Harbour Porpoises (Phocoena phocoena) in Busy Waters: How Recreational Boats Affect Numbers, Swimming Speed, and Surfacing Interval

Abstract

The harbour porpoise (Phocoena phocoena) is affected by anthropogenic activities in Swedish waters, and the individuals found in the Öresund belong to the Belt Sea population, which is classified as ´vulnerable´. Boat traffic is known to have a negative impact on porpoises. Behavioural responses to such disturbances are context-dependent, and locally specific knowledge is needed for effective management plans. So far, the potential impact of boats around Kullaberg, an area with a high density of porpoises, remains to be investigated. In this study, the effects of the number of recreational boats on the number of porpoises observed around Kullaberg are analysed using traditional land-based visual observations. In addition, the impact of speed and proximity of recreational boats on the speed and surfacing interval of porpoises in the area is examined using unmanned aerial vehicles (UAVs). During eight sampling days in June 2024, 1438 porpoise cues and 294 boats were counted. Generalised linear models (GLMs) were created to analyse the influence of the number of boats on the number of porpoises observed. No effect was found. In August 2024, two UAVs were flown simultaneously on predefined routes within the same study area. One drone searched for and followed detected porpoises, while the other drone monitored the presence of boats. All monitoring data was captured as UAV video and used to determine surfacing intervals, measure the speed of boats and porpoises, and calculate the closest distance between porpoises and boats. A total of 91 synchronous flights were conducted, resulting in 32 porpoise observations. GLMs were used to investigate the effects of boats on porpoise speed and surfacing intervals. The results show that the mean speed of porpoises was influenced by the interaction between the mean boat speed and the distance to the boat. Porpoise surfacing interval decreased with decreasing distance to the boats. No effect of maximum boat speed on the behaviour of the porpoises was found. This study shows that recreational boats can influence the behaviour of porpoises, with potential individual- and population-level effects. In addition, it adds to a growing number of studies exploring the potential of UAVs for data collection on marine mammals.

Popular Abstract

Porpoises in Busy Waters: How Boats Affect Behaviour

‘What is a porpoise?’ was the question we heard most often when we collected data for this study at Kullaberg in the northwest part of Scania. Although porpoises are the most common cetacean in Swedish waters, they live incognito. Nevertheless, they are jeopardised by human activities, and all Swedish populations are declining. The main problem is bycatch, but this study found that high speeds of recreational boats can lead to behavioural changes over both short and far distances, which could also affect long-term survival and reproduction.

Have you ever been to Kullaberg in the summer? If so, you will have noticed the large number of recreational boats travelling the waters. What you probably did not realise is that the sea around the Kullaberg peninsula is an important habitat for porpoises belonging to the Belt Sea population, which is found in the inner Danish waters, the Öresund, and the southern Kattegat. The area seems to offer excellent opportunities for feeding and nursing. So far, however, there has been no research into how intense boat traffic might affect the behaviour of the porpoises in this area. This study aimed to investigate exactly this question. A good understanding of the various factors that could have a negative impact on the porpoises is crucial, as the population is categorised as ‘vulnerable’ and in need of effective conservation actions. The more we know, the better the protection measures can be.

Although porpoises are quite common in the waters around Kullaberg, it is easy to miss them when looking out over the sea while walking through the nature reserve. The small whales only emerge from the water for a brief moment, which can make them difficult to observe. To collect data efficiently, this study therefore used a combination of traditional land-based observations to count the animals and drone videos to study their behaviour in relation to the speed and distance of recreational boats. From the drone footage, the speed of the boats and porpoises, the distance between them, and the surfacing interval of the porpoises were extracted. The collected data were then used to investigate whether different boat characteristics affected the number and behaviour of porpoises.

Surprisingly, no effect of the number of boats on the number of observed porpoises was found. However, the mean swimming speed of the porpoises appeared to be influenced by the speed and distance of the nearest boat. When boats were close and travelling at high speed, porpoises reduced their swimming speed. In contrast, when boats were far away and moving quickly, porpoises increased their swimming speed. Additionally, this study indicates that porpoises surface more frequently the closer boats are.

You may wonder why porpoises react in such different ways. The answer probably lies in the way they perceive their surroundings. Humans use their visual senses intensively in everyday life. Porpoises, on the other hand, instead rely on active sensing through click sounds (biosonar) for communication, orientation, and foraging. Motorised boats can disrupt these sensory abilities of porpoises by disturbing the underwater world with noise. This can lead to disorientation, stress, less successful foraging, and higher energy expenditure. The extent of the noise disturbance depends on the speed and proximity of the boats. Thus, faster boats might cause stronger reactions because they are louder underwater. In addition, the increased swimming speed of porpoises when the boats are further away could indicate an avoidance strategy. However, this strategy could become impossible when the boats are closer, as the increased noise pollution might affect their orientation too much. When boats are close, a slower swimming speed could allow porpoises to lessen the energy deficit caused by reduced foraging opportunities. Furthermore, previous research has shown that the presence of boats can increase the metabolic rate of minke whales by inducing stress, which can lead to a higher respiration rate. This may explain why porpoises surface more frequently the closer boats pass them. Boats therefore appear to have the potential to destabilise the energy balance of the whales.

What does this mean for Kullaberg, and what can we do? It seems that the porpoises will not leave their habitat because of the boats. Probably because the feeding and nursing opportunities are so good. However, the results suggest that they might pay an energetic price for it, which could affect their survival and reproduction in the long term. Given the negative population trend of the Belt Sea population of porpoises, this could have dire consequences. Therefore, a well-monitored speed limit for recreational boats seems to be crucial to reduce their impact on porpoises. In addition, porpoises' quality of life could be improved through boat exclusion zones in areas with high numbers of porpoises. These could be important steps to help Sweden's incognito whale thrive again in the waters of the Öresund!

Master´s Degree Project in Biology 60 credits 2025
Department of Biology, Lund University

Advisors: Johanna Stedt, Per Carlsson
Aquatic Ecology/Functional Ecology

• author: Till, Johannes
• supervisor: Johanna Stedt; Per Carlsson
• organization: Degree Projects in Biology
• course: BION03 20241
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9188121
• date added to LUP: 2025-05-07 12:03:24
• date last changed: 2025-05-07 12:03:24