Baltic Herring—More Unique Than Previously Thought

Everyone can see the herring for what it is—one might be tempted to paraphrase a well-known quote from New Athens. It is one of the most abundant fish in the world and, at the same time, a species of immense importance to the functioning of marine ecosystems. Despite intensive fishing, the global population of adult herring is estimated at around one trillion individuals.

The Baltic herring population also remains relatively large, especially compared to the severely depleted Baltic cod population. Herring, however, plays a much more important role than its abundance might suggest. Feeding primarily on zooplankton, it is one of the key links in the food web of the North Atlantic and the Baltic Sea—it passes on the energy stored in plankton to larger fish, seabirds, and mammals.

Its natural predators include cod, salmon, sturgeon, seabirds, seals, and cetaceans. Ultimately, humans are also part of this same food web.

From Science to Politics

Herring from around the world differ only slightly from one another. It is generally accepted to classify oceanic herring into two species: Atlantic and Pacific, even though there are established populations of their hybrids in the waters of Karelia and eastern Norway. Carl Linnaeus, the father of modern plant and animal taxonomy, had already distinguished the Baltic herring (salaka) Clupea harengus membras as a subspecies distinct from the Atlantic herring C. harengus harengus.

Fish from the fully saline waters of the Atlantic are larger and fattier; they mature more slowly and later than their counterparts from the Danish Straits and the Baltic Sea. This affects fishing methods, as well as how the fish are preserved, served, and, ultimately, their taste. Scholars still debate the cause of the leaner flesh of our herring. Is it due to lower plankton productivity in the Baltic Sea? Or rather stress caused by lower salinity (especially in the Gulf of Bothnia and the Vistula Lagoon), or possibly higher water temperatures?

The classification of herring in the Atlantic and its inland seas has been a subject of controversy for centuries. Like sea trout and brown trout, herring has been defined either very broadly or very narrowly—and increasingly so—with dozens of subspecies, varieties, ecotypes, evolutionary lineages, and management units identified. Currently, the following stocks/management units are recognized:

• The Atlantic Ocean, which meets the Atlantic in the spring;
• the Atlantic in the fall;
• the North Sea and the Celtic Sea;
• The Danish Straits;
• the Baltic Sea in the fall;
• the Baltic Sea in the spring;
• Norwegian fjords;
• The Barents Sea and the eastern Norwegian fjords (hybrids with Pacific herring) [1, 6, 8].

A few years ago, the genetic distinctiveness of yet another ecotype was confirmed: the large, fish-eating herring (Slatterstil) of the Baltic Sea [3]. Discussions about the systematics of these silvery fish are not purely academic. On the contrary! They immediately translate into economic and political disputes between fishermen from different countries, as well as between various professional groups (coastal fishing versus fishmeal fishing; net fishermen versus anglers; fishermen vs. NGOs) [4].

The first steps

The first analyses of herring genetic diversity were conducted in the 1980s using the allozyme method. Their results turned out to be blatantly at odds with intuition and the findings of centuries of observations on variations in shape, taste, and behavior. This is because they revealed no variation whatsoever, not even the slightest. They suggested that all the herring in the world form a single, large population in which individuals mate at random. In technical terms, complete panmixia prevails [7].

It was only with advances in genetics that results more consistent with data from other branches of biology began to emerge. Whole-genome sequencing (WGS) confirmed the validity of previous classifications, which were based on geographic criteria, migration times and routes, spawning seasons, and/or the diet of specific herring stocks. Atlantic herring turned out to be more diverse than official catch quotas suggest. Likewise, the Baltic Sea and Danish Straits sprat population breaks down into a number of distinct stocks/management units. And yet it evolved no earlier than 8,000 years ago, along with the entire Baltic Sea! [1, 4]

As with herring

A comparison of the whole-genome sequencing (WGS) data for herring with that of its competitors for the title of most commonly consumed fish yields interesting insights. In the case of Atlantic cod ( Gadus morhua), just as with Baltic silver, the analysis of neutral genetic markers reveals little population diversity. In contrast, analyses of loci important for natural selection—those responsible for the fish’s response to oxygenation, salinity, and water temperature—reveal high diversity within the species, among individual stocks and ecotypes.

The differences between sedentary and migratory, coastal, and offshore ecotypes are plain to see. The genetic basis and root cause of these differences are likely four major inversions in the linkage groups (supergenes) 1, 2, 7, and 12. In the past, these allowed Atlantic cod to adapt to climate change. Perhaps they will enable the cod to survive for millions of years to come? That is, unless we catch them all to extinction, just like the passenger pigeon and Steller’s sea cows [1].

Czy śledzie jedzą ryby?

Unlike with herring

The genetic structure of the herring turned out to be, in a sense, the opposite of that of the eel, *Anguilla anguilla*. In the latter, intraspecific variation is lower than expected. This may be due to the fact that herring reproduce in a manner opposite to that of eels. We have many spawning grounds that formed yesterday, or even this morning, in evolutionary terms (8,000 years ago). Furthermore, they differ in salinity, temperature, substrate type, light levels, and pressure from predators and competitors [2, 6, 8].

