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&#xa0;californiensis (Dana). The range of this freshwater copepod has expanded, and in 2015, S.&#xa0;californiensis was observed in Blue Mesa Reservoir, Colorado, USA, an important kokanee salmon (O.&#xa0;nerka, Walbaum) egg source for sustaining fisheries. Few S.&#xa0;californiensis were detected on kokanee salmon in 2016 (<10% prevalence; 2 adult S.&#xa0;californiensis maximum). By 2020, age-3 kokanee salmon had 100% S.&#xa0;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.&#xa0;californiensis prevalence/intensity. There was evidence that S.&#xa0;californiensis spread rapidly, but their population growth was maximized at the initiation (the first 2-3&#xa0;years) of the invasion. Gills and heads of kokanee salmon carried the highest S.&#xa0;californiensis loads. S.&#xa0;californiensis population growth appears to be slowing, but S.&#xa0;californiensis expansion occurred concomitant with myriad environmental/biological factors. These factors and inherent variance in S.&#xa0;californiensis count data may have obscured patterns that continued monitoring of parasite-host dynamics, when S.&#xa0;californiensis abundance is more stable, might reveal. The rapid proliferation of S.&#xa0;californiensis indicates that in 5&#xa0;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.