Are There Whales in the Baltic Sea?

The Baltic Sea does have whales, though the reality is far more modest than the word “whale” might conjure. The harbour porpoise, a small toothed cetacean rarely exceeding 1.7 metres in length, is the only species that actually lives in Baltic waters year-round. Its population in the inner Baltic has dwindled to a few hundred animals at most, making it one of Europe’s most endangered marine mammals. Larger whale species do occasionally enter the Baltic, but they are rare visitors rather than residents, and the unusual conditions of this semi-enclosed sea make it a difficult place for most cetaceans to thrive.

The Harbour Porpoise Is the Baltic’s Only Resident

When researchers talk about cetaceans in the Baltic, they are almost always talking about the harbour porpoise (Phocoena phocoena). It is the only cetacean that maintains a year-round presence in the region, though its numbers are thought to have declined sharply over the twentieth century.¹ Harbour porpoises are not what most people picture when they think of whales. They are compact, shy animals that surface with a quick rolling motion, lack the dramatic blow of larger whales, and rarely approach boats. Spotting one in the wild takes patience and luck, which is part of why so many people living along the Baltic coast are unaware they share these waters with a cetacean at all.

The harbour porpoise occupies a wide range across the North Atlantic, from the coasts of Norway and Britain down to West Africa and across to eastern North America. The Baltic population, however, is not simply the tail end of some broader North Sea group. Genome-wide analysis has revealed clear genetic separation between North Sea porpoises and Baltic Sea porpoises, and even within the Baltic itself there is a meaningful genetic split between animals in the Belt Sea (the waters between Denmark and Germany) and those in the inner Baltic Sea farther east.² This matters because it means the inner Baltic porpoises are not being replenished by animals drifting in from healthier populations to the west. They are a distinct, largely isolated group, and when their numbers fall, recovery depends on those few remaining animals successfully breeding.

How Many Are Left in the Inner Baltic

Estimating the abundance of harbour porpoises in the open Baltic is a genuine technical challenge. The animals are small, solitary or in small groups, and spend most of their time underwater. Traditional ship-based visual surveys, the kind used to count larger whales in the open ocean, tend to miss porpoises in the vast, often rough waters of the Baltic Proper. To get around this, eight Baltic Sea nations collaborated on one of the largest passive acoustic monitoring efforts ever attempted for a marine mammal. Researchers deployed acoustic loggers at 298 stations across the Baltic Proper from 2011 to 2013, listening for the distinctive echolocation clicks that porpoises produce as they navigate and hunt.

The result was sobering. The point estimate for the Baltic Proper harbour porpoise population came in at about 500 individuals, with a wide confidence interval stretching from roughly 70 to 1,100.³ That wide range reflects the difficulty of counting such a sparse, mobile animal across such a large area. Even the upper bound, though, is remarkably low for a marine mammal occupying an entire sea basin. For context, the Belt Sea population to the west, which shares some genetic ancestry but is largely separate, numbers in the low thousands and is considered vulnerable rather than critically endangered. The inner Baltic population is in far worse shape.

The International Union for Conservation of Nature lists the Baltic Proper harbour porpoise as Critically Endangered, and ASCOBANS (the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas) has a dedicated recovery plan for it, known informally as the Jastarnia Plan after the Polish town where it was first drafted. The plan focuses on reducing bycatch, the single largest known cause of unnatural death in these animals, and on improving monitoring so that population trends can be tracked more accurately over time.

Why the Baltic Is Hostile Territory for Most Cetaceans

The Baltic Sea is, in global ocean terms, an oddity. It is nearly landlocked, connected to the North Sea only through the narrow Danish Straits. Fresh water pours in from over 250 rivers draining a catchment area four times the size of the sea itself. The result is a strong salinity gradient: water near the Danish Straits is moderately salty, while the northern reaches of the Gulf of Bothnia are nearly fresh. Much of the central Baltic has a salinity well below half that of the open Atlantic.

This low salinity has cascading effects on the food web. The Baltic supports fewer marine fish species than the North Sea, and many of those it does support are smaller-bodied or present in lower densities. For a large baleen whale that needs to consume enormous quantities of prey each day, the Baltic simply does not offer the dense aggregations of fish or krill found in more productive, fully marine waters. The sea is also relatively shallow, with an average depth of only about 55 metres, and large portions of it freeze in winter. These physical constraints set a hard ceiling on which cetacean species can sustain themselves here.

