protect the Yellowstone Cutthroat Trout population.

Figure 1. Rainbow and Hybrid Trout (RBT) and Yellowstone Cutthroat Trout (YCT) abundances at the Conant survey reach of the South Fork Snake River, ID, 1982-2020 (credit: IDFG).

Question and Answer

How do Rainbow Trout impact native Yellowstone Cutthroat Trout?

Rainbow Trout impact native Yellowstone Cutthroat Trout in the South Fork Snake River in one of two ways; 1) introgression (hybridization) and 2) competition for limited resources. The IDFG released a blog discussing hybridization and introgression of Rainbow Trout and Yellowstone Cutthroat Trout in the South Fork so I will refrain from digging into that topic. Instead, I will focus on interspecies dynamics and competition between the two species.


Over thousands of years, native Yellowstone Cutthroat Trout have evolved and adapted to the South Fork system. They developed a diverse life history that maximized their reproductive success and survival within a natural South Fork system. In general, Yellowstone Cutthroat Trout in the South Fork exhibit a life history that is centered around migrating into tributaries and spawning after peak flows, typically in late-May through July. This allowed Yellowstone Cutthroat Trout to spawn in clean, cold, and clear headwater habitat but it also increased the chance that eggs didn’t get dislodged from the substrate during peak flows. This was a favorable strategy for a trout in a natural, unaltered South Fork system.

Figure 2. General migration and spawning period for Yellowstone Cutthroat Trout in the South Fork Snake River and accompanying hydrograph of median natural (unregulated) flow for the South Fork Snake River at Irwin, Idaho. Note the majority of Yellowstone Cutthroat Trout spawning occurs on the tail end of peak flows.

Rainbow Trout were stocked in the South Fork up until 1981 and have similar life history characteristics to Yellowstone Cutthroat Trout with one seemingly small, but significant difference to Yellowstone Cutthroat Trout. Rainbow Trout spawn 1-2 months earlier than Yellowstone Cutthroat Trout and tend to spawn in the main river. In a natural South Fork system prior to human alterations (i.e. dams and flow management), spawning earlier and in the main river would have been a poor choice for a trout. Not only are you spawning before peak flows (higher chance your eggs get dislodged from the substrate) you are spawning in the main river (again, higher chance your eggs get dislodged from the substrate).

Figure 3. General migration and spawning period for Rainbow and Hybrid Trout (RHT) in the South Fork Snake River and accompanying hydrograph of median natural (unregulated) flow for the South Fork Snake River at Irwin, Idaho. Note that the majority of Rainbow/Hybrid Trout spawning occurs prior to peak flows.

However, in a regulated system (i.e. system with dams and flow management) it can be advantageous to spawn earlier and in the main river if peak flows are not high. Spawning earlier allows juvenile trout a longer period to grow prior to the upcoming winter when there is a bottleneck for juvenile trout and mortality is high. As a trout, being larger in size increases your chances of survival through the winter because of increased fat reserves and the fact that competitive bouts for ideal holding positions in the river are based on size, so larger fish have better access to cover and food while minimizing effort necessary to maintain the spot. This is why Rainbow Trout have an advantage over Yellowstone Cutthroat Trout in the present regulated system.

Figure 4. Mean migration and spawning period for Yellowstone Cutthroat Trout and Rainbow/Hybrid Trout (RHT) in the South Fork Snake River and accompanying hydrograph of median natural flow (flow if there weren’t dams) and median regulated flow (current flow with dams in place) for the South Fork Snake River at Irwin, Idaho. Note the timing for the regulated hydrograph is consistent with the natural hydrograph but the magnitude of peak flow is ~6,000 cfs lower. The regulated hydrograph enables Rainbow/Hybrid Trout to be successful in spawning earlier in the spring which allows juvenile Rainbow/Hybrid Trout a longer growing season prior to winter.


Can’t you do restoration projects to increase Yellowstone Cutthroat Trout production?

Then there will be more juvenile Yellowstone Cutthroat Trout being produced in the system and you don’t need to remove Rainbows/Hybrids! Unfortunately, no. Restoration projects alone will not address the overarching issue that Rainbow/Hybrid Trout displace and outcompete Yellowstone Cutthroat Trout. I explained above how Rainbow/Hybrid Trout spawn earlier in the spring and this allows juvenile Rainbow/Hybrid Trout to have a longer growing period relative to Yellowstone Cutthroat Trout prior to the first winter. This allows Rainbow/Hybrid Trout to be larger going into the winter when juvenile trout have the highest mortality rates.


Completing restoration projects that increase juvenile Yellowstone Cutthroat Trout production will not provide much benefit if juvenile Yellowstone Cutthroat Trout migrate out of tributaries into the main river and come in direct competition with larger juvenile Rainbow Trout. Below are a series of figures that try to explain this bottleneck concept.

If trout populations in the South Fork Snake River were left unmanaged, Rainbow/Hybrid Trout would outcompete Yellowstone Cutthroat Trout based on the biology and data that during the winter when resources are limited, larger juvenile trout will have the greatest chance of acquiring resources and surviving.

If there are fewer juvenile trout competing for available resources, overall juvenile trout survival will increase over the winter. This is the concept behind removing Rainbow/Hybrid Trout in order to increase juvenile Yellowstone Cutthroat Trout survival (i.e. decrease interspecies competition). If there are more resources available for juvenile Yellowstone Cutthroat Trout, their survival would increase.

Trying to produce more juvenile Yellowstone Cutthroat Trout in a system that is saturated by Rainbow/Hybrid Trout may produce a few more Yellowstone Cutthroat Trout but it is more likely there would just be higher mortality rates of Yellowstone Cutthroat Trout during the winter. The larger juvenile Rainbow Trout will continue to outcompete smaller juvenile Yellowstone Cutthroat Trout for available resources and Yellowstone Cutthroat Trout survival would remain low.