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Cormorant Colony Was Estimated to Consume 4.2–32.4 Tonnes of Fish Annually at Raszyńskie Ponds

Great cormorant in flight against a blue sky
Conservation · Human–wildlife conflict

A study in the Raszyńskie Ponds Nature Reserve in central Poland estimated that the breeding great cormorant (Phalacrocorax carbo) colony consumed between 4.2 and 32.4 tonnes of fish per year during 2015–2024, depending on colony size. The reserve protects important waterbird habitat while also supporting fishponds where carp and other cyprinids account for more than 90% of annual fish production.

Researchers counted 92 occupied cormorant nests in 2024 and combined that count with colony records extending back to 1984. They also analysed 100 regurgitated pellets collected during spring and summer 2024 to identify prey and estimate fish consumption.

Eight fish species were identified

The pellets contained remains of 218 fish representing eight species. Common carp dominated the diet, accounting for 57.8% of identified prey fish and 72.5% of estimated prey biomass, reflecting the fish-production system in the ponds. Estimated prey length ranged from 10.2 to 33.6 centimetres; the largest reconstructed prey were a northern pike (Esox lucius) of 33.6 centimetres and a grass carp (Ctenopharyngodon idella) of 31.1 centimetres.

Pellets collected in spring contained significantly more fish than those collected in summer, averaging 2.5 and 2.3 fish respectively. Estimated daily biomass consumption did not differ significantly between the periods and was about 300 grams per bird in both.

Colony size drove the annual consumption estimate

Using the 2024 daily consumption estimate together with colony size in each year, the researchers calculated that total fish intake ranged from 4.2 to 32.4 tonnes per year between 2015 and 2024. For the 92 occupied nests recorded in 2024, the estimate was 20.1 tonnes. Because common carp accounted for 72.5% of estimated prey biomass, the authors estimated annual carp consumption at 3.04–23.49 tonnes, depending on colony size. These figures are modelled annual consumption estimates rather than direct measurements of all fish eaten. The study therefore describes a management tension within the reserve: the same protection that provides suitable conditions for the cormorant colony can also increase predation pressure on the fish community.

The authors argue that continued monitoring is needed to balance protection of the bird colony with conservation and management of the reserve’s fish populations. Their results provide a quantitative estimate of the scale of cormorant predation rather than treating the conflict only through observations of bird numbers.

Sources and image credit

  1. Karpińska, K., Czerniejewski, P., Brysiewicz, A., Kierasińska, B., Horaczek, T., & Naziębło, A. (2026). Protected fish predator in a protected area: Impact of the Great Cormorant (Phalacrocorax carbo L.) on fish populations in the Raszyńskie Ponds Nature Reserve (Central Poland). The European Zoological Journal, 93(1), 412–425. https://doi.org/10.1080/24750263.2026.2638038Primary open-access peer-reviewed study published online 10 March 2026. Researchers counted the breeding colony and analysed 100 regurgitated pellets from spring and summer 2024, identifying 218 fish from eight species. Applying 2024 consumption rates to annual colony size produced an estimated total fish intake of 4.2–32.4 tonnes per year during 2015–2024.

Image: Charles J. Sharp — great cormorant (Phalacrocorax carbo) in flight at Farmoor Reservoir, Oxfordshire, United Kingdom, photographed 10 July 2014; Wikimedia Commons; representative species image, not a bird from the Raszyńskie Ponds colony · CC BY-SA 3.0 · Source

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