Do sea lice reduce the survival of juvenile sockeye salmon?
2015-08-13
2015-08-13
Godwin, S.C., Dill, L.M., Reynolds, J.D., Krkošek, M. 2015. Sea lice, sockeye salmon, and foraging competition: Lousy fish are lousy competitors. Canadian Journal of Fisheries and Aquatic Sciences. 72(7):1113-1120. [PDF] [News coverage by the Vancouver Sun and The Globe and Mail]
Juvenile Fraser River sockeye migrate north through Johnstone Strait.
Salmon are exceptionally important to the west coast of North America, not only culturally and economically, but also ecologically. Pacific salmon are born in freshwater and spend most of their lives at sea before returning to freshwater to reproduce. They invariably die upon their return to freshwater, but in the process they bring huge quantities of nutrients from the ocean to their freshwater systems and surrounding terrestrial environments.
Humans tend to focus on the return migration of salmon from saltwater to freshwater because this is where we catch most of our wild salmon. In contrast, the juvenile out-migration – where young salmon move from their freshwater systems to the ocean – is little studied and poorly understood.
Migration is usually a taxing event for animals, and the juvenile salmon out-migration is no exception. A large proportion of juvenile salmon die during their early marine life, but little is known about factors that govern their survival. What we do know is that upon entering the ocean and starting their migration, these young fish must cope with many new challenges including increased predation, inconsistent food sources, and pathogens like sea lice.
Sea lice are parasites that feed on the surface tissue of fish. They are native pathogens to salmon in BC, but when fish farms holding high densities of Atlantic salmon are situated on the migration routes of wild salmon, sea lice numbers on the wild fish are often amplified due to this additional reservoir host. Sea louse infection of juvenile Fraser River sockeye has previously been linked to salmon farms. While sea lice numbers are thought to be kept in check on farms by an in-feed pesticide that is used when parasite levels get too high, the primary species of sea louse that infects juvenile Fraser River sockeye is not considered when farms are deciding whether treatment is necessary.
Humans tend to focus on the return migration of salmon from saltwater to freshwater because this is where we catch most of our wild salmon. In contrast, the juvenile out-migration – where young salmon move from their freshwater systems to the ocean – is little studied and poorly understood.
Migration is usually a taxing event for animals, and the juvenile salmon out-migration is no exception. A large proportion of juvenile salmon die during their early marine life, but little is known about factors that govern their survival. What we do know is that upon entering the ocean and starting their migration, these young fish must cope with many new challenges including increased predation, inconsistent food sources, and pathogens like sea lice.
Sea lice are parasites that feed on the surface tissue of fish. They are native pathogens to salmon in BC, but when fish farms holding high densities of Atlantic salmon are situated on the migration routes of wild salmon, sea lice numbers on the wild fish are often amplified due to this additional reservoir host. Sea louse infection of juvenile Fraser River sockeye has previously been linked to salmon farms. While sea lice numbers are thought to be kept in check on farms by an in-feed pesticide that is used when parasite levels get too high, the primary species of sea louse that infects juvenile Fraser River sockeye is not considered when farms are deciding whether treatment is necessary.
Ecological processes like competition and predation determine juvenile salmon survival.
Sea lice are unlikely to directly kill large numbers of juvenile sockeye since these fish are usually relatively large and well-covered with scales when they enter the ocean. It is more likely that sea lice indirectly reduce juvenile sockeye survival by influencing ecological processes like predation and competition. For example, if licier fish are less successful at competing for food, they would be at greater risk of starvation.
We performed an experiment with juvenile sockeye salmon to determine whether fish that are highly infected with sea lice have reduced abilities to compete for food. We caught juvenile sockeye in northern Johnstone Strait during their out-migration from the Fraser River to their offshore feeding grounds. We transported the captured fish to our experiment site nearby in Echo Bay. After an acclimatization period, we put equal numbers of highly infected fish and lightly infected fish in net pens with limited amounts of food for a short length of time. After the experiment, we weighed their stomach contents to determine which ones ate more.
We found that highly infected sockeye were 20% less successful at consuming food, on average, than lightly infected fish. Surprisingly, we weren't able to use many completely uninfected fish in our experiment because there were sea lice on almost every fish that we caught in the wild (>99% prevalence).
We performed an experiment with juvenile sockeye salmon to determine whether fish that are highly infected with sea lice have reduced abilities to compete for food. We caught juvenile sockeye in northern Johnstone Strait during their out-migration from the Fraser River to their offshore feeding grounds. We transported the captured fish to our experiment site nearby in Echo Bay. After an acclimatization period, we put equal numbers of highly infected fish and lightly infected fish in net pens with limited amounts of food for a short length of time. After the experiment, we weighed their stomach contents to determine which ones ate more.
We found that highly infected sockeye were 20% less successful at consuming food, on average, than lightly infected fish. Surprisingly, we weren't able to use many completely uninfected fish in our experiment because there were sea lice on almost every fish that we caught in the wild (>99% prevalence).
A juvenile sockeye salmon ready to be examined for sea louse infection.
Recent research suggests that environmental conditions in Johnstone Strait limit food for juvenile sockeye. For these fish to survive the trip through Johnstone Strait, they may have to have sufficient energy reserves before entering the Strait and be able to capitalize on whatever food they can find. If their ability to compete for this limited food was impaired – say, by sea louse infection – their risk of starvation would increase.
We know now that pathogens like sea lice have the potential to indirectly affect the survival of fish like Fraser River sockeye salmon, but more work needs to be done to figure out how and whether such effects scale up to a population level. Ultimately, given the prevalence of sea louse infection that we observed and the lack of treatment on fish farms for the species of louse that infects juvenile Fraser River sockeye, parasite management practices should be improved on farms along the migration route of these fish.
We know now that pathogens like sea lice have the potential to indirectly affect the survival of fish like Fraser River sockeye salmon, but more work needs to be done to figure out how and whether such effects scale up to a population level. Ultimately, given the prevalence of sea louse infection that we observed and the lack of treatment on fish farms for the species of louse that infects juvenile Fraser River sockeye, parasite management practices should be improved on farms along the migration route of these fish.