Each of 16 mature female sockeye salmonOncorhynchus nerkafrom two river drainages were spawned with individual males, and the progeny from each pair were later challenged with virulent infectious haematopoietic necrosis (IHN) virus. At each of 4 monthly challenges, distinct differences were detected in the susceptibility of the fish to IHN from each family. Average percentage mortality was 98 or more in highly susceptible families, in families of intermediate susceptibility, 85 and in resistant families, 52. Furthermore, the mean time to death was only 9.8 days for fish from the susceptible families but 17.9 days for fish from the more resistant families. These results were similar at each monthly challenge. Challenging the fish at various virus‐dosage levels did not change total mortality, but high dosages hastened the onset of mortality. A genetic basis for resistance is proposed to explain these result
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