Oncorhynchus mykiss aquilarum

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General Information
Common Name: 
Eagle Lake rainbow trout

Conservation Status in California: Class 2, Vulnerable (Moyle et al. 2011).
Eagle Lake rainbow trout (ELRT) do not exist as a self-sustaining wild population because of dependence on hatchery propagation. Continued reliance on hatchery production to maintain the Eagle Lake population will make it increasingly difficulty to re-establish a wild population.

Life History: 

Life History: Eagle Lake rainbow trout are late maturing (usually in their third year for females) and were historically long-lived, up to 11 years (McAfee 1966). Trout older than 5 years are rare in the lake today, although individuals as old as 8-9 yrs have been caught (DFG, unpublished data). Originally, the trout spawned primarily Pine Creek, which flows into the lake on the western shore, and, occasionally, in the much smaller Papoose and Merrill creeks, which feed the southern Eagle Lake. Upstream migrations take place in response to high flows in March, April, or May. In Pine Creek, the main spawning areas were presumably the gravel-bottomed spring-fed creeks, such as Bogard Spring Creek, and headwaters in meadows, such as Stephens Meadows, about 45 km from the lake. Historically, it is likely that the trout spent at least their first 1-2 years of life in these creeks before migrating to the lake, much like coastal steelhead. However, it is possible some became stream resident although capable of producing migratory progeny, similar to steelhead and other lake-dwelling trout populations, such as Goose Lake redband trout (Moyle 2002). In recent years, progeny of adults transported to the upper basin have been found to be as old as four years. It is also possible that ELRT spawned successfully in the lower reaches of Pine Creek, with fry washing into the lake. In 2010, 26 adult spawners were released above the weir in April. In June, fry (30-40 mm TL) were collected from the trap downstream (P. Divine CDFG, pers. comm.). It is not known if these fish can survive in the lake.
Based on growth of yearling ELRT from hatchery plantings, they grew to about 40 cm by the end of their first year in the lake, 45-55 cm in the third, and up to 60 cm in the fifth year (McAfee 1966). These fish could apparently reach 3-4 kg and 65-70 cm FL (McAfee 1966). Data from the last 10 years shows that mature females produce an average of 3,300 eggs (Crystal Lake Hatchery CDFG, unpublished data, 2009). Rapid growth is the result of abundant forage combined with a delay in maturity until 2-3 years of age. This latter trait has made them highly desirable as a hatchery fish (Dean and Chappell 2005).
The life history of these fish has been significantly altered because access to spawning grounds in Pine Creek has been obstructed (see below). Today, as fish move up Pine Creek in the spring, they are trapped at a permanent weir installed by CDFG and artificially spawned. The embryos are then taken to Crystal Lake and Darrah Springs hatcheries where they are reared for 14-18 months. The fish that originated from parents captured in the trap are planted in Eagle Lake at 30-40 cm FL (CDFG, unpubl. data). 180,000-200,000 fish are planted in the lake each year, about half in the lake at the mouth of Pine Creek. In addition, up to 10,000 1+kg ‘bonus’ fish may be planted in the fall and spring for the sport fishery. Progeny of the migrant fish are also reared in other hatcheries in California and planted widely in reservoirs (Carmona-Catot et al,in press).
All trapped fish are marked to prevent sibling crosses (reduce inbreeding), to avoid using fish that have been more than one generation in the hatchery, and to select for longer lived fish to compensate for longevity reductions that may have been caused by past hatchery practices (R. L. Elliott, CDFG, pers. comm. 1998). Formerly, a hatchery program for rearing Eagle Lake rainbow trout was maintained at Mt Shasta Hatchery by using wild-caught fish as brood stock for one generation. The progeny of these fish were originally planted widely in reservoirs of the state and used as a source for brood stock in other hatcheries in California, as well as elsewhere in the western USA. Eagle Lake rainbow trout are prized because of their delayed maturity, rapid growth, and longevity.
Today, all fish used for hatchery production of Eagle Lake rainbow trout are reportedly first generation fish from the Pine Creek trap, although fish from hatchery broodstock were planted in the lake in the past (P. Divine, CDFG, pers. comm. 2009). The fish planted in waters other than Eagle Lake come from the spawning of individuals derived from Pine Creek spawners and raised to maturity in the hatcheries. Fish planted in Eagle Lake originate directly from fertilized eggs taken from Pine Creek spawners. In the past fish of both origins were planted in the lake, as were Eagle Lake rainbow trout of the domesticated hatchery strain.
Despite this long history of hatchery selection, there is evidence that Eagle Lake rainbow trout can still spawn successfully in Pine Creek. Fish that were trucked to the upper reaches of the creek in the 2000s produced young which survived and grew for two years. A thorough survey of Bogard Spring Creek revealed the presence of at least 170 rainbow trout in 2007, with most fish lengths between 105 and 150 mm FL; in 2008, only 25 ELRT were captured with lengths between 130-165 mm FL, while 34 such fish were captured in 2009 (Figure 1; Carmona-Catot et al 2010,in press). These fish survived and grew despite the presence of about 5,300 brook trout in the same reach of stream in which they were found (see management section below for details). There is some evidence that the two year old fish will try to migrate downstream to the lake during periods of high spring flow (P. Moyle, unpublished observations, 2006). In spring 2009, an Eagle Lake rainbow trout was captured in Pine Creek at 800 meters downstream from the confluence with Bogard Spring Creek. This fish was fin clipped in September 2008 in Bogard Spring Creek (Moyle and Carmona, unpublished data). In 2010, a single male trout managed to migrate the entire distance from the weir to the spawning area (T. Pustejovsky, pers. comm.). The diet of the trout varies with age and season. Newly planted trout in their first year in the lake feed mainly on zooplankton, including Daphnia spp. and Leptodora kindti, and on benthic invertebrates, especially leeches and amphipods. By August, most of the trout switch to feeding on young-of-year tui chubs (King 1963, Moyle 2002, Eagles-Smith 2006).

