The effects of temperature and infection by Ichthyophonus were examined in juvenile Pacific herring (Clupea pallasii) maintained under simulated overwinter fasting conditions. In addition to defining parameters for a herring bioenergetics model (discussed in Vollenweider et al. this issue), these experiments provided new insights into factors influencing the infectivity and virulence of the parasite Ichthyophonus. In groups of fish with established disease, temperature variation had little effect on disease outcome. Ichthyophonus mortality outpaced that resulting from starvation alone. In newly infected fish, temperature variation significantly changed the mortality patterns related to disease. Both elevated and lowered temperatures suppressed disease-related mortality relative to ambient treatments. When parasite exposure dose decreased, an inverse relationship between infection prevalence and temperature was detected. These findings suggest interplay between temperature optima for parasite growth and host immune function and have implications for our understanding of how Ichthyophonus infections are established in wild fish populations. 1. Introduction Ichthyophonus hoferi, a highly pathogenic parasite of marine and anadromous fishes, is currently ubiquitous in Pacific herring (Clupea pallasii) populations throughout the NE Pacific [1–3]. Phenotypic [4] and genotypic [5, 6] differences among isolates of I. hoferi suggest that there are multiple sympatric species in the region. Due to this taxonomic uncertainty, here we refer to the parasite by its generic name. Among wild Pacific herring, the prevalence of infection typically increases with age; consequently, the resulting disease (ichthyophoniasis) is thought to affect primarily older age cohorts [2, 7]. However, the impact of Ichthyophonus infections on juvenile Pacific herring has received little attention. In Atlantic herring (Clupea harengus), it has been suggested that time to mortality is shorter for juveniles than for adults resulting in relatively low, but highly variable, prevalence [8]. Prevalence of infection among young-of-the-year (YOY) herring is typically 3–13% in the NE Pacific (USGS unpublished data) with infections detected as early as 4 months after hatch [9]. In addition to causing direct mortality from disease, Ichthyophonus infection may predispose juvenile herring to indirect mortality through predation [10], presumably as a result of decreased swimming performance [11, 12]. This type of indirect mortality is difficult to demonstrate but should be expected considering the
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