Swedish researchers from Stockholm University, Uppsala University, and SLU, together with coastal fishermen, have conducted a unique genetic mapping of herring and Baltic herring in the Baltic Sea. They collected 4,500 spring-spawning fish from 150 locations along the Swedish coast. The results show many more genetically distinct variants than previously known, and in completely new areas. Three genetically distinct main groups of spring-spawning Baltic herring have now been confirmed in three clusters along the coast.
Today, Baltic herring is mainly fished through large-scale trawling in two larger zones further out in the Baltic Sea. The different stocks are mixed in the catch, and in practice, it is not known which variants are being fished up. There is a risk of fishing out entire stocks, such as the Törnrosströmming, without even noticing it. The unusual Baltic herring spawn not in spring, but in midsummer, when the rose blooms. The Törnrosströmming can tolerate warmer water than other Baltic herring, which may mean that it can survive climate changes as the Baltic Sea's temperature becomes warmer.
Researchers are now demanding that fishery management and its zoning be changed, and that the fishing industry also needs to monitor what types of Baltic herring are being fished, to avoid risking fishing out entire sub-stocks. It is important to retain as much genetic variation as possible, such as the Törnrosströmming, which may be important for the species' survival in a warmer climate.
Baltic herring is already heavily pressured today by both overfishing and environmental factors such as eutrophication and climate change. It is a key species in the Baltic Sea ecosystem, and if it disappears, predators such as cod and porpoise may have difficulty surviving. The disappearance of Baltic herring could also contribute to eutrophication due to more plankton. Baltic herring has decreased very sharply in the central and southern Baltic Sea, and become smaller and leaner.
A study from Linnaeus University shows that Baltic Sea cod may not suffer from thiamine deficiency to the extent previously feared. The results were compared with cod from the Åland Sea, one of the few places in the Baltic Sea where the stock still exhibits good growth, and the North Sea where no deficiency has been reported. No clear differences in thiamine levels were found between the examined cod stocks. None of the fish analyzed showed any signs of impaired metabolism. The results are encouraging because cod has decreased sharply in both number and size in the Baltic Sea for a long time.
Thiamine deficiency has long been pointed out as a threat to cod in the Baltic Sea. The deficiency has been reported in salmon since the 1970s and causes increased mortality among young fish. In recent decades, several other fish and bird species in the region have also been found to suffer from the same problem. Thiamine (vitamin B1) is essential for fish metabolism, nerve function, and reproduction. If a female fish has a thiamine deficiency when she lays her eggs, the newly hatched larvae risk dying from malnutrition. Researchers recommend continuing to monitor the cod's thiamine status, as these problems in other species have been shown to vary greatly between different years.
Over the past 50 years, the stock has decreased sharply in number, size, and condition. Behind the decline are factors such as overfishing, deteriorated feeding and spawning grounds, and changes in the ecosystem. Despite strict regulations being introduced, few improvements are noticeable, according to researchers. If further studies confirm our results, we could strike thiamine deficiency from the long list of problems that cod in the Baltic Sea has.
Herring and Baltic herring are the same species, Clupea harengus, with different names depending on where they are fished. North of the Kalmar Strait, herring is called strömming (Baltic herring). Baltic herring is shorter than herring and has a longer head. A new genetic map gives a new picture of Baltic herring in the Swedish Baltic Sea. The old perception of a spring-spawning and an autumn-spawning stock is being replaced.