Humboldt Cutthroat

Oncorhynchus clarki ssp.

Native Trout Fly Fishing

A Humboldt Cutthroat from small desert stream in Nevada.

Introduction

The Humboldt Cutthroat is a minor subspecies of cutthroat and is most closely related to the Lahontan cutthroat. This trout is native to the sage brush country of the Humboldt River drainage in Nevada. Unlike the Lahontan cutthroat which adapted to a lake environment, the Humboldt cutthroat primarily dwell in harsh environment of the region's high desert streams.

Life History Information

Due to the region that it lives in the Humboldt cutthroat has become very well adapted to life in unstable stream environments that experience a high degree of variability in flow and temperature. The Humboldt cutthroat occurs as fluvial, stream resident and lacustrine populations. Although the fluvial component of the Humboldt cutthroat was once widespread throughout the basin, it has been eliminated from much of its native range. Today the strongest populations of this life history form occur in the Mary's River drainage (Elliot and Layton 2004). These fish spawn in small tributary streams, than migrate to larger rivers to feed and over-winter. Most creeks that hold this trout dry up in the desert before connecting to larger streams in the drainage and as such spawning occurs to during spring, corresponding with the highest flows of the season (Trotter 2008). During this time mature cutthroat run up even the smallest streams and seek out clean gravel to spawn in. As soon as spawning is complete the adult fish migrate back down to the river, where there is enough flow to support them throughout the the dry season. Young-of-year cutthroat hatch during the summer and seek out pools to allow them to survive the extreme temperatures, which may exceed 80 degrees Fahrenheit during summer or fall to near freezing in the winter (Trotter 2008). By the following spring the juvenile fish will ride the high flows downstream to the river where they will feed and grow. Humboldt cutthroat are oppurtunistic feeders, with the majority of their diet being composed of aquatic and terrestrial insects although they will feed on fish when they are available (Dunham et. al 2000).

There are only a couple of known lacustrine populations of Humboldt cutthroat, all of which occur in degraded reservoir environments. These reservoirs are very eutrophic and as such experience massive blue-green algae blooms as well as temperatures in excess of 80 degrees Fahrenheit during the summer (Benke 2002). Never the less Humboldt cutthroat survive and even thrive in this environment, reaching sizes of five to ten pounds. Typically these fish will spend one to two years feeding in the reservoir before migrating up one of the small tributary streams for spawning. Spawn timings and migration patterns for these lacustrine populations appear to be very similar to those of fluvial populations.

The stream resident form is doing much better today than the other life history forms. Stream resident populations of Humboldt cutthroat are found in some of the smallest and most unstable streams in the Humboldt watershed. During dry years populations typically decline in these streams, but are quick to rebound once conditions improve. Platts and Nelson (1983) showed that in a small Humboldt basin stream the population of cutthroat increased by three times over two consecutive high water years. It is believed that part of this rapid increases in population size due to a high survival rate for eggs and juvenile fish. According to Trotter (2008) the eggs of Humboldt cutthroat hatch faster than typical of other subspecies of cutthroat. First spawning for these fish typically occurs at an age of two to three years and generally stream resident fish have an average size of 6" to 12" at maturity. The diet for these fish is composed mostly of aquatic and terrestrial insects and the maximum lifespan tends to be around four to five years old (Behnke 2002).

Status

It is believed that during wet years these fish were historically found in as much as 2,200 miles of stream habitat. Today this number has been greatly reduced and depending on conditions Humboldt cutthroat inhabit between 180 and 250 miles in 71 different streams across the basin (Elliot and Layton 2004). As a result of this substantial decline, the Humboldt cutthroat were listed as endangered under the Endangered Species Act in 1969 along with the Lahontan cutthroat, although they were downlisted to threatened in 1975 to increase the feasibility of restoration efforts (CRS 1998). Habitat degradation and the introduction of non-native trout have been the two leading causes of decline for the Humboldt cutthroats, although over harvest has also played a role in some waters. Oddly enough habitat degradation has also been a bit of a blessing in disguise for these cutthroat in some cases. Due to the harsh conditions the these fish have adapted to survive in, they have managed to maintain a hold in many degraded streams in which introduced trout could not survive (Platts and Nelson 1983). In fact today there are only four streams in the upper Humboldt River drainage that are known to contain rainbow-cutthroat hybrids (Elliot and Layton 2004). It is only in the "highest quality" cold mountain streams that Humboldt cutthroat have been completely replaced by introduced species.

 

Like trout native to arid regions across the West, cattle grazing and irrigation have both had a major impact on the populations of Humboldt cutthroat. Irrigation has been an issue for trout in the Humboldt drainage causing low flows and high temperatures and in some cases complete dewatering in the lower reaches of streams. As cattle go, typically the best area for free-range cattle to graze is in the lush riparian zone along streams (Platts and Nelson 1985). This zone is vital to the survival of trout and helps to keep water temperatures cool and stream banks stable, reducing sedimentation. As cattle eat the riparian vegetation the amount of shade drops and water temperature rise to levels lethal to the trout. The other affect is that stream banks become less stable and more prone to incising. During the high flows of spring when the cutthroat are spawning, this equates to an increase in the sediment load and lower survival rate for eggs. The effects of cattle grazing could easily be seen in a stream that I visited for these trout back in 2006. There were sections of this stream where cattle had completely caved the banks of the stream in causing it to run murky and cut itself into a ten to fifteen foot deep canyon. Today the US Forest Service and Bureau of Land Management have began to create fenced off enclosures along the riparian zones of streams containing Humboldt cutthroat and with the improved habitat the populations are beginning to rebound in many watersheds.

Description

Due to a long period isolation between the different populations of Humboldt cutthroat there is a high degree of variation in their features across their native range. As such the description provided here is generalization for the subspecies across its native range. The coloration of this trout tends to be and olive-bronze to copper color on the back which transitions to a pale yellow with hints of red on the side. Humboldt cutthroat have a dull almost dirty looking red or pink stripe along their lateral line and on their gill plates, which tends to be much more pronounced in spawning males (Behnke 2002). They also have a red cutthroat marked below the lower jaw and many stream resident populations will retain bluish purple colored parr marks into adulthood. Humboldts tend to be a little more sparsely spotted than Lahontan cutthroat and have spots spread fairly evenly across their backs and sides of their bodies. A trait that I noticed in the Humboldt cutthroat the I have seen is that they tend to have a streamlined appearance, with the jaw common to more of a point than what I have seen in other subspecies.

Stream Resident Form

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Native Range

Native Trout Fly Fishing

A map of the native range of the Humboldt Cutthroat trout. Data Source: Behnke (2002) and Trotter (2008).