Causes and consequences of dispersal in a marine parasite (Lepeophtheirus salmonis)

This thesis examines the causes and consequences of the dispersal of a marine ectoparasite, the salmon louse (Lepeophtheirus salmonis), in an area of intensive salmon aquaculture where the transmission of lice from farmed salmon may have consequences for the dynamics of adjacent wild salmon populations. Salmon lice are capable of leaving one host in search of another as they approach sexual maturity; while previously thought to be an artefact of the confined conditions characteristic of experiments and salmon farms I show that this is common in nature with at least 50% of lice on juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon moving from one host to another as they mature. I demonstrate that the underlying drivers of this dispersal shift with ontogeny from competition for resources to access to mates. Movement among hosts increases the probability that a louse interacts with multiple hosts potentially increasing the extent of louse transmission from farmed to wild salmon and the spread of other pathogens if lice can act as vectors. In addition, the movement of lice among hosts may have important consequences for predator-prey interactions between salmonids during early marine life. Coho salmon smolts (O. kisutch) selectively prey upon infected pinks. I show that sea lice transfer from pinks to coho during these interactions and as a result coho experience a 2 to 3 fold increase in parasite exposure over what they would otherwise experience through passive exposure to infective larvae from farms. To test for a population level response to this increase in louse exposure I examined coho population dynamics spanning a region of exposure to lice from infected pink salmon and salmon farms and show that populations exposed to recurrent infestations were depressed 7 fold relative to adjacent unexposed populations. These findings highlight the ecosystem context in which louse transmission from farmed to wild fish occurs and suggest that species interactions and parasite behaviour may cause the effects of parasite transmission from farmed to wild fish to propagate up marine food webs with broader consequences than previously appreciated.