The harbour porpoise can tolerate brackish water and feeds on small fish like herring, cod, and gobies that are common in the Baltic. Its small body size means its caloric needs are modest compared to those of a minke whale or a fin whale. That combination of dietary flexibility and low energy requirements is what allows it to persist where other cetaceans cannot stay permanently.

Larger Whales That Occasionally Wander In

While the harbour porpoise is the only resident, other cetacean species do show up in Baltic waters from time to time. A study examining stranded cetaceans along the German North Sea and Baltic Sea coasts between 2016 and 2021 documented seven different species, including sperm whales, minke whales, two species of dolphin, a long-finned pilot whale, a fin whale, and a neonate orca.⁴ The researchers were clear that these strandings could not be attributed specifically to Baltic conditions, since many of the animals were not regular inhabitants of either sea.

Minke whales are probably the most frequent large-whale visitor. They are relatively small for a baleen whale, at around seven to ten metres, and they are common in the North Sea. Individuals occasionally swim through the Danish Straits into the southwestern Baltic, particularly in summer when fish are more abundant. Sightings are reported every few years along the coasts of Denmark, Germany, and Poland, and they generate considerable local excitement precisely because they are uncommon.

Sperm whales, by contrast, are deep-water animals with no business in the shallow Baltic, and their appearances almost always end badly. Strandings of sperm whales along the southern North Sea and adjacent Baltic coasts have occurred sporadically for centuries, and the animals that wash up are typically disoriented individuals that followed prey or currents into waters too shallow for them to navigate safely. Fin whales and orcas are even rarer. An orca sighting in the Baltic makes the news precisely because it is extraordinary.

The key point is that none of these larger species maintain breeding populations in the Baltic. They are accidental tourists, not residents, and many that enter the sea do not survive the trip.

Bycatch and the Threat of Gill Nets

For the harbour porpoises that do live in the Baltic, the single greatest threat from human activity is getting tangled in fishing nets. Bycatch in static fishing gear, particularly gill nets, is the leading human-caused source of mortality in small toothed whales worldwide.⁵ Gill nets are panels of fine mesh set vertically in the water to catch fish by the gills. They are cheap, effective, and widely used by small-scale fisheries across the Baltic. Unfortunately, porpoises hunting the same fish cannot detect the thin monofilament mesh with their echolocation in time to avoid it, and once entangled they drown within minutes.

For a population that may number only a few hundred animals, even a handful of bycatch deaths per year can meaningfully affect the population’s trajectory. Researchers studying bycatch patterns in the southwestern Baltic found that both the age of the porpoise and the lunar cycle influenced the likelihood of entanglement, suggesting that foraging behaviour shifts with light conditions and that younger animals may be more vulnerable. These findings are useful for designing mitigation measures, such as seasonal closures of high-risk fishing areas or mandatory use of acoustic deterrent devices (pingers) on nets, but implementation has been uneven across the Baltic nations.

The political challenge is that gill-net fishing is deeply embedded in the coastal culture and economy of Baltic communities, and porpoise conservation competes with the livelihoods of small-scale fishers who are already under economic pressure. Conservation agreements exist, but enforcement varies and the porpoises continue to die in nets.

Underwater Noise and Other Pressures

Beyond bycatch, the Baltic’s harbour porpoises face a suite of other stresses. The Baltic Sea is one of the world’s busiest shipping corridors, and commercial vessel traffic generates a constant low-frequency hum that pervades the underwater environment. A recent assessment of continuous underwater noise across the Baltic evaluated whether this background noise was harming marine mammals and fish. The study found that low-frequency noise is probably having a negative impact on indicator fish species, but did not find a significant negative impact on marine mammals at the frequencies assessed.⁶

That sounds like good news for porpoises, but the picture is more complicated than a single assessment can capture. The study focused on continuous low-frequency shipping noise in two specific frequency bands. Porpoises echolocate at much higher frequencies, so their communication and hunting clicks may not be directly masked by ship noise. However, impulsive sounds from pile-driving during offshore wind farm construction, military sonar exercises, and underwater explosions to clear old ordnance (the Baltic seabed is littered with World War II munitions) produce intense, broadband noise bursts that can cause temporary hearing loss or drive porpoises out of important habitat. These acute noise events are harder to study at the basin scale but are a recognized concern.

