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Eagle Lake Rainbow Trout

Oncorhynchus mykiss aquilarum

An Eagle Lake Rainbow Trout caught during spring in Eagle Lake, California.

Introduction

The Eagle Lake Rainbow Trout was first described as a subspecies of Rainbow Trout Salmo gairdneri aquilarum in 1917 and is native to Eagle Lake California (Snyder 1917). Eagle Lake is located within the Lahontan basin and would have been expected to contain Lahontan Cutthroat. However, genetics work has indicated that the Eagle Lake Rainbow Trout do not appear to have any Cutthroat genes and are most closely related to Coastal Rainbow Trout from the Feather River (Busack et al. 1980, Simmons 2011). Eagle Lake is a highly alkaline and Eagle Lake Rainbow Trout can tolerate higher levels of alkalinity than any other form of Rainbow or Redband Trout (Behnke 2002). The record Eagle Lake Rainbow Trout was caught in 1988 and weighed 11 ¼ pounds (5.1 kg).

Life History Information

The Eagle Lake Rainbow Trout primarily express an adfluvial life history pattern, although it is possible that some fish may express a stream resident life history in the upper headwaters of Pine Creek. If a stream resident form exists, it is likely quite limited as the upper reaches of Pine Creek are almost exclusively inhabited by Brook Trout currently (Carmona-Catot et al. 2010). Eagle Lake Rainbow Trout generally mature between age-2 to age-3, with some individuals documented up to age-11 (Moyle et al. 2017). Historically Eagle Lake Rainbow Trout spawned in Pine Creek, the primary tributary to the lake, although some other smaller tributaries may also support some level of spawning. The spawning migration into Pine Creek begins when water temperatures reach 40° F (4.4° C) and generally stops when temperatures exceed 50° F (10° C) (CDFW et al. 2015). Most of the Pine Creek watershed is intermittent, only flowing for 2-3 months of the year; during the winter rainy season and spring run-off. Due to this Eagle Lake Rainbow Trout only have a short window from late-February to early-May to migrate upstream in Pine Creek to the perineal spring fed reaches in Stephens Meadows and Bogard Spring Creek about 28 miles (45 km) above the lake. Like other Rainbow and Redband Trout, the Eagle Lake Rainbow Trout is iteroparous and given the chance will migrate back to Eagle Lake after spawning. It is thought that juvenile Eagle Lake Rainbow historically reared in upper Pine Creek for 1-2 years prior to migrating downstream to the lake during spring runoff (Moyle et al. 2017). Eagle Lake Rainbow Trout sampled in Bogard Spring Creek ranged from age-0 to age-4, with their size ranging from 4-6” (10 to 140 cm) at age-2, 6-7” (14 to 19 cm) at age-3 and 9” (22 cm) at age-4 (Carmona-Catot et al. 2011). There is evidence that some spawning occurs in the intermittent reaches of lower Pine Creek, with these fish producing fry migrant offspring, that travel to the lake immediately after swim-up (CDFW et al. 2015). Currently, almost all Eagle Lake Rainbow Trout are spawned in a hatchery environment and little natural production is occurring in the watershed.

Upon entering Eagle Lake, the fish grow quickly, and hatchery yearlings stocked at 10” (25 cm) and are usually about 16” (40 cm) by the end of their first year, 18-22” (45-55 cm) by the end of their second year and 24” (60 cm) by the end of their third year (CDFW et al. 2015, Moyle et al. 2017). The diet of the Eagle Lake Rainbow Trout changes with age and season. Yearling trout feed extensively on zooplankton, such as daphnia as well as aquatic insects, leeches and amphipods. However, by August most Eagle Lake Rainbow Trout transition to feeding on juvenile tui chub (Moyle et al. 2017). During the fall, Eagle Lake Rainbow Trout will often switch back to invertebrates have been observed flipping over rocks to get at aquatic insects and snails living on the underside of them.

Status

The environment that the Eagle Lake Rainbow Trout inhabits would be considered hostile to many salmonids and while the Eagle Lake Rainbow Trout is specially adapted to deal with this challenging environment, human caused alterations have made survival for this trout much more challenging. The initial decline of the Eagle Lake Rainbow Trout was likely due to an extensive commercial fishery, which operated in the late-1800’s until commercial fishing for trout was banned in California in 1917. During this same period, logging, grazing and agriculture all had negative impacts on these trout. These impacts were felt most on Pine Creek, where the stream channel became down-cut, water temperatures and sediment load increased, and the amount of available water decreased (Moyle et al. 2017). As Pine Creek is the primary tributary and already had intermittent stretches, this greatly reduced the success of spawning fish in the stream. In addition to over-harvest and habitat destruction, in the early 1900’s, Largemouth Bass and Brown Bullhead were introduced to the lake and successfully established populations and began preying on and competing with the Eagle Lake Rainbow Trout. With the impacts to these trout building, in 1923 the Bly Tunnel was constructed, which diverted water from Eagle Lake into Willow Creek for irrigation. This coupled with a severe drought in the 1930’s resulted in a significant reduction in lake levels and increase in the alkalinity in the lake. The combined effect of all of these stressors was that by the 1930’s trout had become scare in Eagle Lake and Pine Creek. The only silver lining was that the drop in lake levels and increase in alkalinity resulted in the Bass and Bullhead dying out in the lake and after the diversion failed to live up to expectations, the Bly Tunnel was sealed, and the lake was allowed to refill.

