Graduate Thesis Or Dissertation
 

Distribution and coinfection of microparasites and macroparasites in juvenile salmonids in three upper Willamette River tributaries

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  • Wild fish populations are typically infected with a variety of micro- and macroparasites that may affect fitness and survival, however, there is little published information on parasite distribution in wild juvenile salmonids in three upper tributaries of the Willamette River, OR. The objectives of this survey were to document (1) the distribution of select microparasites in wild salmonids and (2) the prevalence, geographical distribution, and community composition of metazoan parasites infecting these fish. From 2011-2013, I surveyed 279 Chinook salmon Oncorhynchus tshawytscha and 149 rainbow trout O. mykiss for one viral (IHNV) and four bacterial (Aeromonas salmonicida, Flavobacterium columnare, Flavobacterium psychrophilum, and Renibacterium salmoninarum) microparasites known to cause mortality of fish in Willamette River hatcheries. The only microparasite detected was Renibacterium salmoninarum, causative agent of bacterial kidney disease, which was detected at all three sites. I identified 23 metazoan parasite taxa in these fish. Nonmetric multidimensional scaling of metazoan parasite communities reflected a nested structure with trematode metacercariae being the basal parasite taxa at all three sites. The freshwater trematode Nanophyetus salmincola was the most common macroparasite observed at three sites. Metacercariae of N. salmincola have been shown to impair immune function and disease resistance in saltwater. To investigate if N. salmincola affects disease susceptibility in freshwater, I conducted a series of disease challenges to evaluate whether encysted N. salmincola metacercariae increase susceptibility of juvenile Chinook salmon to Flavobacterium columnare and Aeromonas salmonicida infection. These bacteria cause high mortality in juvenile hatchery salmonid populations in the Willamette River, and are a potential threat to wild juvenile salmonids. Juvenile Chinook salmon were first infected with N. salmincola through cohabitation with infected freshwater snails, Juga spp., then challenged through static immersion with either F. columnare or A. salmonicida. Cumulative percent mortality from F. columnare was higher in N. salmincola-parasitized compared to non-parasitized juvenile Chinook salmon. In contrast, cumulative percent mortality from A. salmonicida did not differ between N. salmincola-parasitized and non-parasitized juvenile Chinook salmon. No mortalities were observed in the N. salmincola-only and control groups from either challenge. These results show that a high infection intensity (>200 metacercariae per posterior kidney) of encysted N. salmincola metacercariae does not cause mortality alone, but can increase susceptibility to certain bacterial infections in juvenile Chinook salmon.
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