Aquaculture-driven collapse of white fish communities reduces helminth prevalence in great cormorants

Aquaculture intensification has transformed fish communities in Central European ponds, with common carp (Cyprinus carpio) now dominating harvests at the expense of diverse assemblages of cyprinids and percids. These changes are expected to alter parasite transmission to piscivorous birds, which serve as definitive hosts for helminths. We investigated helminth communities in two major avian predators, the great cormorant (Phalacrocorax carbo, n = 126) and the grey heron (Ardea cinerea, n = 577), from Czech fishponds at two time points: the late 1990s and 2022-2025. In cormorants, dietary analysis revealed a shift from a mixed prey of white fish and carp in 1997 to carp alone in recent years, including individuals exceeding 650 g. This dietary simplification coincided with significant declines in the prevalence and intensity of several dominant digeneans (e.g., Hysteromorpha triloba and Petasiger phalacrocoracis) and the nematode Contracaecum rudolphii, with coinfections now involving fewer helminth species and reduced overall burdens. Grey herons showed a different pattern. Prevalence of major taxa, particularly Apharyngostrigea cornu, Uroproctepisthmium bursicola, Echinochasmus beleocephalus, and Posthodiplostomum cuticola, remained relatively stable across decades. However, mean intensities declined, and individuals with high infection intensities were no longer observed. These contrasting outcomes reflect differences in foraging ecology, with cormorants tightly linked to pond fish stocks, whereas herons exploit broader habitat mosaics. Our findings demonstrate that aquaculture-driven simplification of fish assemblages cascades into parasite transmission networks, causing asymmetric erosion of helminth communities in definitive avian hosts. Helminths of fish-eating birds thus serve as sensitive bioindicators of ecological change in pondscapes dominated by carp production.