The Baltic Sea is a unique, semi-enclosed body of water connected to the North Atlantic through the narrow Danish Straits. Substantial freshwater runoff from surrounding lands creates one of the world’s largest bodies of brackish water. Large baleen and toothed whales are extremely rare visitors to the Baltic. However, a small population of a smaller cetacean species does maintain a resident presence within the sea.
Resident and Common Cetaceans of the Baltic
The only cetacean species with a permanent population in the Baltic Sea is the Harbor Porpoise (Phocoena phocoena). This small marine mammal is often confused with dolphins, but porpoises belong to a distinct family of toothed whales, characterized by spade-shaped teeth and a triangular dorsal fin. The porpoise reaches lengths of about 1.6 meters and weighs around 65 kilograms.
This resident population is divided into two segments. The Baltic Proper Harbor Porpoise population is genetically and geographically distinct from those in the adjacent Belt Sea. This isolated segment is classified as critically endangered by the IUCN Red List and HELCOM due to a drastic decline since the mid-20th century.
A passive acoustic monitoring study estimated the abundance of the Baltic Proper population to be only around 491 individuals. This small number highlights the vulnerability of the population, which faces significant threats primarily from bycatch in fishing gear. Conservation efforts focus on establishing protected areas and mitigating underwater noise from shipping, which interferes with the porpoise’s echolocation used for navigation and hunting.
Environmental Constraints on Whale Habitation
The primary factor limiting the habitation of larger whales is the Baltic Sea’s low salinity, measuring only about one-fifth of the salinity found in the world’s oceans. Salinity levels decrease significantly from the Danish Straits (15–18 practical salinity units, or psu) to the central Baltic Proper (7–8 psu). This brackish water imposes a severe physiological constraint on most marine mammals, which are adapted to the full ocean salinity of around 35 psu.
Marine species must expend considerable energy to osmoregulate, balancing the salt and water content in their bodies, making the environment physiologically stressful. The low salinity results in significantly lower overall biodiversity compared to the North Atlantic, limiting the variety of potential prey species. This reduced biodiversity translates to a lack of the dense, large schools of fish needed to sustain the massive caloric requirements of a large whale.
Another major constraint is the sea’s relatively shallow depth, averaging only about 54 meters, with over a third of the area being less than 30 meters deep. Large whales, such as Fin or Humpback whales, are deep-diving specialists adapted to the vast waters of the open ocean. The lack of deep water restricts their movement, diving behavior, and ability to execute their typical feeding strategies.
Documented Rare Visitors
Despite the challenging environmental conditions, the Baltic Sea occasionally receives visits from larger cetaceans that stray from the North Sea. These visitors are almost always non-resident individuals who enter the Baltic through the narrow Danish Straits. Species most commonly sighted include Minke Whales, Humpback Whales, and Orcas (Killer Whales), which are the largest of the toothed whales.
These sightings are rare, with records of Humpback Whales, for instance, only occurring a few times over the past several decades. The whales that enter are often young, inexperienced, or disoriented individuals that have mistakenly followed prey or currents into the confined sea. For example, a young Humpback whale was sighted near the German island of Rügen in 2008, and multiple sightings have been reported in the western Baltic in recent years.
The presence of these transient visitors is not sustainable, as the combination of shallow water, heavy shipping traffic, and insufficient food resources creates a dangerous environment. Large whales that remain in the Baltic for extended periods frequently struggle to find enough prey to meet their metabolic needs. They must successfully navigate the complex exit back through the Danish Straits to the more suitable waters of the North Sea.