Resistant Trout Research: Study 1

Resistant Trout

Principal Investigators:
Ronald P. Hedrick and Bernie May, University of California-Davis
Mansour El-Matbouli, University of Munich, Germany
George Schisler, Colorado Division of Wildlife
Richard Vincent, Montana Fish, Wildlife and Parks

The observation that the Hofer strain (HR) of rainbow trout has developed a stable genetic resistance to Myxobolus cerebralis infections has provided a unique opportunity to pursue both applied and basic research aimed at controlling whirling disease (WD) in wild trout. This research is being conducted by interactive teams at the University of Munich, the University of California, Davis, the Colorado Division of Wildlife, The Utah Department of Natural Resources and Montana Parks and Recreation are currently involved with the Resistant Trout Project as sponsored by the Whirling Disease Foundation (WDF).

Phase I Research
The initial studies that provide the foundation for this program evaluate the resistance of the HR, a strain of rainbow trout reared in Germany, as compared to known strains of rainbow trout from North America.

Results from both laboratory and field exposures verified that this strain is highly resistant to WD in comparison with all other strains tested. These findings clearly demonstrate that development of resistance to whirling disease in rainbow trout can occur. The implication for waters managed for wild trout is that it offers hope that, given the opportunity, resistance will naturally develop in these populations. Where management for wild trout is not or cannot be practiced, this research also provides opportunities for re-establishing rainbow trout populations or for providing fish for sport fishing that will not contribute to the negative impacts of the disease.

Phase II Research
Resistance to a single disease is only one of many criteria that need to be considered when moving plants and animals from one geographic region to another. If broader ecological implications of movements are not considered, greater harm than good may result. Thus, the second phase of experimentation provides assurances that the HR trout, having developed resistance to whirling disease, have not become extremely susceptible to other microbial pathogens they may encounter. In addition, these trout will be thoroughly examined to determine if pathogens unknown in North American trout are potentially transmitted with them via eggs. Such studies are critical before further discussions on the potential applications of these fish in North America can proceed. Until this is demonstrated, all live fish studies in Phases I and II have been conducted in containment laboratories.

Results of this testing showed disease susceptibility similar to other rainbow trout, in other words, it had neither gained nor lost resistance to other pathogens as a result of selection for resistance to whirling disease. Repeated health inspections of HR held in several laboratories determined that no pathogens exotic to rainbow trout in North America were present in the fish that had been imported. In addition, the Hofer facility is OIE certified as being free of IPNV.

Phase III Research
Phase III is the most complex phase of the project. Traditional selective breeding studies will be designed to determine if resistance to WD is a heritable trait and if the genes associated with this resistance can be identified. This phase requires a substantial commitment to holding numerous family groups and tracking of adults and progeny from selected crosses (a significant genetic program). To that end, the Colorado Division of Wildlife has become a partner in this study and all research is being conducted in a containment facility.

Phase III is viewed as a multi-year project, with new research partners included as opportunities arise. The performance of the selectively bred rainbow trout is being evaluated by geneticists, physiologists, biologists and health experts in the laboratory and under highly controlled field situations to identify WD resistance genes, to understand the basis for resistance and to evaluate potential applications for these fish in WD-effected areas.

Throughout these studies we also continue to examine the resistance of other promising rainbow trout strains. Evaluation of one of these, the Desmet strain of rainbow trout from Montana, is underway.

Investigations conducted in this past year demonstrated a range of resistance among families of F1 hybrids made from crosses of HR with a wild strain of Colorado River (CR) rainbow trout. This indicates the potential for selective breeding as a means to combat whirling disease among both hatchery and wild stocks of rainbow trout.

Implications of Research Results
Any potential application of selectively bred rainbow trout for fisheries management would be dependent on the approval by federal or state agencies responsible for the aquatic resources after appropriate input from appropriate sources. We see a range of applications of selectively bred rainbow trout in the intermountain west, where rainbow trout are not native but prized fisheries exist or have existed. However, this range of applications must be tied closely to the dispositions and protection of native assemblages of fish including other salmonid species. The Whirling Disease Foundation does not play a role in the policy decisions of the states, but we have facilitated discussion of this research in numerous venues so that informed decisions can be made.

While we believe that resistant trout will be one of the solutions identified to reduce the effects of whirling disease in states where its effects have devastated trout populations, it is not the only solution and the Whirling Disease Foundation recommends the following considerations for the use of these fish:

Naturally reproducing wild rainbow trout in their native range must be protected from genetic or biological intrusions of hatchery rainbow trout of any strain. And that dollars spent to benefit these populations are best spent for habitat restoration that is the long-term key to their success.
In areas where rainbow trout fisheries outside of their native range have been established and then have declined or face elimination due to WD, introduction of WD resistant rainbow trout may serve a useful role in restoration. However, programs to rebuild or restore these nonnative fisheries should not occur at the expense of re-establishing native fish assemblages, particularly those fish species that might hybridize with rainbow trout (e.g. cutthroat trout).
The most cautious approaches to re-introducing/re-establishing rainbow trout populations outside of their native ranges is done by using progeny from selective breeding between WD resistant rainbow trout and desired and known rainbow trout broodstocks.

Secondly, the practical applications forthcoming from Phase III of the project include:

This research will result in a greater understanding of the mechanisms involved in the development of disease resistance to whirling disease and explores potential practical applications of this knowledge. This knowledge has potential applications for other myxozoan and infectious diseases in wild and captive fish populations.
Selectively bred rainbow trout with a greater resistance provide a useful management approach in areas where whirling disease occurs and hatchery rainbow are currently used. Hatchery stocking programs in the USA use rainbow trout from eggs from numerous hatcheries around the USA and from such distant locations as Tasmania and South Africa. By providing an example of a methodical scientific approach, we may preclude moves that might result in direct planting of Hofer or other WD resistant rainbow trout strains from Europe into US waters.
Selectively bred WD resistant rainbow trout can be used to re-establish naturally reproducing populations in waters where there are no native salmonids or where native salmonids have been lost and can no longer be restored (e.g. significant and irreversible habitat changes that cannot be reversed). In Colorado there are several examples of rainbow trout fisheries that were established and are now lost.
Stocking trout with enhanced resistance will ultimately result in addition of fewer spores back into heavily affected systems. It is expected that this will reduce parasite abundance over time so that survival of less resistance species/strains is not compromised.

We recognize that the potential applications of the findings from the project will differ between geographic regions depending on numerous biological, economic and political factors. It has been the intent of the WDF to provide the best science available to those who will have to make the decisions on the use of these fish.

Resistant Trout: New Rainbow Trout Strains Provide Management Options for Colorado

Principal Investigators:
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.

Literature Cited:

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.