Records from more than forty years have shown how the role of small fish in the Baltic coastal ecosystem has changed recently. Barely a few cm long stickleback fish eats the fry of predators, which in adulthood measure several tens of centimeters. According to ecologists, the two species have swapped places in the food chain, drastically changing the rest of the ecosystem.

From Prey by a Predator

In the 1980s, perch and pike predominated along the Baltic coast. Despite being freshwater fish, they can survive in areas where freshwater mixes with the ocean. From the 1990s, however, the three-spined stickleback (Gasterosteus aculeatus) began to predominate over their predators, and from 2014 they controlled the area up to 21 kilometers from the coast.

By JaySo83 – Own work, CC BY-SA 3.0,

Thanks to ecological measures in the region, the quality of the local water has improved. This has led to an increase in the population of fur seals, which, together with cormorants, feed on pike and perch. Fishing has focused more on cod, which eat sticklebacks.

Sticklebacks can adapt very well to life in the ever-warmer Baltic water. They eat young pike and perch. After overgrowth, they became an enemy to their original predators.

The Baltic Coast Ecological Impact

The analysis also describes the growth of overgrown algae. Algae-eating snails disappear because sticklebacks feed on the snails. “The disappearance of larger predators can cause cascading effects to algae, and that these changes can take place on a huge spatial scale-like falling dominoes,” said Boris Worm, a marine biologist at Dalhousie University.

By Ron Offermans, CC BY-SA 3.0,

According to him, these problems are very problematic and harm the region, he describes them as a “slow disaster that runs through the entire Baltic Sea”.

It was not the first time that an inverted relationship between a predator and prey had been recorded. Large herring populations in the North Sea have probably reduced the numbers of cod, their natural predators, by eating their young.

Featured photo by Piet Spaans (Viridiflavus) – Own work, CC BY 2.5,