The populations of some 37 fish and soil animal species in the Oosterschelde estuary have fallen by an average of 28%, statistics agency CBS said on Wednesday. The claim is based on 30 years of monitoring by divers in estuary, south of Rotterdam.

The Zeeland estuary is a tidal area where fresh and salt water meet, creating a unique and biodiverse environment, and has been designated a European protected habitat.

The populations of 15 species were found to have more than halved between 1994 and 2023, including such typical Oosterschelde dwellers as plaice, bullheads and a number of shellfish and crustaceans such as mussels and European lobsters.

Global warming is one of the causes some fish no longer thrive in the Oosterschelde, CBS spokesman Vinodh Lalta told broadcaster NOS. “They go to the deeper, colder waters of the North Sea and don’t come back in the winter. That affects the whole food chain,” he said.

The warmer water has increased populations of eight other species, including dog snails and flat oysters. “Warmer water attracts exotic species, which is not necessarily a bad thing but could turn out to be,” he said. “We have only to look at the American crayfish which have no predators and eat everything they meet.”

Another reason for the decline may be the use of steel industry waste to fill holes in the basalt blocks used to strengthen the dykes. This, researchers think, may be polluting the habitat used by sea anemones and shellfish.

In December, Wageningen University started a new four-year investigation into the decline of lobsters and mussels in the estuary itself.