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Harbor porpoises are being harmed by traffic from boats and ships

As summer approaches, harbor porpoises in coastal waters face increased challenges as the waters they inhabit become crowded with recreational boats and ships.

While people enjoy water skiing, fishing, and cruising around scenic coasts, bays and fjords, these activities significantly disrupt the lives of these magnificent mammals.

Harbor porpoises and motorboats

Harbor porpoises, medium-sized cetaceans that primarily feed on small fish, spend about 60% of their time hunting.

Research from Aarhus University, led by a team including postdoc Laia Rojano Doñate, reveals that disturbances from motorboats can severely impact these animals’ ability to hunt effectively.

“Harbor porpoises spend a lot of energy in a day. Not because they spend it hunting for fish, but because they need it to stay warm,” explains Rojano Doñate.

“Living in cold waters requires a lot of energy for thermoregulation. That’s why it’s a big problem if they are disturbed by motor boats and stop eating.”

How harbor porpoises hunt

Harbor porpoises primarily hunt small fish using echolocation. They emit high-frequency clicks and listen for the echoes bouncing back from their surroundings to locate prey.

Just before capturing their prey, they produce a rapid series of clicks known as a “buzz,” which helps them pinpoint the exact location of the fish.

Their diet mainly consists of small fish such as herring, cod, and whiting, but they also occasionally consume squid and small crustaceans.

To meet their daily energy needs, harbor porpoises need to catch around 2,000 small fish. This high intake is essential because they burn a significant amount of energy maintaining their body temperature in cold waters.

As agile swimmers, they can make quick, precise movements to catch their prey. Harbor porpoises spend about 60% of their time hunting, highlighting the importance of uninterrupted foraging to their survival and overall health.

Constant disruption

Rojano Doñate and her colleagues have tracked several harbor porpoises over the years to study their behavior and the effects of human disturbances.

In 2018, they discovered that the animals were near boat noise 80% of the time, a startling figure given their need to hunt 60% of the time.

The tracking devices used not only recorded surrounding noise but also measured dive depth and GPS location. The data showed that some porpoises ceased hunting when boats approached.

“When hunting for fish, they make a buzzing sound just before catching prey. This sound helps them navigate in the dark waters below. But when motorboats were around, they sometimes stopped buzzing,” Rojano Doñate says.

“This constant disturbance can prevent them from catching enough fish, ultimately threatening their survival.”

Energy expenditure

Harbor porpoises require a significant amount of energy daily. They need to catch around 2,000 small fish each day to meet their energy needs, much like cows grazing throughout the day.

Studies in both captivity and the wild have quantified the energy expenditure of harbor porpoises, showing that each breath they take burns around 5 kilojoules of energy.

With approximately 4,000 breaths a day, they need 20 megajoules of energy just to survive.

Declining population

For years, the harbor porpoise population in Kattegat appeared stable at around 40,000 individuals. However, recent counts show a dramatic decline to just 14,000.

This decline is attributed to several factors, including reduced fish stocks, increased pollution, and entanglement in fishing nets.

“Getting disturbed by boats is not the only cause. More pollution and getting caught in fishing nets are also part of the explanation of why harbor porpoises are disappearing fast,” Rojano Doñate explains.

“Additionally, the growing grey seal population, which preys on harbor porpoises, contributes to their decline.”

Conflict between human activity and harbor porpoises

Harbor porpoises, despite being part of the whale family, spend a significant amount of time in coastal waters. In Kattegat, these shallow waters are also popular with recreational boaters, creating a conflict between human activity and porpoise survival.

“We might not even notice it, but when we venture out on the sea on a sunny summer day, we might be disturbing a lot of animals,” Rojano Doñate says.

“To address this problem, we need to change our habits. Establishing zones where motorboats are prohibited and encouraging responsible boating behavior could make a significant difference.”

In conclusion, while summer activities bring joy to many, they also bring challenges to the harbor porpoises.

By understanding the impact of our actions and taking steps to mitigate them, we can help ensure the survival of these vulnerable marine creatures.

The full study was published in the journal Science Advances.


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