Harbour porpoises declining in North Sea, study suggests

Aerial surveys reveal while some animals may have moved to new territories, overall numbers have fallen over last two decades

Harbour porpoises are common in the North Sea, but numbers are falling in waters around Germany, worrying scientists (Ecomare/Wikimedia)

The number of harbour porpoises which live in the North Sea is declining, scientists who have conducted long-term surveys of marine life in the area warn.

As human activity in the area has increased during the 20th and 21st centuries, with rises in shipping, fishing and offshore energy infrastructure,  the impact is feared to be taking a critical toll on numerous species.

The harbour porpoise is regarded as a “sentinel species” – animals which indicate the health of an ecosystem and point to potential risks, in a similar manner to canaries in coal mines.

According to a study which examined the German North Sea, in some areas populations were declining by up to 3.8 per cent a year.

"The trend seen here is concerning," said Dr Anita Gilles of the University of Veterinary Medicine in Hannover, Germany, one of the study's authors.

What is “particularly troubling”, the researchers said, is the fact harbour porpoises have even experienced a strong decline in protected areas, such as the Sylt Outer Reef special area of conservation, which was specifically designated to keep marine life safe.

Meanwhile in southern parts of the North Sea, the population increased, indicating a possible shift in distribution.

But despite this glimmer of hope, overall, the harbor porpoise population declined by 1.79 per cent a year in the German North Sea.

In order to get an accurate abundance estimate, Dr Gilles and her colleagues used a system in which the surveyed area was divided into smaller blocks, then observed by plane.

“Knowing the abundance of a population is at the heart of ecology, but extremely challenging for mobile species in a rapidly changing marine environment like the North Sea,” she said.

They said the system is a standard method to determine wildlife population sizes, and includes measures to ensure accuracy, such as doubling back to account for diving porpoises.

The study is also noteworthy for its two-decade time span.

"After almost 20 years of systematic data collection ... we now have a wealth of data at hand," said Dr Gilles.

Though the results of the investigation paint a bleak picture the scientists hope the research can help drive legislation to aid conservation efforts.

“Abundance, distribution, and trends are key for EU and other legislative instruments, and for marine conservation management in general,” said Dr Gilles.

The authors warned their study was limited only to the German North Sea, meaning population counts and observed trends in the broader North Sea are missing, and further research is needed.

Dr Gilles and her colleagues believe the decline of the porpoises could be due to an increase in human activities, a change in prey availability, or that their distribution patterns have changed.

“Most probably it is a mixture of different causes and cumulative effects,” said Dr Gilles. “We urgently need more research into the drivers of change.”

In recent weeks, a “surge” in the number of supertrawler fishing vessels off the UK coast has been blamed for soaring numbers of deaths of dolphins and porpoises.

These floating factories, which can be over 100m long and trail nets over a mile in length, are able to pump hundreds of tonnes of fish aboard a day and process and freeze them while staying at sea for months at a time. They have also been linked to high levels of bycatch.

More than 1,000 porpoises died in the nets of these largely EU-registered vessels in UK waters in 2019 alone.

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