Effects of fish on water mites (Acariformes: Hydrachnidia): consequences for the habitat value of culture ponds

Semi-natural earthen ponds used for fish culture potentially offer habitats for aquatic macroinvertebrates. However, only a few macroinvertebrate taxa have been documented to be able to coexist with fish. Macroinvertebrate interactions with farmed fish are complicated by the ecological heterogeneity brought about by culture practices. We investigated water mite abundance and assemblage composition in cyprinid ponds in which age/size classes of fish with different biomass density and size structure are reared separately. These differences in fish status (standing biomass and individual body size) result in contrasting levels of fish trophic impact on pond communities. We sampled water mites in two layers of the water column: the benthic zone and near the water surface. The relative densities, species richness and Shannon diversity of water mites in the bottom catches were greater in ponds with high standing biomass of large-sized fish than in ponds with low biomass of young fish, but not in the near-surface catches. Redundancy analysis revealed that fish status, total phosphorus concentration and water conductivity were the most important factors shaping water mite assemblages. Non-metric multidimensional scaling also indicated differences in species composition with respect to the fish status of ponds. Our results suggest that some water mites benefit from the trophic dominance of fish and find suitable habitats in culture ponds. Divergences in water mite assemblages driven by fish indicate that ponds differing in fish stocks may complement one another to support high beta diversity of fish farms.