Early marine ecology of Pacific salmon: interactions with sea lice.
Date
2011-11-29
Authors
Price, Michael Harold Howard
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Abstract
Pacific salmon (Oncorhynchus spp.) are key elements of ecological systems, and play an important role in the cultural foundation of human societies. All species of wild salmon face multiple, simultaneous threats, with habitat degradation likely playing a key role in survival. Open net-pen salmon farms can degrade important nursery marine habitat for wild juvenile salmon by disrupting natural salmonid host-parasite dynamics. The first two chapters in this thesis examine louse parasitism of wild juvenile chum (Oncorhynchus keta), pink (O. gorbuscha), and sockeye salmon (O. nerka) in relation to their marine migration past salmon farms. I compare sites of low and high exposure to salmon farms, and include two areas without farms on British Columbia’s central and north coasts to assess baseline infection levels. Louse prevalence and abundance were lowest and most similar to natural baseline levels at low exposure sites, and highest at high exposure sites in all farm regions. A significantly greater proportion of the lice infecting juvenile chum and pink salmon were Lepeophtheirus salmonis at high exposure sites. Caligus clemensi was the principal louse species infecting all juveniles in areas without salmon farms, and at low exposure sites within salmon farm regions; C. clemensi was also the dominant louse to infect juvenile sockeye that migrated past farms. Mixed-effects modelling results showed that exposure to salmon farms was the most consistent factor to explain the variation in louse infection levels, and support my hypothesis that salmon farms are a major source of sea lice on juvenile wild salmon in regions with salmon farms.
I discovered that juvenile sockeye at one particular location within the Georgia Strait hosted unusually high lice levels; this location was situated at a distance from salmon farms, but near a farm salmon processing facility. Upon further investigation, I found live sea lice, Lepeophtheirus salmonis, mucus, and fish tissue in effluent discharged from the processing facility. Sea lice transmitted from this source may pose a threat to wild salmon populations, and the release of potentially untreated offal, including blood water, is of considerable concern. These results form the third chapter in my thesis.
Given the challenges facing juvenile salmon in general, and sockeye from the Fraser River in particular (i.e., 2009 was the lowest return on record), and because poor habitat conditions within Georgia Strait are considered the major cause of the recent decline in Fraser River sockeye, this raises the question as to whether food limitations are a factor. The final chapter in my thesis examines the prey assemblage, diet composition, and foraging selectivity of juvenile sockeye, and investigates whether food limitations can be detected during early migration through Georgia Strait. Juvenile sockeye demonstrated high prey diversity, with preference for particular prey. Prey were more concentrated in the north, which may help explain migratory behavior of juveniles through the study region, and temporal similarities in sockeye foraging success may reflect short-term food resource stability. Moreover, I could not find evidence of food limitations that might suggest juvenile sockeye were strongly food deprived during the years of this study.
Finally, my thesis explores how best to conserve salmon populations given the multitude of stressors. Because stressors often interact to produce compound effects and unpredictable results, ranking the overall threats in order of severity may not be useful. Instead, the most successful ranking system may be in terms of reducing harm where possible. For juvenile salmon during their early marine migration, risks posed by salmon farms can be more easily mitigated than the far-reaching effects on ocean productivity of climate change and ocean acidification, or predator removal. I recommend we begin here.
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Keywords
Pacific Salmon, Conservation, Pathogens, Sea Lice, Salmon farms, Marine ecology