The postrelease mortality of rainbow trout Oncorhynchus mykiss caught on scented artificial baits was compared with postrelease mortalities of rainbow trout caught on traditional artificial flies. In all, 457 fish were captured on flies, 505 on artificial baits fished actively (ABA), and 511 on artificial baits fished passively (ABP) in five replicate experiments. Water temperature, fish length, time played, time out of water, hook location, leader treatment, and bleeding intensity were recorded for each fish captured. Mortalities were recorded daily over a 3‐week holding period. Overall mortalities were 3.9% for fly‐caught fish, 21.6% for fish caught on ABA, and 32.1 % for fish caught on ABP Differential mortality among gear types resulted largely from differences in the number of fish hooked in the gill arches or deep in the esophagus (critically hooked) in each group. Overall, critical bookings were 3.9% for the fly‐caught group, 45.7% for the ABA group, and 78.3% for the ABP group. The Akaike Information Criterion, a model selection procedure, was used to develop a logistical regression model that best fit the mortality data. Parameters that reduced mortality probability include using flies rather than synthetic baits, hooking the fish in a noncritical location, and cutting the leader on critically hooked fish. In addition, as fish length increased, mortality probability decreased. Length of time played and length of time out of water contributed to mortality, as did increasing water temperatures and bleeding intensity.

A whirling disease resistant domestic strain of rainbow trout Oncorhynchus mykiss (German [GR]) was crossed with a susceptible wild strain of rainbow trout (Colorado River [CR]). Thirty-two families of F 1 crosses, along with five pure GR and two pure CR rainbow trout families, were then evaluated for resistance to whirling disease after experimental exposure to Myxobolus cerebralis. The pure domesticated GR strain was verified to have strong resistance to the parasite. In contrast, wild CR rainbow trout were highly susceptible. Crosses of these two strains resulted in offspring with a range of susceptibility. The resistance to whirling disease in some families was similar to that of pure GR rainbow trout, while other families were as susceptible as pure CR rainbow trout. Infection severity was significantly greater in the CR strain than in the pure GR and the GR 3 CR strains, as measured by both microscopic pathological scores (histology) and myxospore counts. Infection severity in the reciprocal crosses (CR female 3 GR male) was not significantly different from that of the pure CR rainbow trout as measured by histology but was significantly different with respect to lower myxospore counts. Future studies will examine the stability of inheritance of whirling disease resistance and the potential use of selective breeding to control whirling disease in free-ranging rainbow trout populations.

We used a quantitative genetics approach and estimated broad sense heritability (h 2 b ) of myxospore count and the number of genes involved in myxospore formation to gain a better understanding of how resistance to Myxobolus cerebralis, the parasite responsible for whirling disease, is inherited in rainbow trout Oncorhynchus mykiss. An M. cerebralis-resistant strain of rainbow trout, the German Rainbow (GR), and a wild, susceptible strain of rainbow trout, the Colorado River Rainbow (CRR), were spawned to create 3 intermediate crossed populations (an F1 cross, F2 intercross, and a B2 backcross between the F1 and the CRR). Within each strain or cross, Comparison of means and a joint-scaling test suggest that resistance alleles arising from the GR strain are dominant to susceptible alleles from the CRR strain. Resistance was retained in the intermediate crosses but decreased as filial generation number increased (F2) or backcrossing occurred (B2). The estimated number of segregating loci responsible for differences in myxospore count in the parental strains was 9 ± 5. Our results indicate that resistance to M. cerebralis is a heritable trait within these populations and would respond to either artificial selection in hatcheries or natural selection in the wild. KEY WORDS: Myxobolus cerebralis · Oncorhynchus mykiss · Parasite · Salmonid · Heritability · Quantitative genetics Resale or republication not permitted without written consent of the publisherDis Aquat Org 102: [97][98][99][100][101][102][103][104][105][106] 2012 process presumably led to the production of hatchery-derived rainbow trout strains that are resistant to Myxobolus cerebralis. El-Matbouli et al. (2002) found that, under experimental laboratory conditions, a German strain of rainbow trout (German Rainbow, GR) was at least as resistant to M. cerebralis as brown trout Salmo trutta, which evolved with the parasite in its European home range (Hoffman 1970). The GR strain was also found to be more resistant to M. cerebralis than either the North American Trout Lodge (TL) or Colorado River Rainbow (CRR) trout strains (Hedrick et al. 2003, Schisler et al. 2006. Resistance likely arose through the growth and reproduction of the GR strain under continuous exposure to the parasite in Bavarian hatcheries (Hedrick et al. 2003).Resistance to disease in animals is often a complex, polygenic trait (Grenfell & Dobson 1995) that results from a series of complex interactions among the host, pathogen, and environment (Snieszko 1974, Hedrick 1998. The mechanisms for resistance to Myxobolus cerebralis seen in the GR strain, like those seen in trout resistant to a similar myxosporean, Ceratomyxa shasta, are suspected to be polygenic (Hedrick et al. 2001). Studies examining differential gene expression in resistant and susceptible strains of rainbow trout have identified several genes potentially involved in resistance (Severin & El-Matbouli 2007, Baerwald et al. 2008, Severin et al. 2010. Baerwald et al. (2011) discovered a major quantitative trait lo...

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