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 affec t the prevalence and levels of bacterial kidney disease (BKD) in proge ny fish during hatchery rearing. Smelts from that study were subjected to standardized fish health and condition evaluation procedures 2 wee ks before the conclusion of hatchery rearing and release of the fish f or migration to the Pacific Ocean. The results suggested that the gene ral health of the smelts in the progeny group from parents that had lo w R. salmoninarum infection levels or tested negative for R. salmonina rum (low-BKD group) was better than that of the smelts 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 smelts from the lo w-BKD group. Subgroups of smelts from the study were acclimated to tan ks of seawater for extended holding. After a 22-day acclimation period and 98 days in full-strength (29 ppt salinity) seawater, total mortal ity 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 hi gh-BKD group occurred after the fish were transferred to full-strength seawater. Testing of kidney tissues from all dead fish by the FAT rev ealed 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 detectab le numbers of R. salmoninarum. We concluded that brood stock segregati on by use of the ELISA and the FAT can affect mortality and the R. sal moninarum status of progeny chinook salmon for as long as 21 months af ter hatching, even after the fish have been transferred to seawater.