Host tissue stoichiometry links nutrient availability to parasite abundance in a piscivorous fish

Understanding how ecosystem nutrient availability influences parasites is challenging, especially for trophically transmitted parasites in hosts occupying higher trophic levels. Host tissue nutrient stoichiometry may elucidate these effects by indicating nutrient availability within a host’s environment and/or mediating these effects on parasites. We assessed these possibilities by manipulating nutrient availability in 11 experimental ponds stocked with piscivorous largemouth bass for 3 months and quantifying the role of pyloric caeca stoichiometry in mediating the relationship between basal nutrient availability, the abundance of both fish and invertebrate consumers (including potential intermediate hosts), and the abundance of the trematode parasite, Crepidistomum spp., in this tissue. Fertilization increased the total phosphorus (P) and decreased the carbon (C):P and nitrogen (N):P ratios in the water column and fine particulate organic matter of ponds. While we did not detect effects of fertilization on intermediate trophic levels, N:P ratios in bass pyloric caeca were inversely related to both total water column P levels and the abundance of Crepidistomum spp. in these tissues. Overall, the nutrient ratios of a key tissue involved in predator digestion mirrored fertilization-induced shifts in basal nutrient availability and explained more variation in the abundance of a trophically transmitted parasite than any other variable we quantified. Whether tissue nutrient ratios directly influenced the abundance of this parasite or simply reflected shifts in the ecosystem that mediated parasite abundance remains unclear. Regardless, digestive tissue nutrient stoichiometry appears to provide an informative link between basal nutrient availability and parasite abundance in these tissues.