Different prevalences of Renibacterium salmoninarum detected by ELISA in Alaskan chinook salmon Oncorhynchus tshawytscha spawned from freshwater and seawater

Soluble antigen of Renibacterium salmoninarum (Rs) was detected by a polycl onal enzyme-linked immunosorbent assay (ELISA) at significantly higher prev alences in adult chinook salmon Oncorhynchus tshawytscha that matured in fr eshwater than in the same cohort of fish spawned after maturation in seawat er. The cumulative results were consistent during 4 yr of comparison at the Little Port Walter Hatchery on Baranof Island, Alaska, USA. Possible cause s for this difference are discussed. Maturation of chinook salmon broodstoc k in seawater has become a practical strategy at this hatchery to reduce th e prevalence of Rs-positive parent fish and the numbers of culled eggs.