Mortality of Coho Salmon (Oncorhynchus kisutch) Associated with Burdens of Multiple Parasite Species

Multiple analytical techniques were used to evaluate the impact of multiple parasite species onthe mortality of threatened juvenile coho salmon (Oncorhynchus kisutch) from the West Fork SmithRiver, Oregon, USA. We also proposed a novel parsimonious mathematical representation ofmacroparasite distribution, congestion rate, which i) is easier to use than traditional models, and ii) isbased on Malthusian parameters rather than probability theory. Heavy infections of Myxobolusinsidiosus (Myxozoa) and metacercariae of Nanophyetus salmincola and Apophallus sp. occurred inparr (subyearlings) from the lower mainstem of this river collected in 2007 and 2008. Smolts(yearlings) collected in 2006 - 2009 always harbored fewer Apophallus sp. with host mortalityrecognized as a function of intensity for this parasite. Mean intensity of Apophallus sp. in lowermainstem parr was 753 per fish in 2007 and 856 per fish in 2008, while parr from the tributaries had amean of only 37 or 13 parasites per fish, respectively. Mean intensity of this parasite in smolts rangedbetween 47 - 251 parasites per fish. Over-dispersion (variance to mean ratios) of Apophallus sp. wasalways lower in smolts compared with all parr combined or lower mainstem parr. Retrospectiveanalysis based on smolt data using both the traditional negative binomial truncation technique and ourproposed congestion rate model showed identical results. The estimated threshold level for mortalityinvolving Apophallus sp. was at 400 - 500 parasites per fish using both analytical methods. Unique tothis study, we documented the actual existence of these heavy infections prior to the predictedmortality. Most of the lower mainstem parr (approximately 75%) had infections above this level.Heavy infections of Apophallus sp. metacercariae may be an important contributing factor to the highover-wintering mortality previously reported for these fish that grow and develop in this section of theriver. Analyses using the same methods for M. insidiosus and N. salmincola generally pointed tominimal parasite-associated mortality.

This research was funded, in most part, by an Oregon Department of Fish and Wildlife(ODFW), USA, Fish Health Graduate Research fellowship (agency grant 010-7032-IAA-FISH) toJ.A.F.