Piscivorous herring – a unique fish in the Baltic Sea ecosystem

“This unique piscivorous (fish-eating) herring may play an important role in the Baltic Sea ecosystem and is therefore especially important to preserve, says Ulf Bergström, researcher at the Swedish University of Agricultural Sciences (SLU).

Together with a large group of research colleagues, he has studied the herring’s genetics, growth, otolith chemistry, diet, and relationship with the three-spined stickleback, another very common small fish species in the Baltic Sea. The results have recently been published in two scientific papers, in Nature Communications and ICES Journal of Marine Science.

Genetic studies reveal the unique nature of predatory herring

The presence of unusually large herring along certain stretches of the Baltic Sea coast has long been known. According to fishermen in northern Uppland, this large herring, locally called "slåttersill", is known to spawn just before midsummer along this coastal stretch in the northern Baltic Sea.

With the help of local fishermen and county administrative boards, the researchers collected "slåttersill" and other large herring along the Swedish Baltic coast and, among other things, sequenced the entire genome of the herring.

“Our analyses show that there are at least two genetically distinct groups of piscivorous herring in the Baltic Sea. The so-called “slåttersill” belongs to the northern group, which has so far been found in the southern Gulf of Bothnia. The southern group is found from the Stockholm archipelago and further south to Kalmar” says Jake Goodall, researcher at Uppsala University and the lead author of the article published in Nature Communications.

Piscivorous herring could help control stickleback populations

Herring primarily eat plankton, but when the researchers analyzed the stomach contents of these large herring, they discovered they consumed far more small fish than expected. The larger the herring, the higher the proportion of fish in their diet. One species was particularly common in their stomachs – the three-spined stickleback.

The populations of three-spined stickleback in the Baltic Sea has increased dramatically in recent decades, significantly affecting the coastal ecosystem. Stickleback eats the eggs and larvae of species like European perch and northern pike, directly impacting these populations. Indirectly, large stickleback populations promote the growth of filamentous algae, exacerbating the effects of eutrophication.

“The large herring seem to consume large amounts of stickleback juveniles in coastal areas. In the open sea, we also observed a negative correlation between the density of large herring and sticklebacks over time. Where there were plenty of large herring, stickleback populations were  lower, suggesting that piscivorous herring may help control stickleback populations through predation” says Serena Donadi, researcher at SLU and the lead author of the study published in ICES Journal of Marine Science.

The researchers conclude that the decreasing number of large herring that has been observed in the Baltic Sea may be a contributing factor to the increase in sticklebacks. The study on the interaction between herring and stickleback also shows that the rise in stickleback may partly be due to reduced competition for food. Stickleback, small herring, and sprat largely eat the same food, and when herring and sprat populations decline, sticklebacks thrive.

Preserving genetic diversity is crucial

Leif Andersson leads the research group at Uppsala University that conducted the genetic analyses. The group has previously shown that herring in the Atlantic and Baltic Sea consist of several genetic groups (ecotypes) adapted to different environmental conditions such as salinity and climate. Spring-spawning and autumn-spawning herring in the Baltic are two of these groups. The piscivorous herring is most closely related to the spring-spawning herring and can be considered a subgroup of this population.

“Our investigations show that there is significant genetic diversity within a common species like herring. This valuable diversity is likely crucial for how herring and Baltic herring will cope with accelerating climate change and may be essential for the stability of the Baltic Sea ecosystem and our food security”, says Leif Andersson.

The researchers believe their results should have implications for management.

“The current EU fisheries management in the Baltic Sea does not account for local populations. For example, herring in the central Baltic Sea is managed as a single large stock, and there is a clear risk of overfishing some local populations, including of the newly described fish-eating ecotype. Moving the trawl limit further offshore for large-scale herring fisheries could be an important measure to protect the piscivorous herring, which tend to stay closer to the coast, and ensure their role in the ecosystem”, says Ulf Bergström.

References

Goodall J, Pettersson ME, Bergström U, Cocco A, Delling B, Heimbrand Y, Karlsson M, Larsson J, Waldetoft H, Wallberg A, Wennerström L, Andersson L 2024. Evolution of fast-growing piscivorous herring in the young Baltic Sea. Nature Communications

Donadi S, Olin A, Casini M, Eklöf J, Erlandsson M, Fredriksson R, Kaljuste O, Larsson N, Nilsson J, Bergström U 2024. Reduced predation and competition from herring may have contributed to the increase of three-spined stickleback in the Baltic Sea. ICES Journal of Marine Science.

Contact

Ulf Bergström, SLU
ulf.bergstrom@slu.se, +46(0)73-802 2532

Leif Andersson, Uppsala universitet
leif.andersson@imbim.uu.se, +46(0)70 425 0233