In the 20th century, the Baltic Sea underwent a major environmental change. The Atlantic cod (Gadus morhua) is probably the species that has been most affected by this development. Because it is the most common predatory fish in the Baltic Sea, this has also resulted in significant changes to the entire ecosystem. Cod has pelagic liquid roe, which in the Baltic Sea require a salinity of about 11 per mil to float. This is an adaptation to the brackish water, and the roe of cod native to Västerhavet (the Kattegatt and Skagerrak straits) require a significantly higher salinity. In the Baltic Sea, they sink to the seafloor like a stone.
As a result of eutrophication, the stocks of Baltic herring increased in the second half of the 20th century. For a few years at the end of the 1970s, large saltwater inflows from Västerhavet created good conditions for cod spawning. There was also an abundance of prey fish, and as a result the stock of cod absolutely exploded.
Between 1970 and 1980, the spawning stock increased from 200,000 tonnes to 700,000 tonnes, becoming one of the largest in the world. Fishing was relatively unregulated at the time, and between 1980 and 1985, an unimaginable 300,000-400,000 tonnes of Baltic cod were annually. This can be compared to the total global cod catch, which during the same period was around 2 million tonnes per year. This means that approximately 20 per cent of the world’s cod catch originated in the small, inland Baltic Sea. However, this was a period of rapid transition, and by the early 1990s the spawning biomass had sunk to less than 100,000 tonnes, while catch had decreased to under 50,000 tonnes.
In 1994, Peter Arvidsson caught the largest Swedish cod ever recorded, near a shipwreck south of Abbekås in the southern Baltic sea. The fish was 122 cm long and weighed a total of 32.42 kg.