Cormorant predation devastates Baltic cod and plaice populations, study finds

Researchers from Denmark's DTU Aqua conducted a two‑year tagging study of 5,400 fish (2,783 cod and 2,644 plaice) in the western Baltic Sea. Using PIT‑tags, they monitored how many fish were consumed by cormorants (great cormorant colonies). The results show that cormorants are responsible for up to 88 % mortality in cod and up to 84 % in plaice within a 2‑7 month period after tagging. Predation varies with size, with 52‑88 % of young cod and 14‑84 % of plaice being eaten. The method is the first extensive use of PIT‑tag scanning in an open coastal marine area, providing robust evidence that cormorant predation is far greater than previously accounted for. The study notes that other factors—climate change, low‑oxygen zones, habitat loss, pollution and historic overfishing—also contribute to fish declines. In Sweden, populations of the great cormorant have risen sharply since the 1980s, with nest counts increasing from 40,958 in 2012 to higher numbers in recent surveys.