Decline in harbour porpoises in the North Sea

Categories // Whale & Dolphin General News

Decline in harbour porpoises in the North Sea
Photo credit: Andy Gilbert

A long term study conducted in the North Sea has concluded that the number of harbour porpoises in the area is declining.

It is feared that human activity, which has increased in the past two centuries, has had a significant impact on numerous species in the North Sea and after examining the German North Sea, the study found that harbour porpoise numbers were declining up to 3.8% a year in some areas. Human activity, which could be having an impact on these numbers, include a rise in shipping, offshore energy structures and fishing, resulting in a change of prey availability. “Most probably it is a mixture of different causes and cumulative effects,” said one of the study’s authors, Dr Anita Gilles of the University of Veterinary Medicine in Hannover.

Harbour porpoise are considered as a species that is used to show the health of an ecosystem and indicate any potential risks. They are the smallest cetacean and only species of porpoise to be found in European waters. They are shy animals, rarely breaching out of the water and avoid vessels where possible. Over the past 40 years there have been significant declines in the European populations of harbour porpoise. Due to their preferred habitat of shallow, coastal zones the harbour porpoise is under threat from high levels of chemical pollution, vessel traffic, underwater noise, the depletion of prey by overfishing and their primary threat; accidental capture in fishing nets (bycatch).

In regards to the study, Dr Gilles has said “The trend seen here is concerning” and researches have expressed particular concern that this species has declined in protected areas, such as the Sylt Outer Reef Special Area of Conservation (SAC) which was specifically designated to keep marine life safe.

The Sylt Outer Reef SAC is situated in the German North Sea and covers an area of 5.314km2 with water depths ranging from 8 – 48m. It’s significant for its size and biodiversity. Sandbanks and reefs provide habitats for many different fish, sea bird, and marine mammal species. The area’s classification as an SAC was mainly due to the range of habitats present and distribution of harbour porpoise, which it is especially important for. The densest concentrations of harbour porpoise in the entire German North Sea have been seen here and it is also considered to be a major calving and mating habitat, with regular sightings of mother and calf pairs. These factors combined make the Sylt Outer Reef a key site for the conservation of harbour porpoise.

However, it is not all bad news. There is indication of a possible shift in distribution to southern parts of the North Sea where population has increased and overall the population has only declined by 1.79% a year.  

Though the results of this study are cause for concern, scientists hope that research such as this, can help determine legislation in the future and help conservation. “Abundance, distribution, and trends are key for EU and other legislative instruments, and for marine conservation management in general,” said Dr Gilles.

The study was only carried out in the German North Sea meaning that further research is needed to determine population counts and trends in the wider North Sea.

Recently there has been an increase in the number of large fishing trawlers (supertrawlers) off the coast of the UK which have been blamed for a rise in the number dolphin and porpoise deaths. These large ships can be over 100m long and trail nets which are over a mile in length. They spend months on end at sea and have also been linked to high levels of bycatch; it was recorded that more than 1000 harbour porpoises died in UK waters in 2019 having got caught in the nets of EU registered vessels.

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

ORCA regularly surveys the North Sea, often recording harbour porpoise on the crossing between Newcastle and Ijmuiden; over 200 individuals have been recorded by ORCA Marine Mammal Surveyors in just one crossing!