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 populations 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.