Proliferative Kidney Disease (tetracapsuloides Bryosalmonae) in Merced River Hatchery Juvenile Chinook Salmon: Mortality and Performance Impairment in 2005 Smolts

Merced River Hatchery (MRH) juvenile Chinook salmon, from tagged lots used in the 2005 Vernalis Adaptive Management Program (VAMP) study, were brought to the California-Nevada Fish Health Center wet lab 6 days prior to the first VAMP release and reared for 50 days at water temperatures similar to the San Joaquin River. At the time of transport, a fish health inspection showed that the population was generally healthy but had a low prevalence of an early stage infection by the myxosporean parasite, Tetracapsuloides bryosalmonae. This parasite has been detected in Merced River salmon for several decades and causes Proliferative Kidney Disease (PKD). The level of clinical PKD, as demonstrated by a combined kidney lesion and anemia score, markedly increased starting at 29 days post-transfer (dpt) from MRH. Severe disease occurred in the study population after the last VAMP coded wire tag fish was recovered in the Chipp’s Island trawl on 27 May. A total of 76 study salmon died (27% cumulative mortality) due to PKD beginning at 36 dpt through the final sample at 50 dpt. Both dpt and disease state correlated with a decline in hematocrit and plasma magnesium as well as an elevation in circulating white blood cell number and plasma protein concentration. There was no observed PKD effect on time to exhaustion during a 120-min swim challenge until 50 dpt. Smolt development measurements indicated that the study fish were in an advanced stage of smoltification. Similar to swim performance, saltwater adaptation was not impaired until 50 dpt. Tetracapsuloides bryosalmonae was observed in 40% (17 of 43) of the kidney imprints collected from VAMP tag salmon recovered in the Chipp’s Island trawl. These results indicate that while T. bryosalmonae infection is prevalent in Merced River out-migrant salmon, it may not have a significant effect on VAMP recoveries. However, PKD could be a significant mortality factor for Merced River salmon smolts during their early seaward entry phase. CALIFORNIA FISH AND GAME 58