Porpoises ‘buzz’ less when boats are nearby

Harbour porpoises “buzz” less when boats and ships and nearby – suggesting a drop in feeding and socialising, new research shows.

Scientists used underwater microphones in a narrow strait called the Little Belt (Denmark) then mapped porpoise sounds against vessel traffic.

Buzzes (short sounds used when porpoises feed and socialise) reduced by up to 45% during busiest vessel traffic periods.

Harbour porpoises are small but have a rapid metabolism and eat almost constantly – so reduced activity could have major impacts.

The researchers say conservation measures such as reduced boat speeds could help protect porpoises.

“Our findings suggest chronic boat traffic may be limiting porpoises’ ability to feed and socialise effectively in important coastal habitats,” said Rachel Lennon, who co-led the study during a master’s degree in Marine Vertebrate Ecology and Conservation at Exeter’s Penryn Campus.

“Porpoise activity dropped most significantly during summer daytime hours, when the Little Belt is busiest with vessels.

“In moments when vessels were not present, porpoise activity increased.”

Co-lead author Shannon Merkle said: “We know from other studies that disruption to foraging can affect porpoises’ health, body condition and ultimately reproduction and survival.

“While we did not directly assess the impact of boat traffic on porpoise populations, the disruption we found is likely to affect their ability to cope with other environmental stressors.

“It’s also important to note that porpoises cannot simply move elsewhere – as other areas may not have the needed density of prey that exists in the narrow part of the Little Belt.”

Several thousand larger vessels and an unknown number of recreational vessels pass each year through the Little Belt, which is one of three straits that connects the Baltic Sea and the North Sea.

The Belt Sea population known to use the Little Belt strait are “Endangered”, making threats to this population particularly concerning, according to environmental protection organisation HELCOM.

Co-author Jonas Teilmann, professor at Aarhus University, said: “This study adds to the growing understanding of how underwater noise and the increasing vessel traffic affects our environment, and in particular a noise-sensitive species like the harbour porpoise.

“Our hope is that the results of this study will be used in future managing of the number of vessels, how fast they may move and their distribution in protected areas.”

The study was carried out in collaboration with Aarhus University, and industry partner Seiche. 

The paper, published in the journal Marine Mammal Science, is entitled: “Seasonal and diurnal patterns of harbor porpoise (Phocoena phocoena) behavior and the disruptive effects of vessel presence in a high-traffic coastal habitat.”