Atlantic and Baltic herring are crucial plankton-eating fish within the northern Atlantic Ocean and Baltic Sea ecosystems. A study published in Nature Communications, led by scientists from Uppsala University (Sweden), documents the evolution of genetically distinct, fish-eating herring in the Baltic Sea, a relatively young body of water formed after the last glaciation.
These herring play a vital role in the ecosystem, linking plankton production with various organisms, including predatory fish, seabirds, marine mammals, and humans. Previous research has identified several ecotypes of herring, showing genetic adaptations related to climate, salinity, and spawning seasons.
Historically, Carl Linnaeus, an 18th-century taxonomist and professor in Uppsala, classified the Baltic herring as a subspecies of the Atlantic herring, adapted to brackish water. The Baltic herring is smaller and has less fat than its Atlantic counterpart. The current research began when a local fisherman informed the principal investigator about a special type of herring that spawns just before midsummer and is comparable in size to the Atlantic herring.
Leif Andersson, a professor at Uppsala University, stated, “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.”
The research team conducted a detailed analysis of morphology, growth patterns, fat content, and environmental pollutants. A notable finding was the damaged gill rakers in the large herring, which likely indicates a dietary shift from plankton to fish, possibly including the common stickleback.
Additionally, these large herring exhibited higher fat content and lower levels of dioxin, a harmful pollutant, suggesting a beneficial profile for human consumption. The study revealed at least two distinct subpopulations of fish-eating herring in the Baltic Sea, one located north of Stockholm and the other south.
Jake Goodall, a researcher at Uppsala University, remarked, “Our genetic analysis demonstrates that there are at least two distinct subpopulations of fish-eating herring in the Baltic Sea.”
The evolution of fish-eating herring in the Baltic Sea raises questions, particularly since similar herring are not found in the Atlantic Ocean. The Baltic Sea has existed for approximately 8,000 years, and limited marine fish have colonized its brackish waters, which have a salinity of 2-10‰ compared to about 35‰ in the Atlantic Ocean.
Andersson hypothesized, “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.”