Evolution of fast-growing fish-eating herring in the Baltic Sea

Atlantic and Baltic herring are typical plankton-eating fish of central importance for the northern Atlantic Ocean and Baltic Sea ecosystems. A new study published in Nature Communications led by scientists from Uppsala University (Sweden) documents the discovery of the evolution of genetically distinct, fish-eating herring in the Baltic Sea, a young water body that has only existed since the end of the last glaciation.

Atlantic and Baltic herring have a key role in the ecosystem, acting as a critical link between plankton production and other organisms, like predatory fish, sea birds, sea mammals, and humans. Previous research from the Uppsala group has documented that herring is subdivided into a number of ecotypes that show genetic adaptation related to, for instance, climate conditions, salinity, and preferred spawning season.

Larger than the common plankton-eating Baltic herring

Linnaeus, the founder of taxonomy and professor in Uppsala in the 18th century, defined the Baltic herring as a subspecies of the Atlantic herring adapted to the brackish water in the Baltic Sea. The Baltic herring is much smaller and has less fat than the Atlantic herring. The current project was initiated when the principal investigator was informed by a local fisherman at the coast northeast of Uppsala that there is a special type of herring “that always spawns just before midsummer and which is as big as the Atlantic herring,” thus much larger than the common plankton-eating Baltic herring.

“When I learned that the locals are aware of a specific population of very large Baltic herring that always spawns in the same area year after year, I decided to sample and explore their genetic constitution. Now we know that this is a genetically unique population that must have evolved over hundreds, if not thousands, of years in the Baltic Sea,” says Leif Andersson, Professor at the Department of Medical Biochemistry and Microbiology at Uppsala University, who led the study.

The researchers carried out a careful analysis of morphology, growth pattern, fat content, and presence of environmental pollutants. A striking finding was that the large herring exhibited damaged gill rakers. The plankton-eating Baltic herring uses the gill rakers to sieve plankton, while the observed gill damage in large herring likely reflects a switch to a fish diet, probably including the common stickleback, which has sharp spines for predation protection.

Another interesting finding was that the large herring had a significantly higher fat content and significantly reduced level of dioxin, a problematic chloro-organic pollutant in the Baltic Sea. Both these observations and the much faster growth rate are consistent with a switch to a fish diet. The relatively low dioxin content makes this fish-eating Baltic herring interesting for human consumption.

Two distinct subpopulations of fish-eating herring

After finding that the large fish-eating herring is genetically unique, the researchers decided to perform whole genome sequencing of the large herring together with previously collected large herring from different parts of the Baltic Sea. The stomach content of this second set of large herring showed that these individuals were feeding on small fish.

“Our genetic analysis demonstrates that there are at least two distinct subpopulations of fish-eating herring in the Baltic Sea; one occurs north of Stockholm, and the other occurs south of Stockholm,” says Jake Goodall, researcher at Uppsala University and first author on the publication.

One interesting question is why fish-eating herring have evolved in the Baltic Sea, when there is no evidence for such herring in the Atlantic Ocean. The Baltic Sea is a very young water body that has only existed for about 8,000 years, after the end of the last glaciation period. Only a limited number of marine fish have been able to colonise the brackish Baltic Sea, where salinity is in the range of 2-10‰ compared with about 35‰ in the Atlantic Ocean.

“We hypothesise that fish-eating Baltic herring have evolved due to a lack of competition from other predatory fish, for instance, mackerel and tuna, which do not occur where we find fish-eating herring. Thus, these herring take advantage of an underutilised food resource in the Baltic Sea,” says Leif Andersson.