Abstract
Novel egg‐laying boards were found to be effective in the biological control of the freshwater fish louse Argulus foliaceus in a 12·9 ha rainbow trout Oncorhynchus mykiss fishery which had a high prevalence and intensity of infection of juvenile parasites in the early spring of 1999. Approximately 228 000 egg clutches were harvested during an extensive 14 week period of egg laying which peaked in June 1999. In contrast, only 1566 clutches were harvested in 2000, when egg laying activity showed a bi‐modal distribution, peaking in May and again in July and August. Egg laying activity decreased 145‐fold compared to 1999. Argulus foliaceus prevalence and mean intensity also decreased nine‐fold and six‐fold, respectively. The ratio of female to male A. foliaceus on rainbow trout in consecutive years was 2·9: 1 and 2·1: 1. Estimates of the size of the female A. foliaceus population based on egg‐laying activity in 1999 exceeded that derived from measurements of prevalence and intensity of infection, whereas in 2000, this was more in balance. A minimum temperature of 10° C was identified for egg laying, which occurred continuously from May to October in a broadly synchronous manner. This produced almost two generations each year, with juveniles, adults and eggs undergoing anabiosis during winter.