Yellowstone Cutthroat Trout

Oncorhynchus clarkii bouvieri

Yellowstone cutthroat trout (YCT) are spring/early summer spawners with three main life history patterns found among and even within populations. Resident fish remain in a relatively small stream area their entire life, while fluvial and adfluvial fish migrate from mainstem rivers and lakes, respectively, to tributaries to spawn.

Historic Range Relief Map

Historically, these fish occupied much of the Yellowstone River basin spreading across southern Idaho, Montana, northwestern Wyoming, and small regions of Nevada and Utah. Today, however, genetically pure YCT are found in less than half of their historic range and are mostly restricted to waters within the boundaries of Yellowstone and Grand Teton National Parks and surrounding Forest Service lands. Despite an earlier petition, the species is not Federally listed under the ESA, but is widely characterized as a “sensitive species” or “species of special concern” by agencies.

The 88,000-acre Yellowstone Lake still houses the largest population of cutthroat – of any kind – to be found anywhere in the world. This stronghold, however, is now in jeopardy, as the highly piscivorous lake trout was introduced illegally and repeatedly into the lake in the 1980s and ‘90s. Aside from the Yellowstone Lake population, other YCT populations have also been impacted by various introduced salmonids including rainbow trout, which pose a hybridization threat, as well as brown and brook trout. Whirling disease and the New Zealand mud snail are also now present in drainages within the Yellowstone National Park and may present future threats to the species.

For the CSI, YCT were scored for various factors falling under the 4 basic categories of Range-wide Condition, Population Integrity, Habitat Integrity, and Future Security, and these scores were used to prioritize management actions.

Key CSI Findings

• 49% of watersheds within historic range are currently occupied (= 511 of 1042)
• 58% of extant populations had a total CSI score > 80 (out of 100)
• Median Range-wide Condition score (extant populations only) = 22/25 (range 10-25)
• Median Population Integrity score (extant populations only) = 18/25 (range 6-24)
• Median Habitat Integrity score = 19/25 (range 5-25)
• Median Future Security score = 21/25 (range 13-24)
• % of watersheds prioritized for specific management actions:
  • 31% protection
  • 50% reintroduction
  • 19% restoration

Many watersheds have lost YCT populations so only Habitat Integrity and Future Security could be scored, leading to a low total CSI score for these watersheds. Several specific threats to YCT were identified through the CSI. Habitat Integrity scores were negatively affected by poor land stewardship and water quality, while many Population Integrity scores were lowered by introgression with exotic species (threatening genetic integrity) and vulnerability to disease. Under the Future Security category, climate change and introduced species posed the biggest threats to the future welfare of the species. Due to the loss of local populations in many watersheds still retaining relatively high-quality habitat, reintroductions are a priority for management activities. Reintroductions must, however, be planned in conjunction with management of exotic species, the primary threat to YCT.

Written by Helen Neville, TU, 11/16/06

Conservation Strategies Map

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