Habitat Requirements: 

Habitat Requirements: Eagle Lake rainbow trout spend most of their life in Eagle Lake, a large (24 km long by 3-4 km wide), highly alkaline lake. The lake consists of three basins, two of them averaging 5-6 m deep, the third averaging 10-20 m with a maximum depth of about 30 m. The shallow basins are uniform in their limnology and water temperatures may exceed 20°C in the summer. The deep basin stratifies, so in late summer most of the trout are in the deeper, cooler water of this basin. Otherwise, they are found throughout the lake.
Eagle Lake rainbow trout are stream spawners. They formerly migrated up to 45-50 km upstream to spawn in the gravelly upper reaches of Pine Creek and its tributaries. Juveniles then spent their first and/or second year in the stream before moving into the lake during high run-off periods that reconnected headwaters to the lake. During the summer, upper Pine Creek is a cold spring-fed trout stream, flowing at .03-0.14 m3/s through meadows and open forest, with modest gradients. Bogard Spring Creek is also a spring-fed creek, with flows of 0.01-0.02 m3/s. The meadow streams have deep pools and glides with deeply undercut banks, providing abundant cover for trout. The Pine Creek watershed is described in detail by Pustejovsky (2007). Unfortunately, the trout present today in the creek are almost entirely alien brook trout, in high densities (Carmona et al. 2010).
Environmental tolerances of Eagle Lake rainbow trout are high for a trout and similar to those of coastal rainbow trout. In Eagle Lake, they live in highly alkaline water (pH 8.4-9.6), in which dissolved oxygen is usually at or close to saturation. They have been observed foraging in shallow water at temperatures of 22-23°C but generally retreat to deeper, cooler (


Distribution: Eagle Lake rainbow trout are endemic to Eagle Lake, Lassen County, and its main tributary, Pine Creek. They have been planted in numerous waters throughout California, where they are maintained from hatchery stocks originating from trout captured at the weir and fish trap at the mouth of Pine Creek. In the past, hatchery trout have been exported to other states and to Canada. It is unlikely that naturally reproducing populations of genetically ‘pure’ Eagle Lake trout are present in any of these planted waters, although data are largely absent.

