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Harbor porpoises are severely threatened by fishing activities

In a collaborative effort, researchers from Denmark, Germany, and Sweden are bringing urgent attention to the plight of harbor porpoises in the Danish coastal waters and the western part of the Baltic Sea. 

The research underscores the critical challenges these marine mammals face, specifically highlighting the significant decline in the numbers of what is known as the Belt Sea population of harbor porpoises. 

Shrinking population of harbor porpoises 

The Belt Sea population has seen its numbers plummet from an estimated 40,000 individuals in 2012 and 2016 to a mere 14,000 by 2022, marking a stark decrease that signals a dire situation for the species in this region.

“The population shrinks with 2.7 percent annually, which is concerning for the harbor porpoises,” said senior author Signe Sweegaard, the Head of Section of marine mammal research at the Department of Ecoscience at Aarhus University

Multiple threats must be addressed

“Multiple factors contribute to this decline. By-catch due to net fishing, where porpoises become entangled and drown, is a significant issue. Additionally, deoxygenation, pollution and a lack of fish play a role.”

Sveegaard stresses the importance of addressing these threats promptly, advocating for a reduction in net fishing or the implementation of acoustic alarms on all nets as viable solutions to mitigate the immediate risks to the porpoises.

Aerial surveys used to track harbor porpoises 

The methodology for tracking the population of harbor porpoises involves comprehensive aerial surveys initiated in 2005. Researchers systematically divide the ocean into segments and scout these areas from the air, counting visible porpoises. 

“We count all the porpoises we can spot in the ocean. From other studies, we know that harbor porpoises stay at the surface approximately 10 percent of the time,” explained Sveegaard.

“By multiplying the number of spotted porpoises in the area and adding the multiplied numbers for all the studied areas up, we estimate the whole population.”

Concerning decline in harbor porpoise numbers

Sveegaard acknowledges the uncertainties inherent in this method but underscores its value in tracking population trends over time, revealing a clear and concerning decline in harbor porpoise numbers.

Despite these challenges, the harbor porpoises of the Belt Sea are recognized as a distinct population that spans the coastal region divided among Denmark, Sweden, and Germany. 

Genetic analyses distinguish these porpoises from those found in the North Sea and the Baltic Sea, indicating a unique population that migrates in search of food. This migratory behavior results in uneven distribution across the Belt Sea, with porpoises clustering in areas where food is abundant, such as the Femern Belt and parts of the Øresund, Storebælt, Lillebælt, and the Kattegat along the Swedish west coast.

Impact of the fishing industry 

The impact of the fishing industry, particularly through by-catch in net fishing, emerges as a significant threat to these porpoises. “We mention the fishing industry and by-catch specifically because it’s the only way to make a change here and now. If the politicians decide to limit net fishing, it will quickly reduce the by-catch of harbor porpoises and give the population time to heal,” Sveegaard said. 

The current political debate surrounding commercial fishing practices, especially the discussion on limiting bottom trawl fishing, is a point of worry for Sveegaard. While bottom trawl fishing is known to harm marine ecosystems and thereby affect the availability of food for harbor porpoises, she fears that restrictions on this practice may lead to increased reliance on nets, exacerbating the by-catch problem.

Slow population recovery

The biology of harbor porpoises compounds their vulnerability to these threats. With a reproductive rate of just one calf per year, the population’s recovery is slow, and the young, particularly susceptible to entanglement in fishing nets, bear the brunt of this risk. 

“The harbor porpoises reach maturity when they are about four years old, but they leave their mother after only one year. For three years, they swim around the ocean all on their own, not being able to reproduce. They are inexperienced and are much more likely to get caught in a net than the adult harbor porpoises,” Sveegaard explained. This pattern poses a significant risk to the population’s future, as it impedes the growth and stabilization of their numbers.

North Sea harbor porpoises 

While the harbor porpoises in the Belt Sea face declining numbers, their counterparts in the North Sea present a contrasting picture, with a population that has remained stable over the years. The North Sea porpoises benefit from a larger habitat, enabling them to follow fish shoals and adapt to environmental changes

However, the distinct differences between the populations, including adaptations for feeding in different marine environments, suggest that the North Sea porpoises cannot simply replace those in the Belt Sea should the latter’s population collapse.

Immediate action is needed

Sveegaard emphasizes the critical need for immediate action to protect and stabilize the harbor porpoise population in the Belt Sea. Highlighting the unique adaptations and behaviors of these porpoises, she advocates for conservation efforts that address the specific challenges they face. 

“If the harbor porpoises of the Belt Sea disappear, they might never come back. Therefore, we need to do something now to protect and stabilize the population,” she concluded. 

The study is published in the journal Frontiers in Marine Science.


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