Parasites perform poorly at high host densities: an experimental test in the salmon louse <i>Lepeophtheirus salmonis</i>

The link between host density and parasite population size is well-established. However, while high host density facilitates parasite transmission, it may negatively impact parasites through more intense competition and/or more intense host immune defence. We assessed the performance of individually tagged female salmon lice (Lepeophtheirus salmonis), an ectoparasite of Atlantic salmon (Salmo salar), under experimental low and high host density conditions, the latter mimicking salmon aquaculture conditions. Lice at high density exhibit greater pre-reproductive mortality, reduced growth, delayed maturation, smaller adult sizes and lower fecundity. Adult size in salmon lice correlates negatively with age at maturity and positively with early fecundity, lifetime fecundity and lifespan. Thus, lice in high-density host populations perform poorly across all life-history traits, probably owing to stronger immune responses of hosts initially exposed to higher parasite doses. This study suggests density-dependent regulation of parasites in intensive agriculture, where hosts are farmed at high densities. Our results further suggest that salmon lice in aquaculture may be selected for greater resistance to host immune defence. As host populations now consist overwhelmingly of farmed fish, continued research is essential to understand the implications for wild salmonids, which are frequently infected by lice originating from farms.