Abundance Trends: 

Trends in Abundance: Naturally spawned Eagle Lake rainbow trout were once abundant in the lake. According to Purdy (1988), "In the spring months of the 1870s and 1880s, when trout were spawning, huge quantities were being caught. It was not unusual to hear that wagon loads of trout, some weighing as much as 600 pounds, were being brought into Susanville where they were sold at local markets for twenty-five cents a pound (p. 14)." This exploitation occurred at the same time as extensive logging in the drainage, heavy grazing in the meadows, and the first construction of railroad grades and roads across the meadows and streams, all of which altered stream channels. When the ELRT was described by Snyder (1917), he noted its numbers were low. Although commercial fishing for trout was banned in California in 1917, ELRT populations remained low, presumably because of the poor condition of Pine Creek and the establishment of predatory largemouth bass and brown bullheads in the lake. By 1931, trout were scarce in the lake and Pine Creek (Snyder 1940).
During the 1930s, trout populations were stressed as lake levels dropped dramatically when diversion of water through Bly Tunnel combined with prolonged drought to presumably reduced access of spawning trout to Pine Creek. In 1939 biologists in Lassen National Forest expressed concern over impoundments further reducing flows of drought-stricken Pine Creek (Pustjevoksy 2007). Meanwhile, logging, railroad construction, and other activities further degraded the Pine Creek watershed. On the bright side, high alkalinities brought on by dropping lake levels also eliminated bass from the lake, although bullheads persisted into the 1970s. Even with the return of wetter conditions, the trout populations showed little sign of recovery. In 1949 and 1950, DFG collected 35 and 75 adult ELRT, respectively, from the mouth of Pine Creek, spawning them for hatchery rearing (Dean and Chappell 2005). The 258 progeny from the 1949 fish were planted in Pine Creek, where brook trout had recently become established, but probably did not survive. The spawning of fish in 1950 was more successful and the hatchery-reared progeny were planted in the embayment at the mouth of Pine Creek. In 1951-1958, some artificial propagation also took place although the records are not clear as to how many fish were produced (Dean and Chappell 2005). Prior to hatchery propagation, trout presumably persisted only because occasional wet years permitted successful spawning despite degraded stream channels and the presence of brook trout in the spawning reaches (McAffee 1966). It is possible that these actions by DFG biologists prevented extinction of ELRT although it is equally possible, based on recent genetic evidence, that a small migratory population persisted until all access to upstream areas was blocked in 1995.
In 1959, an egg taking station was built at the mouth of Pine Creek, including a wooden weir/dam to block upstream passage of most fish (Dean and Chappell 2005), Regular trapping operations began in 1959, when 16 trout were captured and spawned; in the next five years the numbers captured varied from 45 to 391 (McAfee 1966). From 1959 through 1994, a few trout were able to make it over the barrier during wet years, apparently allowing some natural spawning (Pustejovsky 2007, Moyle, unpublished data).
In 1995, the weir was rebuilt to prevent upstream movement of all ELRT (Pustjevoksy 2007). The life history of the trout then became entirely under human control. At present, eggs and milt are stripped from the fish at the egg taking station. The embryos are then transported to Crystal Lake Hatchery, from where they are distributed to other hatcheries across California (Carmona et al. in review). Originally, trout were marked to prevent using fish that been used for spawning in previous years, to prevent sibling crosses and thus minimize inbreeding, and to select for longer lived fish to compensate for longevity reductions that may have been caused by past hatchery practices (R. L. Elliott, California Department of Fish and Game (DFG), pers. comm. 1998). Today, the main process is simply to stock only progeny of wild-trapped fish in the lake. At the Mount Shasta Hatchery, broodstock trout are reared from eggs collected at the Pine Creek egg taking station; once mature, the broodstock are used for multiple spawning seasons to produce trout which are stocked widely across California, mainly in reservoirs.
