Oncorhynchus mykiss aguabonita

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General Information
Common Name: 
California golden trout

Conservation Status in California: Class 1, Endangered (Moyle et al. 2011).
The California golden trout has high likelihood of extinction in its native range in 50-100 years, or sooner, due to introgression with non-native trout species. It meets the criteria for listing as threatened species under both state and federal ESAs.

Life History: 

Life History: California golden trout live in cold, clear alpine streams. They have comparatively slow growth rates due to the truncated growing season and the low productivity of the high elevation streams of their native range (Knapp and Dudley 1990, Knapp and Matthews 1996). In streams, they are usually 3-4 cm at the end of their first summer of life, 7-8 cm SL at the end of their second summer, 10-11 cm SL at the end of their third summer and grow 1-2 cm per year thereafter; they reach a maximum size of 19-20 cm SL and a maximum age of 9 years (Knapp and Dudley 1990). In alpine lakes, individuals from introduced populations grow to 4-5 cm FL, 10-15 cm FL, 13-23 cm, and 21-28 cm at the end of their first through fourth years, respectively (Curtis 1934); they can reach 35-43 cm FL by the seventh year. The largest on record from California weighed 4.5 kg, from Virginia Lake, Madera County in 1952. However, most records of growth of golden trout in lakes are suspect because the populations were established from introductions and hybridization with rainbow trout is common.
Golden trout spawn when they are three or four years old, when water temperatures exceed 10°C, with daily maximums of 16-18°C in late June and July (Stefferud 1993; Knapp and Vredenburg 1996). Average daily temperatures for spawning are around 7-10°C. They spawn in gravel riffles in streams. Spawning behavior is typical of other members of the rainbow trout group although they spawn successfully in finer substrates (decomposed granite) more than most other trout (Knapp and Vredenburg 1996). Females produce 300-2,300 eggs, the number depending on body size (Curtis 1934). Embryos hatch within 20 days at an incubation temperature of 14°C. The fry emerge from the gravel two to three weeks after hatching, at which time they are about 25 mm TL. In introduced lake populations, fry move into the lakes from spawning streams when they are about 45 mm TL.
In streams, golden trout are active at all times of day and night but tend to stay in the same areas for long periods of time (Matthews 1996a). They feed on both terrestrial and aquatic invertebrates, mostly adult and larval insects, taking whatever is most abundant. In lakes, they feed mainly on benthic invertebrates, especially midge pupae (Chironomidae) (T. Armstrong, unpublished data). Although the bright coloration makes them highly visible, there are very few natural predators in the range occupied by this subspecies (Moyle 2002). Their tendency to be more active during the day than most trout also suggests low predation. Thus, the bright coloration may have evolved for reproductive advantage. However, bright coloration has also been implicated as providing camouflage against the bright colors of the volcanic substrates in the clear, shallow streams (Needham and Gard 1959). When these trout are removed from the mountainous streams and brought down to low elevation streams, they may lose the brightness and take on dull gray and red colors (Needham and Gard 1959). In lakes, they become paler in color, often appearing silvery.

Habitat Requirements: 

Habitat Requirements: Golden trout evolved in streams of the southern Sierra Nevada, at elevations above 2,300 m. The unglaciated valleys of the Kern Plateau are broad, flat, and filled with glacial alluvium, which results in wide meadows through which the streams meander. The streams are small, shallow, and have only limited riparian vegetation along the edges. The exposed nature of the streams is largely the result of heavy grazing of livestock on a fragile landscape, which began in the 1860s, causing compaction of soils, collapse of stream banks, and elimination of riparian plant cover (Odion et al. 1988, Knapp and Matthews 1996, Matthews 1996b). The stream bottoms are mostly volcanic sand and gravel, with some cobble. The water is clear and mostly cold, although summer temperatures can fluctuate from 3 to 20°C (Knapp and Dudley 1990). California golden trout generally prefer pool habitat and congregate near emergent sedges and undercut banks (Matthews 1996a).
Environmental tolerances are presumably similar to those of coastal rainbow trout.


