Parasites of invasive freshwater fishes and the factors affecting their richness

Freshwater environments are threatened by nonnative species introductions, often involving fishes. Parasites and pathogens introduced with fishes have the potential to infect native taxa and should be investigated. We examined 726 fishes representing 6 invasive species from 27 ponds in California to evaluate how parasite richness, infection prevalence, and parasite abundance varied with host body size, host species, pond identity, and environmental variables. We identified 14 parasitic taxa based on morphological and molecular techniques. Overall, 60% of fish were infected with ≥1 parasite taxa: 6 trematodes, 3 monogeneans, 3 nematodes, 1 larval cestode (unidentified), and 1 acanthocephalan. Micropterus salmoides had the highest average infection prevalence (85% ± 0.08 SE, n = 17 site–year combinations), the highest average infection abundance (292.51 ± 119.10 SE parasites/fish), and the 2nd- highest γ diversity (10 taxa; Lepomis macrochirus had 11 total parasite taxa). In contrast, Gambusia affinis consistently supported the lowest parasite abundance, infection prevalence, and richness. Across all fish species, larger body size was associated with a broader diversity of parasites, but host sex had no effect. Increased pond area, pH, and aquatic community diversity also were correlated positively with parasite richness in fish populations, consistent with potential influences of colonization opportunities and the ‘diversity begets diversity’ hypothesis. A shortage of historical information on the native ranges of observed parasites precluded an assessment of whether these infections were introduced with their fish hosts. Nonetheless, they have the potential to directly or indirectly affect interactions between introduced fishes and native fauna, particularly given the near-absence of native lentic fishes in this region and the fact that several infections are shared with sensitive endemic taxa, such as amphibians.