Brood stock segregation for the control of bacterial kidney disease can affect mortality of progeny chinook salmon (Oncorhynchus tshawytscha) in seawater

Segregation of spring chinook salmon (Oncorhynchus tshawytscha) brood stock based on the measurement of maternal Renibacterium salmoninarum infection levels by the enzyme-linked immunosorbent assay (ELISA) and the fluorescent antibody technique (FAT) was previously shown to affect the prevalence and levels of bacterial kidney disease (BKD) in progeny fish during hatchery rearing. Smolts from that study were subjected to standardized fish health and condition evaluation procedures 2 weeks before the conclusion of hatchery rearing and release of the fish for migration to the Pacific Ocean. The results suggested that the general health of the smolts in the progeny group from parents that had low R. salmoninarum infection levels or tested negative for R. salmoninarum (low-BKD group) was better than that of the smolts in the progeny group from female parents with high R. salmoninarum infection levels (high-BKD group). Testing by the ELISA showed that the overall severity of R. salmoninarum infection also was lower in the smolts from the low-BKD group. Subgroups of smolts from the study were acclimated to tanks of seawater for extended holding. After a 22-day acclimation period and 98 days in full-strength (29 ppt salinity) seawater, total mortality was 12% in the low-BKD group and 44% in the high-BKD group. All of the mortality in the low-BKD group and 85% of the mortality in the high-BKD group occurred after the fish were transferred to full-strength seawater. Testing of kidney tissues from all dead fish by the FAT revealed that 85% of the fish that died in the high-BKD group had high R. salmoninarum numbers, indicating that BKD was the cause of death. In contrast, none of the fish that died in the low-BKD group had detectable numbers of R. salmoninarum. We concluded that brood stock segregation by use of the ELISA and the FAT can affect mortality and the R. salmoninarum status of progeny chinook salmon for as long as 21 months after hatching, even after the fish have been transferred to seawater.