Porpoises in the Baltic Sea

Vulnerable Animals Caught Between Nets, Noise, and Climate Change

Porpoises are the small, often inconspicuous inhabitants of our coastal waters—yet their situation is anything but unproblematic. In the Baltic Sea, particularly in the inland sea-like areas around Rügen and the Darß, this species faces a serious existential crisis.

While some individuals in captivity can live to a relatively advanced age, wild porpoises usually have only a short life span: many die as early as seven or eight years of age.

Do you know Freija?

Anyone who knows Freija gains a different perspective: Freija has lived at the Fjord&Bælt research station in Kerteminde, Denmark, since 1997 and, at around 30 years old, is considered the oldest known porpoise in the world. Her story shows just how much we can learn from individual animals. Through targeted work with Freija and two other animals, researchers have gained important insights into perception, stress responses, and behavior.

These findings are directly incorporated into conservation measures: for example, acoustic signals (pingers) for fishing nets are being tested, tracking tags are being developed to analyze movement patterns, and studies are being conducted on reactions to ship traffic.(See also: www.ardmediathek.de/video/Y3JpZDov...)

Ghost nets and underwater noise threaten porpoises

Despite such signs of hope, population trends remain alarming. According to estimates, the number of porpoises in the western Baltic Sea fell from about 42,000 to just around 14,000 between 2016 and 2022. The smaller and more isolated population in the inner Baltic Sea is even more critically affected: it is estimated that only between 100 and 1,000 individuals live there. These groups are so numerically depleted that random losses and local disturbances can have a dramatic impact on the population’s survival.

Several factors are compounding the threat. Gillnets are among the deadliest hazards: porpoises often become entangled and suffocate unnoticed as bycatch. In addition, the disappearance of key prey fish such as herring is eroding the food supply. Declines in fish stocks, in turn, are often the result of overfishing and ecological changes, which are also exacerbated by climate change. Warmer water temperatures are shifting fish spawning times and migrations—food chains are becoming unbalanced, and it is becoming more difficult for specialized feeders like the porpoise to find sufficient food.

Another, often underestimated stress factor is underwater noise. Ship traffic, construction activities, and recreational boats generate noise that disrupts the animals’ echolocation and communication. Studies show that noise can influence behavior, habitat use, and stress hormone levels.
 
Porpoises rely on subtle acoustic signals; dense shipping routes can impair their hunting behavior or drive them out of important habitats.

The combination of these factors leads to a precarious situation: reduced food availability, increased mortality from bycatch, and chronic stress caused by noise and habitat change are causing long-term decline in populations. Conservation measures must therefore be multifaceted. These include technical solutions such as improved net gear and pingers; spatial management measures such as temporary fishing closures in key habitats and the establishment of protected areas; the reduction of ship noise through slower speeds or quieter propulsion systems; and the restoration of fish stocks through sustainable fisheries policies.

The Fjord&Bælt Research Station

Research provides the basis for such measures: field studies, acoustic monitoring, and telemetry reveal where porpoises are particularly vulnerable and which disturbances they avoid. The work at stations like Fjord&Bælt is valuable because controlled experiments and long-term observations yield insights that are difficult to obtain purely in the wild. At the same time, these facilities and documented individual stories—such as that of Freija—make the problem tangible to the public and increase the willingness to take action.

However, protecting porpoises also requires political decisions and public support. Conservation strategies should integrate species and habitat management, fisheries regulation, and maritime spatial planning. Voluntary rules for shipping, mandatory technical standards for fisheries, and targeted research and monitoring are necessary to ensure stable populations in the long term.

Finally, there is the question of what each of us can do: Consumers can help by choosing sustainable fish, supporting conservation projects, or participating in local nature conservation initiatives. Every measure aimed at cleaner, quieter, and more fishing-friendly seas benefits the porpoises—and thus the entire ecosystem of the Baltic Sea.

ARD Mediathek report on Freija (available until June 14, 2028): https://www.ardmediathek.de/video/Y3JpZDov...

Sources
- Fjord&Bælt (information on Freija and research): https://www.fjord-baelt.dk
- HELCOM / Baltic Marine Environment Protection Commission (reports on the harbor porpoise population and its conservation status): https://helcom.fi
- IUCN (general information on Phocoena phocoena and threats): https://www.iucn.org