American Shad and Altered Parasite/Disease Dynamics in Oregon Waters

One factor that contributes to the establishment of exotic species in new environments is a release from pathogens that infected them in their native range.  This release contributes to decreased morbidity and mortality as well as to increased condition and fecundity in recently-introduced populations.  However, the presence of exotic species may alter the dynamics of endemic pathogens, which may eventually colonize the new species as alternative hosts or vectors. Recent studies have shown that American Shad in the Pacific Northwest have been colonized by the by the endemic marine parasites Ichthyophonus and Anisakis, which do not infect them in their native range. We report the comparison of populations of American shad sampled in 1975 and 2003-2007 in the Columbia, Willamette and Umpqua rivers with populations in their native range. Fecundity and fitness (K) of Willamette shad in 2005/06 was significantly lower than for PNW populations in 1975, and is now similar to East coast stocks at latitude of the source population in the Hudson. However, absolute and relative (worm/kg host) infection levels with Anisakis were not correlated with K, GSI, or fecundity, indicating the recent colonization event has already come to host-parasite equilibrium. Implications for fish, wildlife, and human consumers from the ecological expansion of formerly marine parasites into freshwater systems are discussed.