Epizootiology of Parvicapsula minibicornis in Fraser River sockeye salmon, Oncorhynchus nerka (Walbaum)

Late-spawning Fraser River sockeye salmon, O. nerka, stocks have suffered significant pre-spawn mortality associated with an unusually early freshwater migration pattern and the myxosporean parasite, P. minibicornis. Surveys of migrating adult salmon from several spawning populations were conducted in 1999 and 2000 to determine the extent of infection with P. minibicornis, when and where the parasite first became detectable during migration, and whether early migrating stocks might be used as sentinels to assess risk of infection in late-spawning stocks. Posterior kidney, preserved in 95% ethanol, was examined for P. minibicornis in stained histological sections and using a polymerase chain reaction (PCR) test. The prevalence of this parasite in all Fraser River sockeye salmon stocks examined was high (range 47-100% infected). In contrast, P. minibicornis was not detected in the fish tested from the 2 sockeye salmon stocks outside the Fraser River drainage in either 1999 or 2000. The parasite was also not detected histologically or by PCR in the kidney tissue of the fish from the Fraser River that were sampled in salt water or early during their freshwater migration up the river. These findings and the progression in the prevalence and intensity of infection as the fish from 3 stocks (early Stuart, Weaver Creek and Cultus Lake) were monitored over time, suggest that the salmon acquired the parasite either in the lower Strait of Georgia or in the lower Fraser River before the confluence of the Harrison River. In both 1999 and 2000, the parasite was present in all Fraser River sockeye salmon stocks sampled, which suggests that early Stuart salmon may be valuable as a sentinel stock for the presence of the parasite in later-spawning stocks.