Lahontan Cutthroat Trout

Oncorhynchus clarkii henshawi

The Lahontan cutthroat trout has a long evolutionary history of isolation and adaptation in the Lahontan basin in Nevada, eastern CA and southern Oregon, and is one of the four major sub-species of cutthroat trout. Its eastern counterpart, the “Humboldt cutthroat trout”, is actually considered to be a separate un-described sub-species but both forms are treated together here as Lahontan cutthroat trout (LCT), as they are by management and regulatory agencies.

LCT historically expressed a variety of movement life histories, including resident, fluvial, and lacustrine (lake) forms. The western lake form is uniquely adapted to persist in the desert terminal lakes of the Lahontan basin. It has an unusually high tolerance for alkaline and saline waters and its position as top predator in the food web made it the largest of the cutthroat trout: it holds the world-record size of 41 lb from Pyramid Lake. The eastern form is adapted to the highly variable desert and montane stream environment of the Great Basin. However, non-native fishes, and habitat fragmentation and degradation have led to the decline of many populations. Only 8.6% of the historical stream habitat is currently occupied and the fish has been lost from almost 99% of its historic lake habitat; it was one of the first species to be listed under the Endangered Species Act (first as Endangered and later as Threatened) in 1975.

Key CSI Findings

• 25% of occupied subwatersheds had total CSI scores of 71 points or higher (of a total of 100), though the majority (59%) had a moderate score of 61-70.
• LCT have been extirpated from many subwatersheds and subbasins, but within remaining subwatersheds range-wide condition scores showed they are fairly well-distributed and occupy a relatively high amount of 2nd order and larger habitats.
• The overall density and extent of populations was low, but LCT generally maintain high genetic integrity and low disease risk.
• Habitat integrity was high in many parts of the historical range, indicating many possible opportunities for reintroductions.
• Much of the historic LCT range will be highly impacted by climate change, with almost every part of the range being at high risk for at least one, if not several, factors.

Context Map

Non-native fishes are a major concern for LCT and have been implicated in the majority of recent extirpations. However, most LCT populations remain pure (93% of subwatersheds scored highest for genetic purity) and in some cases LCT have been able to resist or potentially coexist with non-natives such as brook trout, especially in highly variable eastern habitats where LCT are possibly better adapted. Since much of the historical LCT habitat remains in fairly good condition with moderate to high Habitat Integrity scores, there may be ample opportunity to restore or reintroduce LCT into habitats where they have been extirpated if non-natives are aggressively controlled. The majority of conservation populations are in isolated small stream reaches which could be restored and reconnected to improve population sizes and extents.

Self-sustaining original strains of LCT still persist in 2 historic lake habitats (Independence and Summit Lakes), both of which are threatened by non-native fishes. In other historic lakes where LCT could potentially be restored, non-natives, isolation from spawning tributaries and water quality need to be addressed to ensure successful conservation of this important ecotype of LCT.

Our analyses show much of the historic LCT habitat will be at high risk from climate change. Large portions of higher-elevation habitats are at moderate to high risk from fire as well as flooding, and all habitats but those in the higher elevations in the Sierra Nevada mountains and a few isolated ranges are at moderate to high thermal and drought risk. Collectively, almost every part of the historic range is at high risk for at least one or more factors. Bolstering populations by improving and reconnecting watersheds to ensure access to multiple localized habitats and refugia and encouraging migratory life histories will be important for securing LCT in the future.

Prepared by Helen Neville 10/15/09, with general background information taken from
Behnke, R.J., 2002 Trout and Salmon of North America, and the USFWS 5-year review (2009).

Table 1. CSI scoring result summary for Lahontan cutthroat trout

Conservation Strategies Map