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Helminth Burdens Fell in Cormorants and Grey Herons as Czech Fishponds Became Carp-Dominated

Wild grey heron and great cormorant together near water
Research · Disease, parasites & microbiology

A long-term comparison of fish-eating birds in Czech fishponds found declining helminth parasite burdens in both great cormorants (Phalacrocorax carbo) and grey herons (Ardea cinerea), although the changes were much stronger in cormorants. The study was published online on 5 March 2026 in the International Journal for Parasitology.

Researchers compared helminth communities from the late 1990s with birds examined in 2022–2025. The material included 126 great cormorants and 577 grey herons. The study was designed around major changes in Central European fishponds, where aquaculture has increasingly favoured common carp (Cyprinus carpio) while diverse communities of other cyprinid and percid fishes have declined.

Carp-dominated diets coincided with sharp parasite declines in cormorants

In great cormorants, dietary data showed a clear shift. In 1997 the birds ate a mixture of so-called white fish and carp, whereas recent samples were dominated entirely by carp, including fish weighing more than 650 g. This change coincided with significant declines in the prevalence and intensity of several common helminths, including the digeneans Hysteromorpha triloba and Petasiger phalacrocoracis and the nematode Contracaecum rudolphii. Cormorants also carried fewer parasite species simultaneously.

Heron prevalence stayed steadier while infection intensity fell

Grey herons showed a different pattern. The prevalence of several major helminth taxa remained relatively stable across the decades, including Apharyngostrigea cornu, Uroproctepisthmium bursicola, Echinochasmus beleocephalus and Posthodiplostomum cuticola. Mean infection intensities nevertheless declined, and heavily infected individuals were no longer recorded in the recent material.

The authors link the contrast between the two birds to their feeding ecology. Cormorants depend more directly on fish stocks within the ponds, while grey herons use a broader range of habitats and food sources. They conclude that the simplification of fish communities caused by carp-dominated aquaculture can alter parasite transmission through the food web, with different effects on different fish-eating bird species.

Sources and image credit

  1. Heneberg, P., & Sitko, J. (2026). Aquaculture-driven collapse of white fish communities reduces helminth prevalence in great cormorants. International Journal for Parasitology. Advance online publication. https://doi.org/10.1016/j.ijpara.2026.104820Primary peer-reviewed study available online 5 March 2026. It compared helminth communities in 126 great cormorants and 577 grey herons from Czech fishponds between the late 1990s and 2022–2025; parasite declines were strongest in cormorants after the shift to carp-dominated diets, while mean infection intensities also declined in grey herons.

Image: José Prego — wild grey heron (Ardea cinerea) and great cormorant (Phalacrocorax carbo) together in the Moura area, Alentejo, Portugal, photographed 16 January 2022; Wikimedia Commons / Flickr; both study species shown together, not birds from the Czech study · CC BY 2.0 · Source

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Disease, parasites & microbiology

SPECIES IN THIS STORY

Species in this story

Great Cormorant Phalacrocorax carbo Explore species Grey Heron Ardea cinerea Explore species

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