Chemical pollution is another long-standing issue. The Baltic is heavily industrialised on all sides and receives agricultural runoff from a vast drainage basin. Persistent organic pollutants, heavy metals, and endocrine-disrupting chemicals accumulate in the food web and concentrate in top predators like porpoises. Whether pollution directly reduces survival or reproductive success in Baltic porpoises is hard to disentangle from other threats, but the combination of a small, stressed population and chronic pollutant exposure is not reassuring.

How Researchers Track a Nearly Invisible Animal

One of the recurring challenges in Baltic porpoise conservation is simply knowing where the animals are and how many remain. Visual surveys from ships or aircraft work reasonably well in the Belt Sea, where porpoises are somewhat more abundant and the survey area is manageable. In the vast Baltic Proper, though, visual methods are impractical. The animals are too sparse, the sea too large, and weather windows too short.

Passive acoustic monitoring has become the workhorse method. Autonomous underwater recorders, often called C-PODs or similar devices, are moored at fixed stations and continuously log the echolocation clicks of passing porpoises. By calibrating the detection range of each device and combining data across hundreds of stations, researchers can build a picture of porpoise distribution and estimate abundance without ever seeing one. The multinational survey that produced the roughly 500-animal estimate for the Baltic Proper relied on exactly this approach, deploying loggers at 298 stations and measuring the click rates of tagged individuals to convert acoustic detections into population numbers.³

This acoustic method has limitations. It cannot distinguish individual animals, so a single porpoise that lingers near a station for days could inflate the detection rate. It also depends heavily on knowing how far each device can “hear” and how often porpoises click under different conditions. But for a rare, cryptic animal in a huge body of water, it is the best tool available, and the technique has proven its value for other marine mammal populations as well.

Can You Spot a Porpoise If You Visit the Baltic

If you are planning a trip to the Baltic coast and hoping to see a cetacean, temper your expectations. In the southwestern Baltic and Belt Sea, where porpoise density is highest, sightings do happen, especially in calm weather from May through October. The waters around the Danish islands of Funen and Zealand, the German coast of Mecklenburg-Vorpommern, and parts of the Swedish west coast are the most likely spots. Some harbours and headlands have earned local reputations as porpoise-watching sites, though nothing approaches the reliability of whale watching in, say, Iceland or Norway.

In the central and eastern Baltic, from the coast of Poland through Lithuania, Latvia, Estonia, and Finland, your chances drop close to zero. The inner Baltic population is so small and thinly spread that even dedicated researchers running acoustic monitoring gear go months without a detection at most stations. A casual visitor standing on a pier in Gdańsk or Tallinn could spend a lifetime looking without seeing a porpoise.

Larger whales are essentially off the table. A minke whale in the Baltic is a once-in-several-years event, and when one does appear it tends to show up briefly in the southwestern corner before heading back toward the North Sea. If your goal is to see whales in a northern European sea, the Norwegian coast, the waters around Iceland, or even Scotland’s Moray Firth are incomparably better bets. The Baltic’s appeal for marine wildlife watching lies more in its grey seals and seabirds than in its cetaceans.

The Genetic Isolation Problem

One of the more troubling aspects of the Baltic porpoise situation is the degree of genetic isolation. The genomic data showing distinct separation between North Sea, Belt Sea, and inner Baltic populations means that if the inner Baltic group continues to shrink, it cannot simply be “topped up” by immigration from healthier western stocks.² While porpoises are physically capable of swimming through the Danish Straits and across the Belt Sea, the genetic evidence suggests they rarely do so in numbers sufficient to blend the populations. Whatever drives this fidelity to their home waters, whether it is learned migratory routes, habitat preference, or social structure, it means the Baltic Proper porpoises are functionally on their own.

Small, isolated populations face well-known risks beyond direct threats like bycatch. Inbreeding reduces genetic diversity, which can lower disease resistance and reproductive success. Random demographic fluctuations, a few bad breeding years in a row, an unusually harsh winter, a disease outbreak, can push a population below the threshold from which recovery becomes extremely unlikely. For a population estimated in the low hundreds, these risks are not hypothetical. They are the kinds of pressures that have driven other small marine mammal populations, like the vaquita in Mexico’s Gulf of California, toward functional extinction.

Whether the Baltic Proper harbour porpoise can avoid that trajectory depends on whether the countries surrounding the Baltic can reduce bycatch quickly enough to give the population room to stabilize. The science on what needs to happen is fairly clear. The politics of making it happen across nine different nations with competing economic interests is where the difficulty actually lies.