Levels of Renibacterium salmoninarum antigens in resident and anadromous salmonids in the River Ellidaár system in Iceland

In relation to stock enhancement programmes, wild salmon broodfish have been routinely screened for the presence of Renibacterium salmoninarum antigens (Rs‐Ag) for decades. A sudden increase in the prevalence of Rs‐Ag experienced caused extensive problems to this industry as eggs from positive fish are discarded. The prevalence and level of Rs‐Ag were examined in resident and anadromous salmonids in the River Ellidaár system and the progress of Rs‐Ag in a cohort of salmon followed. Both prevalence and Rs‐Ag levels were high in resident salmonids and emigrating salmon smolts in the river system. When the smolts re‐entered their home river as adults the following summer, they were almost free of Rs‐Ag, but the longer they stayed in the river, the more Rs‐Ag they acquired; the majority being positive at spawning. This study demonstrates a high level of Rs‐Ag in salmonids in the River Ellidaár system which significantly reduces in the salmon during its seawater phase. Accordingly, it seems ideal to sample salmon broodfish as soon as possible after ascending the river and subsequently transfer to Rs‐free environment for storage until stripping, which could result in lower Rs‐prevalence and minimize the problems that stock enhancement programmes have faced due to Rs‐positive wild broodfish.