Although progeny of first generation broodstock were planted in Eagle Lake in the past, DFG currently stocks only trout from eggs annually collected at the station (P. Divine, DFG, pers. comm., 2009; DFG, unpublished data). Each year, approximately 180,000 - 200,000 trout weighing about 0.23 kg (0.5 lb) each and 20,000 1kg+ “trophy fish” are stocked in Eagle Lake for the fishery. To provide fish for planting, hundreds of trout are trapped each year and between 1 and 6 million fertilized eggs per year are taken for hatchery rearing. Thus in 2009, 1,737 females were spawned, producing 5,985,880 eggs, for the hatchery, while in 2008 the take was 2,757,420 eggs, and in 2007, 1,113,980 eggs (Paul Divine, CDFG, personal communication 2009). There is no recent evidence of natural reproduction contributing to the lake population; the fish captured by anglers usually show signs of a year or more in a hatchery environment, mainly fins with distorted fin rays or missing fins.
Prior to 2006, in some years, DFG also has stocked ca. 1000 “half pound” fish in Pine Creek to reduce the brook trout population through predation (Dean and Chappell 2005), although there were no studies to confirm that it worked. Subsequent sampling suggests that few of these fish persisted for long in the creek (Moyle, unpublished data).
Actual population size of trout in Eagle Lake has not been studied but it is presumably dependent on the stocking allotments every year. Creel censuses indicate that catch per hour from 1983 through 2007 ranged from 0.2 to 0.6, with a mean of 0.3, while average length of fish caught increased over the years (Carmona-Catot et al., in press). The number of mature females captured at the trap while migrating and spawned by the DFG ranged from ca 600 to 1,700, although no estimates were made of size of the entire spawning run. The number of eggs collected averages about 2.2 million, with annual takes of 607,000 to 6 million (P. Divine, CDFG. pers. comm.); the egg quotas are developed every year by DFG hatchery personnel in order to achieve the hatchery goals.
Genetic studies provide some insights into minimum population sizes in the lake. Carmona-Catot et al. (in press) found individuals in the lake population had an FIS or inbreeding value, of 0.064, significantly higher than zero, although no genetic evidence of a bottleneck was detected. The effective population size (size of breeding population) was estimated at 1125 fish, with a confidence interval from 151- infinity, indicating in all years there was a fairly large population contributing to reproduction. Given the presumed small number of fish used to establish the original hatchery-based population, it is interesting that no genetic bottleneck was detected. The original bottleneck could have been masked by the number of generations that have passed since the bottleneck and efforts of the hatchery breeding program to maximize genetic diversity (by breeding as many individuals as possible), as seen in the population’s now high effective population size. It is also possible that the population left in the lake in the 1950s was larger than trapping efforts on Pine Creek indicated and multiple years of naturally-spawned fish contributed to the initial hatchery stock. The slight if significant FIS value is still something to be concerned about and to monitor, although it is comparable to levels found in other lake-stream systems in the region such as Goose Lake (Simmons 2011).
Overall, the population appears to be stable because it is maintained entirely by hatchery production, which may be selecting against fish capable of reproducing naturally. For example, Chilcote et al. (2011) show that wild populations of three species of anadromous salmonids from the Pacific Northwest have greatly reduced ability to be self-sustaining when fish of hatchery origin are also present. While the effects of hatchery rearing on trout populations are sometimes overstated, there is ample evidence that it does have an impact on the genetics and behavior of fish released into the wild affecting their ability to persist on their own in the wild (e.g., Waples 1999, Araki et al. 2007, 2008, Kostow 2008). Recent evidence suggests that fitness reductions may not just be limited to fish raised in the hatchery but instead continue into subsequent generations (Araki et al. 2009). The continued dependence of the Eagle Lake rainbow trout on hatchery production assumes that the hatchery program will always be adequately funded and maintained, that a hatchery-dependent population is always going to be desirable, and that restoring the natural life cycle is a secondary, not a primary, goal for conservation of this fish.


Description: This subspecies is similar to other rainbow trout in gross morphology (see Moyle 2002), but differs slightly in meristic counts, especially in having finer scales than coastal rainbow trout. It also is unique in possessing 58 chromosomes rather than the 60 typical of other rainbow trout (Busack et al. 1980).