WINTER ET AL.. RESISTANCE OF COHO SALMON AND STEELHEAD TROUT 



used Alsea coho salmon in which the AA genotype 

 was also the most susceptible to BKD. Because of 

 similar transferrin results in the B x S cross and 

 Alsea stock, the data were combined. For the 

 combined data, the AC (28% mortality) and CC 

 (24% mortality) genotypes were significantly 

 (P<0.01) more resistant to BKD than was the AA 

 genotype (62% mortality). Within both the stocks 

 and transferrin genotypes, differences between 

 replicates were not significant. 



The second BKD experiment with coho salmon 

 gave results similar to those of the first on the 

 basis of transferrin genotypes (Figure IB). Unfor- 

 tunately, the AA genotype was not included in the 

 Alsea comparison because we lacked sufficient 

 fish. No stock comparison was made because the 

 Big Creek stock came directly from the hatchery, 

 at a time when 91.5% of the mortalities in produc- 

 tion fish at Big Creek were due to BKD (J. Con- 

 rad^). The probability that the Big Creek coho 

 salmon used in the experiment had previously 

 been exposed to BKD was therefore very high. 



In the third BKD study, which involved the four 

 steelhead stocks and a second Rogue stock reared 

 at the hatchery (Figure IC), mortalities in all the 

 test groups began to increase at a high rate 3 wk 

 after the study began because of a secondary infec- 

 tion with Aeromonas hydrophila. This trend con- 

 tinued for another 4 wk, at which time mortalities 

 leveled off, and the study was terminated. A com- 

 parison of the resistance of the different stocks is 

 not fully valid because the fish in the different test 

 tanks were obviously not challenged equally with 

 a secondary infection of A. hydrophila. However, 

 there were no significant differences (P>0.10) be- 

 tween replicates, and the mortality of the Siletz 

 steelhead trout (72%) was significantly lower 

 (P<0.05) than that of all other stocks except the 

 Alsea. Because mortality in the Rogue stock was 

 extremely high (96%), a transferrin genotype 

 comparison was not considered. The AC and CC 

 genotypes within the Alsea stock were equally 

 susceptible to the double infection of BKD and A. 

 hydrophila. Although percentage mortality is a 

 better measure of an organism's ability to tolerate 

 disease, mean time to death is also an indication of 

 resistance to diseases, especially chronic ones such 

 as BKD. There were no differences in mean time to 

 death (days) among either the Rogue or Alsea 

 steelhead transferrin genotypes (numbers of fish 



'J. Conrad, Oregon Department of Fish and Wildlife, 

 Clatskanie, OR 97015, pers. conunun. February 1978. 



in parentheses): Rogue— AA, 28.5 (30); AC, 30.0 

 (41); and CC, 29.7 (19); Alsea— AC, 30.4 (21); and 

 CC, 30.0 (62). The importance of transferrin was 

 probably reduced by the double infection. 



Vibriosis 



In the first experiment in which coho salmon 

 were exposed to V. anguillarum (Figure 2A), the 

 Big Creek stock (38 % mortality) was significantly 

 more resistant (P<0.005) than the Alsea stock 

 (62% mortality) (transferrin was not considered in 

 this comparison). There was a significant differ- 

 ence (P<0.005) in mean weight (<'-test, Snedecor 

 and Cochran 1967: 1 14 ) between the Alsea and Big 

 Creek fish. However, there were no significant 

 differences (P>0.10) in resistance to vibriosis 

 among four weight classes (5.1-10.0, 10.1-15.0, 

 15.1-20.0, and 20.1-25.0 g) within either stock. 

 The difference in resistance between the two 

 stocks appears to be genetic. In a second test, the 

 resistance trend between the Alsea and Big Creek 

 stocks was reversed (Figure 2B), though at a lower 

 level of significance (P<0.07) than the previous 

 experiment. However, the Alsea coho salmon used 

 in this second test came directly from the hatch- 

 ery. Though it is unlikely that any of these fish 

 would have been previously exposed to V. anguil- 

 larum in freshwater, a difference in susceptibility 

 to vibriosis still existed. These conflicting results 

 thus demonstrate that the environment has a 

 strong effect in determining resistance to vib- 

 riosis. In both the Alsea and Big Creek stocks, no 

 differential resistance was shown by the transfer- 

 rin genotypes, although the AA genotype was not 

 included in the Alsea transferrins (Figure 2B). 



In the first of the two vibriosis experiments with 

 steelhead trout (Figure 2C), the North Santiam 

 steelhead trout were the least susceptible to vib- 

 riosis of all the stocks (P<0.05). The Alsea 

 steelhead trout, though exhibiting a higher mor- 

 tality (87%) than the North Santiam fish, were 

 still significantly more resistant than the remain- 

 ing two stocks (P<0.05). Because mortality was 

 high in the Smith Farm- and hatchery-reared 

 Rogue stocks (96%), transferrin genotype differ- 

 ences and the effects of rearing environment on 

 resistance were not considered. However, no dif- 

 ferences in resistance were observed among 

 genotypes within the Alsea stock. These results 

 using steelhead trout are similar to those observed 

 in the coho salmon exposed to vibriosis. 



The second vibriosis experiment (Figure 2D), 



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