Resistant Trout: New Rainbow Trout Strains Provide Management Options for Colorado
G. L. Schisler, K. A. Myklebust and R. P. Hedrick
Recently, rainbow trout strains have been identified that exhibit resistance to whirling disease. One strain in particular that has very strong resistance, named the “Hofer” rainbow, has been imported from Germany. Rainbow trout have been reared in fish culture facilities as food fish in that country since their original importation from the United States in the late 1800’s. Because whirling disease originated in Europe, some rainbow trout strains reared there have developed resistance to the parasite. The “Hofer” rainbow trout are being evaluated for use as a standard domestic catchable rainbow trout for put-and-take waters in Colorado. Experiments have determined that the infection severity due to whirling disease, as measured by number of mature parasites developed among infected fish, is typically much lower in the Hofer rainbow trout than in other standard domestic strains. The Hofer strain has also demonstrated outstanding growth when compared with other domestic strains in laboratory and hatchery studies conducted by the Colorado Division of Wildlife.
Aggressive feeding behavior and lack of predator avoidance, typical of many domestic strains of domestic rainbow trout, are potential drawbacks when considering the use of Hofer rainbow trout for establishing wild populations. The Colorado Division of Wildlife has historically used the Colorado River rainbow trout for wild self-sustaining populations in the state. This strain is very long lived, exhibits wild behavior, and has the ability to reproduce successfully in many rivers in Colorado. Unfortunately, the Colorado River rainbow trout strain is very susceptible to whirling disease, and most wild populations in the state have been reduced or eliminated due to the disease.
The Colorado Division of Wildlife has been conducting breeding experiments to determine if the resistance from the Hofer rainbow trout can be inherited in the offspring of crosses of the Colorado River rainbow and Hofer rainbow strains. The first of these experiments was initiated in 2004, with very promising results. Offspring of these crosses demonstrated varying levels of resistance, with some as resistant as the Hofer parents. A recent article (Sheisler 2006) published in the Journal of Aquatic Animal Health contains more details on the experimental crosses.
Preliminary results of experiments in the wild with this first generation of crosses are also encouraging. First generation crosses stocked into the Gunnison River as fingerlings survived their first year in the river at the same rate as pure Colorado River rainbow trout that were stocked at the same size and age. Infection severity after one year in the river was 30 times lower in the first generation crosses than in the pure Colorado River rainbow trout. If these fish can reproduce successfully in the wild, there is hope for re-establishing wild rainbow trout populations in Colorado.
Schisler, G. L., K. A. Myklebust and R. P. Hedrick. 2006. Inheritance of Myxobolus cerebralis resistance among F1-generation crosses of whirling disease resistant and susceptible rainbow trout strains. Journal of Aquatic Animal Health 18:109-115.