Rapid proliferation of the parasitic copepod, Salmincola californiensis (Dana), on kokanee salmon, Oncorhynchus nerka (Walbaum), in a large Colorado reservoir

Abstract

Ecologically and economically valuable Pacific salmon and trout ( Oncorhynchus spp.) are widespread and susceptible to the ectoparasite Salmincola californiensis (Dana). The range of this freshwater copepod has expanded, and in 2015, S . californiensis was observed in Blue Mesa Reservoir, Colorado, USA, an important kokanee salmon ( O . nerka , Walbaum) egg source for sustaining fisheries. Few S . californiensis were detected on kokanee salmon in 2016 ( S . californiensis maximum). By 2020, age‐3 kokanee salmon had 100% S . californiensis prevalence and mean intensity exceeding 50 adult copepods. Year and kokanee salmon age/maturity (older/mature) were consistently identified as significant predictors of S . californiensis prevalence/intensity. There was evidence that S . californiensis spread rapidly, but their population growth was maximized at the initiation (the first 2–3 years) of the invasion. Gills and heads of kokanee salmon carried the highest S . californiensis loads. S . californiensis population growth appears to be slowing, but S . californiensis expansion occurred concomitant with myriad environmental/biological factors. These factors and inherent variance in S . californiensis count data may have obscured patterns that continued monitoring of parasite–host dynamics, when S . californiensis abundance is more stable, might reveal. The rapid proliferation of S . californiensis indicates that in 5 years a system can go from a light infestation to supporting hosts carrying hundreds of parasites, and concern remains about the sustainability of this kokanee salmon population.

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