WINTER ET AL,: RESISTANCE OF COHO SALMON AND STEELHEAD TROUT 



Perhaps stock resistance to acute diseases such 

 as vibriosis depends more on which stock has 

 an environmental advantage at the time of in- 

 fection, rather than on genetic make-up. Also, 

 when mortalities in experiments are high, resis- 

 tance comparisons are difficult to make because 

 any immunity that was present may have been 

 overwhelmed. Genetic factors are probably more 

 important in chronic diseases such as BKD. For 

 example, Zinn et al. (1977) observed appairent 

 genetic resistance to infection by Ceratomyxa 

 shasta, normally not an acute condition, among 

 hatchery strains of chinook salmon. 



It is also evident that a stock may be resistant to 

 one disease and not to another. Although the 

 Siletz steelhead trout were most resistant to the 

 double infection of BKD and A. hydrophila, they 

 showed the greatest susceptibility to V. anguil- 

 larum. Ehlinger (1977) observed that certain 

 selected brook trout strains, though resistant to 

 furunculosis, were more susceptible to gill disease 

 than was the native stock. Consequently selection 

 of stocks for resistance to several diseases would 

 be difficult (Mclntyre 1977), except possibly when 

 the pathogens are closely related (Hutt 1970). 



Judging by the present results, it appears that 

 the importance of transferrin genotypes in resis- 

 tance to disease is stock specific. Differences 

 among genotypes were only observed in the Alsea 

 and B X S coho salmon infected with BKD. Wein- 

 berg (1974) noted that different host species may 

 vary in the extent to which they rely on iron- 

 specific nutritional immunity. Although only the 

 most common genotypes were compared within 

 each stock, it is unlikely that other genotypes 

 would have shown greater resistance to BKD; 

 their frequencies within the stocks would have 

 been increased by natural selection if the disease 

 plays an important role as a selective agent. How- 

 ever, it is apparent that factors other than disease 

 may select for different transferrin genotypes. In 

 Ukranian carp, Cyprinus carpio, general survival 

 rates were highest among individuals with the AC 

 genotype (Balakhnin and Galagan 1972). There is 

 also an association of transferrin phenotype with 

 weight gain in juvenile rainbow trout that may be 

 due to the linkage of the transferrin locus with a 

 gene or gene complex affecting growth (Reinitz 

 1977). The association of resistance to BKD with 

 transferrin genotype may also be due to a gene 

 linkage; if so, transferrin serves only as a marker. 

 Mclntyre and Johnson (1977) observed higher 

 growth rates and better survival in AA than in AC 



transferrin genotypes of Big Creek coho salmon. 

 While the frequency of the C allele is high in the 

 Alsea stock , that frequency is depressed in a mixed 

 population at Big Creek where Alsea coho salmon 

 have been used to supplement the broodstock (J. 

 D. Mclntyre unpubl. data). Although BKD selects 

 for the C allele in the Alsea coho salmon, the 

 advantage of this allele is offset by some other 

 more important selective factor, such as growth 

 rate, within the Big Creek stock. 



It is also conceivable that transferrin genotypes 

 provide resistance to different diseases, or not at 

 all — as with vibriosis. The ability to synthesize 

 iron chelators — compounds necessary to remove 

 iron from transferrin — is considered a virulence 

 factor for certain pathogens (Arnold et al. 1977). 

 Perhaps the iron chelators of V. anguillarum re- 

 move iron from transferrin more efficiently than 

 do those of BKD bacteria. This more efficient re- 

 moval would explain to some extent the lack of 

 differential resistance to vibriosis among 

 genotypes within both coho salmon and steelhead 

 trout stocks. Pratschner ( 1978) observed differen- 

 tial resistance among transferrin phenotypes to 

 vibriosis and several other diseEises in coho salmon 

 from the Skagit River, Wash. The AA phenotype 

 exhibited greater susceptibility to vibriosis and 

 cytophagosis but greater resistance to furun- 

 culosis while the CC phenotype was most resistant 

 to vibriosis and very susceptible to furunculosis 

 and cytophagosis. The disparity between 

 Pratschner's and our results with respect to vib- 

 riosis may be due to the stock-specific nature of 

 transferrin. Possibly differences among transfer- 

 rin genotypes are more significant in a chronic 

 disease such as BKD, and less so in an acute dis- 

 ease such as vibriosis — or perhaps the rapid death 

 rate following exposure to V. anguillarum com- 

 pressed the results too much to allow differences to 

 be observed. Because of the short time span in- 

 volved to vibriosis infections, the benefit of such 

 differences to individual fish would be negligible. 



Keeping in mind such considerations as selec- 

 tion for transferrin genotypes by different factors 

 such as growth or disease, it becomes clear ( as with 

 stocks) that selectively breeding for certain trans- 

 ferrin genotypes would not be advisable. Though 

 selection for one particular genotype might pro- 

 vide resistance to BKD, it might also entail lower 

 growth rates or even greater susceptibility to 

 other diseases. Mclntyre (1977) cautiously rec- 

 ommended selective breeding for disease resis- 

 tance only in propagated fish being held under 



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