Furthermore, the same spawning ground is not the same over time. For example, the fauna of the Pomeranian Bay and the Vistula Lagoon has changed completely over the last hundred years (our native species of worms and mayflies have given way to newcomers from the Black Sea and the Far East) [2, 6, 8]. Meanwhile, the herring fishery there remains just as productive as it was decades ago.

In geological time, the herring instantly changes the patterns (length, route, and timing) of its migrations, as well as its spawning behavior. The eel, on the other hand, sticks to a single spawning ground in the Sargasso Sea—a remnant of the Tethys Ocean—even though it lies thousands of kilometers away from its feeding grounds in Greenland, the Masurian Lakes, or the tributaries of the Nile. It also maintains the timing of its migrations up and down rivers. The Sargasso Sea acts as a melting pot where genetic adaptations to local environments converge. Adult eels cope thanks to phenotypic plasticity, while herring do so thanks to their diverse genomes [1].

A herring that stands out from the rest of its family

Most marine fish are pelagophilic, spawning in the upper layers of the water. This, however, does not apply to our subject. Its fertilized eggs must settle on some hard surface: a gravelly or rocky bottom, shipwrecks, or vegetation. Technically speaking, the herring belongs to the lithophytophilic (from the Greek: rock-and-plant) reproductive group, just like the freshwater and aquarium species we’re more familiar with.

Spawning in estuaries (the Vistula Estuary, the Szczecin Lagoon, and the Pomeranian Bay) and even in coastal lakes (e.g., Lake Dąbie) distinguishes the sałaka from its cousins, the alosa (river herring) of the genus Alosa, which migrate far upstream—in the past, as far as Passau in the Bavarian Alps. In the Antipodes, everything is turned on its head, so the Australian herring, or Potamalosa richmondia, spends most of its life in freshwater but spawns in the open sea (much like true eels) [1].

The Power of Evil Against One Person

Not only were our ancestors’ observations just as accurate as ours, even though they were made using primitive methods, but their methods of mining Baltic silver were also just as efficient as those of today. German scientists discovered, to their horror and amazement, that the famous autumn herring had been nearly wiped out by the end of the Middle Ages and the beginning of the Renaissance—quite a few centuries before the Industrial Revolution. There were no GPS-equipped fishing boats or refrigerated trawlers for fish meal back then, but in Falsterbo alone, 45,000 fishermen were employed.

What about Gdańsk, Słupsk, Lübeck, and Stockholm? The decline of the fall herring was caused by a combination of overfishing and climate change. Back then, we were in the Little Ice Age; today, we are experiencing global warming [5].

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Main photo: Chesapeake Bay Program/Flickr

While writing this article, I used:

1. Andersson, L., Bekkevold, D., Berg, F., Farrell, E. D., Felkel, S., Ferreira, M. S., … & Pettersson, M. (2024). How fish population genomics can promote sustainable fisheries: a road map. Annual Review of Animal Biosciences, 12(1), 1–20.
2. Ezhova, E., Żmudziński, L., & Maciejewska, K. (2005). Long-term trends in the macrozoobenthos of the Vistula Lagoon, southeastern Baltic Sea. Species composition and biomass distribution. Bulletin of the Sea Fisheries Institute, 1(164), 55–73.
3. Goodall, J., Pettersson, M. E., Bergström, U., Cocco, A., Delling, B., Heimbrand, Y., … & Andersson, L. (2024). Evolution of fast-growing piscivorous herring in the young Baltic Sea. *Nature Communications*, 15(1), 10707;
4. Goodall, J., Pettersson, M. E., Andersson, A., Dahlin, I., Ryman, N., Ståhl, G., … & Laikre, L. (2026). The population structure in the Baltic herring reflects natural selection and local adaptation. Proceedings of the National Academy of Sciences, 123(11), e2526500123.
5. Lehmann, O., Schmidt, J. O., & Voss, R. (2021). On the Decline of the Herring Stock in the Sound: Evidence for Pre-Industrial Overfishing of Baltic Sea Herring. Hansische Geschichtsblätter, 139, 239–285.
6. Porębski, J., & Kaczewiak, C. (1993). Long-term studies of spring herring (Clupea harengus L.) in the Pomeranian Bay. Studies and Materials, Marine Fisheries Institute, Gdynia, Series S, 40–49.
7. Ryman, N., Lagercrantz, U. L. F., Andersson, L., Chakraborty, R., & Rosenberg, R. (1984). Lack of correspondence between genetic and morphological variability patterns in Atlantic herring (Clupea harengus). Heredity, 53(3), 687–704.
8. Strzyżewska, K. (1969). A Comparative Study of Herring Spawning Populations off the Polish Baltic Coast. Proceedings of the Marine Fisheries Institute, Gdynia A 15, 211–277.