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Estimated Great Cormorant Predation Reached 52–88% in Tagged Cod and 14–84% in Tagged Flounder

Great cormorant at Østerø Lake in Nyborg Municipality, Denmark
Research · Ecology

A PIT-tag study in Genner Bay in southern Denmark produced very high predation estimates for great cormorants (Phalacrocorax carbo; the study concerns P. c. sinensis) in some size classes of cod and flounder. Across size classes, researchers estimated that cormorants consumed 52–88% of the tagged cod and 14–84% of the tagged flounder. For each release, the relevant follow-up period ran from tagging until the cormorants had left the colony, roughly 2–7 months, so these figures are not annual predation rates for the full fish populations.

In 2022 and 2024, the team tagged 2,783 cod (Gadus morhua) measuring 11–38 cm and 2,644 flounder (Platichthys flesus) measuring 12–30 cm. The fish were caught in pound nets and released after tagging. Each passive electronic PIT tag carries a unique code. Researchers then scanned nearby cormorant colonies and roosts for tags deposited after tagged fish had been eaten. Two feeding trials were used to estimate how often a tag from an eaten fish would actually be detected, allowing the predation estimates to be corrected for missed tags.

Predation varied with fish size and was particularly size-selective in flounder. In cod, predation generally increased from the smaller fish towards the intermediate-to-larger size classes before dropping again among the largest fish. In flounder, the contrast was sharper: predation was highest in intermediate size classes and fell steeply in the largest fish. The PIT approach therefore provided unusually direct evidence of which tagged individual fish had entered the cormorants’ diet, rather than inferring total consumption only from pellets or colony size.

The authors interpret the results from the open coastal system as evidence of a potential mortality bottleneck for cod and flounder in western Baltic areas with dense cormorant populations. They do not conclude, however, that cormorant predation by itself determines fish population size. Similar studies combined with population modelling are needed before a cause-and-effect relationship between cormorant predation and fish population size can be established.

Sources and image credit

  1. Cormorant predation on PIT-tagged cod and flounder in an open coastal systemPrimary peer-reviewed open-access article in Fisheries Research 300 (August 2026), article 107819. The publisher gives August 2026 as the issue month rather than a verified day. In 2022 and 2024, 2,783 cod and 2,644 flounder were PIT-tagged in an open coastal area of the western Baltic. Corrected predation estimates ranged from 52–88% across cod size classes and 14–84% across flounder size classes; the authors describe a potential mortality bottleneck but state that population modelling is needed before a cause-and-effect relationship with fish population size can be established.
  2. Afslutningsrapport på FAF-projektet omhandlende skarvers prædation på torsk, skrubbe og ålPrimary project report documenting the PIT-tag field setup at Genner Bugt/Kalvø in southern Lillebælt and scanning of the nearby Hopsø colony; used to verify the study locality and field context.

Image: Kaffe42 — great cormorant (Phalacrocorax carbo) photographed at Østerø Lake, Nyborg Municipality, Denmark, 18 October 2020; Wikimedia Commons. Representative Danish species image; not a bird from the Genner Bay PIT-tag study. · CC BY-SA 4.0 · Source

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