Distribution: California golden trout are endemic to the South Fork of the Kern River (SFKR), which flows into Isabella Reservoir and to Golden Trout Creek (GTC) (including its tributary, Volcano Creek),which flows into the Kern River (Berg 1987). Initially (1909 and earlier) California golden trout were collected from Golden Trout Creek and transported north by pack train, extending their range by some160 km by 1914 (Fisk 1969). They were also translocated into many other waters within and outside California, including Mulkey Creek and the Cottonwood Lakes not far from the headwaters of Golden Trout Creek and headwaters of South Fork Kern River (Stephens et al. 2004). The Cottonwood Lakes served as a source of golden trout eggs for stocking other waters, beginning in 1917, and are still used for aerial stocking of lakes in Fresno and Tulare Counties (Stephens et al. 2004). As a result of stocking in California, these fish are now found in more than 300 high mountain lakes and 1100 km of streams outside their native range (Fisk 1969). Unfortunately, many, if not most, of native and transplanted populations, including the golden trout from Cottonwood Lakes that have been used as brood stock for transplants, have hybridized with rainbow trout (Moyle 2002, Stephens et al. 2004). Golden trout are also widely distributed in lakes and streams of the Rocky Mountains, but most populations there are also likely hybridized with either rainbow or cutthroat trout. It is possible that a few unhybridized populations still exist from early transplants in the Sierras and elsewhere, but they are likely to have limited genetic diversity due to small numbers used to establish these populations.

Abundance Trends: 

Trends in Abundance: California golden trout populations suffered major declines during the 19th and first half of the 20th Century from overfishing and heavy grazing. Invading brown trout displaced California golden trout, including hybrids, from all reaches below artificial barriers, so the golden trout are now confined to a few kilometers of stream in the Golden Trout Creek watershed and in the South Fork Kern watershed. Within their native range, California golden trout occur at both low densities (0.02 - 0.17 fish per m2 in streams) (Knapp and Dudley 1990) and at high densities (1.3-2.7 fish per m2). Low densities are most likely to be in found in grazed reaches of steam with little cover and food. Presumably, densities were much higher on average before livestock began grazing the drainage. Although California golden trout were widely introduced outside their native range during the 19th and 20th century, the introduced populations should not be regarded as contributing to golden trout conservation because most (if not all) have hybridized with coastal rainbow trout.
Knapp and Dudley (1990) estimated that golden trout streams typically support 8-52 fish/ 100 m of stream, although a recent estimate for Mulkey Creek, a tributary to the South Fork Kern River, was 472 fish/100m (Carmona-Catot and Weaver 2006). If the Knapp and Dudley figures are accepted as correct then in 1965, when the first major CDFG habitat management plan was issued (CDFG 1965), there would have been 2,400-15,600 individuals in Golden Trout Creek (30 km) and 4,000-26,000 in the South Fork Kern (50 km). Curiously, the high numbers in the South Fork Kern River are found in reaches that have been degraded by grazing, presumably because the reaches contain decomposed granite substrates that are used for spawning (S. Stephens, pers. comm. 2008). The lack of cover in these reaches selects for smaller fish, which are more numerous.
If unhybridized fish exist only in 5 km of Volcano Creek, then there are only 400-2600 ‘pure’ golden trout left today, a drop of at least 95% from historic numbers. Nevertheless, because golden trout had already been eliminated from most of lower South Fork Kern River by 1965, where populations would have been most dense, the 95 percent decline figure may still be valid, even if populations with low amounts of introgression are counted.
All or nearly all California golden trout in the upper South Fork Kern River and Golden Trout Creek are introgressed with non-native rainbow trout to some degree. However, the levels of introgression are different in the two streams. On the South Fork Kern River there is a cline of introgression from the lower Kennedy Meadows area (94%) upstream to the headwaters (8%). In contrast, in many reaches of Golden Trout Creek, levels of introgression are low, with only one or two fish out of 40 fish hybridized at low levels (Cordes et al. 2006; M. Stephens 2007). Nevertheless, genetically ‘pure’ populations exist in only a few kilometers of streams and this is likely to continue for the short term ( Overall, unhybridized California golden trout are much less abundant than they have been in the past. In areas where they still persist, numbers are undoubtedly higher than they were the days of heavy harvest and grazing, but these numbers are still presumably less than historic highs because of continued grazing and other human impacts.


Description: The California golden trout is named for its bright colors. Behnke (2002) describes their coloration as follows: “The color of the back is brassy or copper, becoming bright golden yellow just above the lateral line. A deep red stripe runs along the lateral line and the golden yellow body color intensifies below. A deep crimson color suffuses the ventral region from the anal fin to beneath the lower jaw… (p. 105).” Fish from Golden Trout Creek are particularly brightly colored. Young and most adults have about 10 parr marks centered along the lateral line. The parr marks on adults are considered to be a distinctive characteristic (Needham and Gard 1959), but they are not always present, especially in larger fish from introduced lake populations. Large spots are present, mostly on the dorsal and caudal fins and on the caudal peduncle. The pectoral, pelvic, and anal fins are orange to yellow. The anal, dorsal, and pelvic fins have white to yellow tips, preceded by a black band. Basibranchial teeth are absent and there are 17-21 gill rakers. Other characteristics include 175-210 scales along the lateral line, 34-45 scales above the lateral line, 8-10 pelvic rays, 25-40 pyloric caeca, and 58-61 vertebrae (Schreck and Behnke 1971).