Pathogens are emerging as considerable threats to wildlife around the world. Often, the only mortality considered to be caused by pathogens is that which results directly from disease, but pathogens can also cause indirect effects that increase host mortality and reduce host fitness. Pathogen infection can impact host behaviour, growth, and competitive ability, each of which can in turn affect the survival of the host.
One species of conservation concern that may be particularly vulnerable to pathogen infection is sockeye salmon. Juvenile sockeye pick up high numbers of ectoparasites called sea lice during their early marine migration, part of which is due to the presence of open-net salmon farms along their migration route that act as parasite reservoirs.
In British Columbia, Fraser River sockeye are an iconic set of stocks that together make up one of the most ecologically, economically, and culturally important salmon runs in the world. Fall 2012 marked the conclusion of a 26 million dollar federal judicial inquiry into a two-decade decline in Fraser River sockeye productivity. The final report of this inquiry explicitly stated the need for research on the "cumulative effects" of "pathogens on Fraser River sockeye".
Are lousy sockeye lousy competitors?
In the spring of 2013, I performed a competitive foraging experiment on a floating field station using juvenile Fraser River sockeye collected during their marine out-migration through Johnstone Strait, BC. I found that high intensities of sea louse infection were associated with decreased competitive abilities in juvenile sockeye (Godwin et al. 2015 CJFAS)
Organisms with lower competitive abilities should have reduced foraging success in equal-opportunity, prey-limited environments. In the spring of 2014, I ran field surveys to determine whether wild-feeding sockeye exhibit a similar relationship between foraging success and louse load, and to assess whether this relationship is accentuated in times of lower prey availability.
Do infected juvenile sockeye grow at a slower rate?
Growth is a crucial component of early marine life for Pacific salmon. As salmon grow, their swimming speed and visual acuity increase and they are able to feed on larger prey and avoid smaller predators. To maximize survival, salmon need to get large, fast.
Individuals with reduced competitive ability (e.g. juvenile sockeye infected with sea lice) may consume less food and experience a lower growth rate than those that are better foragers. Sockeye having to use their immune system to respond to a pathogen infection might also experience a trade-off in resource allocation between immune function and growth.
Otoliths are calcified structures in the inner ear of many fish that exhibit incremental daily growth similar to annual rings in trees. I analyzed daily growth rings in wild juvenile sockeye otoliths and found that infected juvenile sockeye grow more slowly than uninfected individuals (Godwin et al. In review).