With the decline in trout abundance in the Eagle Lake Basin, California turned to hatcheries to help supplement the population in the lake. In 1959, a permanent egg-taking station and trap was constructed at the mouth of Pine Creek and the trout population in the lake became dependent on hatchery stocking (CDFW et al. 2015). Given the degraded nature of Pine Creek, this action likely saved the Eagle Lake Rainbow Trout, but also came with several negative side effects. With the Eagle Lake Rainbow Trout being dependent on hatchery spawning and rearing for over 60 years, the population has become domesticated, as hatchery practices select for fish that are adapted to being reared in a hatchery over those adapted to surviving in the wild. Additionally, due to artificial spawning and a low number of trout that was used to found that hatchery program, there has been some level of inbreeding in the population leading to genetic issues. Despite this, the Eagle Lake Rainbow Trout still thrive once stocked in the lake and the genetic diversity of the population is higher than expected and improved management practices in recent years have likely reduced fitness loss issues (Carmona-Catot et al. 2011). However, given the length of time the Eagle Lake Rainbow Trout has been dependent on hatchery propagation, it is currently unclear how successful they would be at spawning in the wild if the habitat and flows in Pine Creek can be improved. In 2006 and 2007, the California Department of Fish and Wildlife (CDFW) transported mature Eagle Lake Rainbow Trout into the perineal stream reaches in the upper watershed and the fish appeared to spawn successfully (Carmona-Catot and Moyle 2013). Additionally, preliminary tests in 2010 and 2011, passed fish upstream of the trap in April to spawn naturally and fry were collected at the trap migrating downstream in June indicating some level of successful spawning (Moyle et al. 2017). Additionally, there is evidence that at least some of the trout passed over the trap have been able to navigate the 28 miles (45 km) to their historical spawning grounds in the perineal reaches of Pine Creek.

While Eagle Lake Rainbow Trout show signs that with improvements to the habitat in Pine Creek, they could become self-sustaining once again, there is one major hurdle that needs to be overcome first; introduced Brook Trout. Brook Trout were stocked in Pine Creek in the 1930s through 1949 to improve recreational fishing and quickly established a population. Brook Trout densities in the upper reaches of Pine Creek and its tributaries are among the highest observed and competition with juvenile Eagle Lake Rainbow Trout is expected to severely limit any reintroduction efforts. If stream resident Eagle Lake Rainbow Trout exist in upper Pine Creek, they are currently at extremely low abundance likely due to competition with Brook Trout. In Redband populations, stream resident life histories are critical to maintaining the populations in years when lake conditions are not conducive to trout and reestablishing this life history is likely to be an important step in restoring the Eagle Lake Rainbow Trout. Carmona-Catot et al. (2010) showed that electrofishing removal of Brook Trout in Bogard Spring Creek, the primary tributary to Pine Creek, was successful at significantly decreasing the abundance of these trout and could be an effective tool for restoring Eagle Lake Rainbow Trout. However, they also detected Brook Trout from Pine Creek moving into Bogard Spring Creek, and as such it would be imperative to also eliminate the Brook Trout population in that stream as well. While Brook Trout are likely a major limiting factor this may the full extent of this is poorly understood. In other streams where both Brook Trout and Rainbow or Redband Trout are found, the Rainbow or Redband Trout are generally the dominant species (Fausch 1988, Miller et al. 2014), so the true effects of the Brook Trout population on Eagle Lake Rainbow Trout are unknown.

Recently, significant habitat improvements have been made to Pine Creek, in an effort to improve the flow and habitat for Eagle Lake Rainbow Trout (Pustejovsky 2007, CDFW et al. 2015). However, climate change makes the efficacy of efforts unclear present a significant threat to the Eagle Lake Rainbow Trout. In recent years, California has been hit by several extreme droughts. The drought of 2013-2015 resulted in the complete drying of the north basin of Eagle Lake and the trout still do not seem to have fully recolonized the area. In 2021, drought conditions were extreme enough that the boat basin at the Eagle Lake marina was dry in late-May, when the lake should have been near its highest levels. Beyond the lake levels, these droughts result in Pine Creek flowing to Eagle Lake only for short windows or not at all, reducing the likelihood of reestablishing a self-sustaining population. It is expected that snowpack in the basin will be reduced by 65-97% by 2100 and as such flows will be more likely to exist earlier in the winter during the rainy season with lower summer base flows (Thompson 2009). As such spawners are likely going to need to shift their runs earlier to adapted to changes in stream flows. These droughts also bring an increased risk of wildfires and several large wildfires have burned the area around Eagle Lake and Pine Creek, such as the Whaleback Fire, which burned 18,703 acres, putting further stress on the Eagle Lake Rainbow Trout.

The Eagle Lake Rainbow Trout have been petitioned to be listed under the Federal Endangered Species Act three times but have been determined to not merit listing each time (USFWS 1995, USFWS 2012 and USFWS 2016). The primary reason why the Eagle Lake Rainbow Trout has avoided being listed during the most recent review, is due to a multi-agency conservation strategy adopted in 2015 (CDFW et al. 2015). The State of California currently considers the Eagle Lake Rainbow Trout a State Species of Special Concern and while proposed listings have been denied, the Eagle Lake Rainbow Trout is currently far from self-sustaining and without human intervention would likely go extinct.

Description

The Eagle Lake Rainbow Trout is similar in appearance to Coastal Rainbow Trout, but generally have finer scales than Coastal Rainbow Trout. Eagle Lake Rainbows have a greenish blue to bronze color on their backs and their sides are silvery, transitioning to white on the belly. These trout have a rosy to reddish bronze colored stripe along their lateral line and on their gill plates. They have small irregular shape spots, which are often concentrated on the upper half of the body and on the dorsal, caudal and adipose fins, but may also be present on the lower body on some fish. The dorsal fin is a greenish brown, while the lower fishes may be a peachy yellow color to a purplish red with a white tip on the dorsal, pelvic and anal fins. The snout is rounded, especially on larger fish and the caudal fin it squared and not forked.

Adfluvial Form

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Eagle Lake Rainbow Trout Links

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