Scientists raise the alarm: Too many harbour porpoises die each year in fishing nets

In a recently published research paper they highlight the challenges faced by harbour porpoises in Danish coastal waters and in the western part of the Baltic.

The harbour porpoises in that area are considered part of a single population, referred to by scientists as the Belt Sea population. Unfortunately, the number of harbour porpoises in this population has significantly declined. In 2012 and 2016, there were approximately 40,000 individuals, but by 2022, only 14,000 remained.

And it's not getting better, explains Signe Sveegaard, Senior Advisor and Head of Section of marine mammal research at the Department of Ecoscience at Aarhus University. She's also one of the researchers behind the new results.

"The population shrinks with 2,7 percent annually, which is concerning for the harbour porpoises. 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," she says and continues:

"Improving the polluted marine environment and fish availability will take many years. However, an immediate solution lies in limiting net usage in commercial fishing or mandating the use of acoustic alarms on all nets."

Counting Porpoises from the Air

Since 2005, researchers have conducted continuous counts of harbour porpoises. They divide the ocean into smaller segments and systematically fly over these areas scouting for porpoises on the surface.

"We count all the porpoises we can spot in the ocean. From other studies, we know that harbour porpoises stay at the surface approximately 10 percent of the time. 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," says Signe Sveegaard and continues:

"It's the best way of counting harbour porpoises, but the method is still fraught with uncertainty. That aside, we do have comparable data from 2005 till today, which means we can calculate a trend. And the trend is quite clear: The harbour porpoise is in decline.

As a matter of fact, the harbour porpoises fared better from 2011 until 2016. In those years, the population was growing, but after 2016, the trend was broken. Now the researchers observe fewer harbour porpoises than in 2005 when they started counting."

Harbour Porpoises in the Belt Sea

Today, around 14,000 harbour porpoises live in the Belt Sea.

The Belt Sea is a coastal area divided by Denmark, Sweden and Germany.

The harbour porpoises in the area are considered one population, even though some of them live hundreds of miles apart. They are genetically different from the populations in the North Sea and the Baltic Sea.

Harbour porpoises migrate to find food. This means that the harbour porpoises are not evenly distributed in the Belt Sea, but crowd together in areas that have an abundance of food. They are mainly found in the Femern Belt, the northern part of Øresund, Storebælt, Lillebælt and in Kattegat along the Swedish west coast.

Researchers Worried about the Fishing Industry

Even though the fishing industry isn't the only thing affecting the harbour porpoises, the researchers are especially focused on the threat that commercial fishing poses.

"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 harbour porpoises and give the population time to heal," says Signe Sveegaard.

She's worried about the political debate about commercial fishing right now because the focus is on limiting the use of bottom trawl.

"Right now, they debate whether the use of bottom trawl is to be limited. Studies clearly show that bottom trawl is very harmful to ecosystems in the sea and thereby affects the availability of food for harbour porpoises.

"That said, I'm worried about limiting the use of bottom trawl because the fishing industry most likely will use more nets instead. If that happens, it will probably lead to even more by-catch of harbour porpoise."

Young Porpoises Caught in the Net

While populations of herring and codfish, which lay millions of eggs, are able to quickly bounce back, harbour porpoises only have one calf a year. That means it takes many years for a harbour porpoise population to grow and stabilize, Signe Sveegaard explains.

Right now, the harbour porpoise population is under pressure because the calves are the ones who usually get caught in the nets from fishing vessels.

"The harbour 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 harbour porpoises.

"That is a huge problem because the new generations don't survive long enough to have kids. In time, it could make the whole population collapse."

Cousins in the North Sea Fare Better

While the harbour porpoises in the Belt Sea are declining, it looks quite different for the porpoises in the North Sea. Here, the population has been stable for many years. From 1994 till 2022, the population stayed between 300.000 and 400.000.

"The harbour porpoises of the North Sea fare better. It's probably because they have more space to move around and follow the shoals of fish. And they have moved. When we started counting them, they were mostly living in the northern part of the North Sea. Today, many of them have gone south past Dogger Bank towards the English Channel," Signe Sveegaard says.

In the Belt Sea, the harbour porpoises can't move around in the same way as their cousins in the North Sea. The area is much smaller, and furthermore, they are specialized in catching fish in shallow waters.

"We know from studies of the skulls of harbour porpoises that the populations are quite different from each other. The population in the Belt Sea has a downturned beak, whereas in the North Sea, the beaks are more horizontal.

"It's probably because the population in the North Sea are specialized in catching fish directly from the water column. In the Belt Sea, the porpoises have instead specialized in catching bottom-dwelling fish.

Because the harbour porpoises of the North Sea have adapted to a different environment, it's unlikely that they will be able to repopulate the Belt Sea if the population here disappears. That's why it's extremely important to protect the harbour porpoises where they live," she explains.

"If the harbour porpoises of the Belt Sea disappear, they might never come back. Therefore, we need to do something now to protect and stabilize the population."