Drivers of the small-scale spatio-temporal distribution of harbour porpoises (Phocoena phocoena) investigated via UAV around Kullaberg, Sweden

Abstract

The harbour porpoise (Phocoena phocoena) is elusive and challenging to study. Recent reports indicate negative population trends in the Belt Sea population, which is found around the peninsula of Kullaberg in southern Sweden. In this study, the driving factors of the small-scale distribution of porpoises around Kullaberg were investigated by use of an unmanned aerial vehicle (UAV). Over 17 weeks from June through September 2023, transects were flown twice per week, covering most of the water surface of Western Kullaberg in a 600 m radius from the shore each sampling day. Porpoise density increased from June through September and decreased from mornings to afternoons, indicating high activity at dawn. Presence of motor boats had a negative effect on porpoise density, possibly due to noise disturbance affecting navigation, communication and foraging activity. The northern side and tip of the peninsula had significantly higher porpoise density than the southern side. 64 % of porpoises were found outside the 300 m marine reserve. Expanding the reserve could improve conservation effort effectiveness. 6 % of individuals were calves, which corresponds to the mean for the Belt Sea population. This frequency may be an underestimation due to a lower probability of detecting smaller body sizes, especially at rough sea states. This study corroborates the idea of Kullaberg as a site of high ecological value to the Belt Sea population of porpoises with a relatively high mean density of 0.95 ind km-2. Further exploration of porpoise habitat use and vulnerability to disturbance using UAV methods can provide integral information on how to effectively conserve porpoises around Kullaberg.

Popular Abstract

High up to look deep – Drone monitoring of harbour porpoises

Drones are of increasing relevance, not only as tools to produce expensive-looking videos inexpensively, but in research on animal movement, behaviour, morphology, life history and population structure. In this project, an unexplored method of using drones to assess the density and distribution of harbour porpoises (Phocoena phocoena) was tested at Kullaberg nature reserve, Sweden.

Harbour porpoises are small toothed whales and the only whales commonly found in Swedish waters. When looking out on the water in Sweden and Denmark, you might be very likely to spot a dorsal fin coming up out of the water – this is very likely a harbour porpoise. However, these sightings might become rarer in the future since recent scientific reports indicate that the Belt Sea and Baltic Sea populations are under the threat of bycatch, chemical pollution and noise pollution and therefore declining.

At Kullaberg nature reserve in north-western Scania, previous projects and reports from local guides show that harbour porpoises are present in high densities and possibly use the area to calve, nurse and mate from May to September. Therefore, in this project we investigated the distribution of harbour porpoises over space and time and in response to different environmental and anthropogenic factors such as wind speed, waves, temperature, habitat, motor boat presence, sunshine and time of day. Factors such as sunshine and waves could also be relevant when considering possible method bias of using drones as waves and the reflection of sunshine on the water surface could affect visibility of porpoises on the drone video footage. From previous research, we also know that harbour porpoises are not affected by drones hovering above them, so our research was no disturbance to the porpoises.

The drone was flown 188 times from early June until the end of September 2023 on 30 sampling days or about twice a week. On each sampling day, the drone captured about 90% of the water surface in the marine reserve around Western Kullaberg in a 600 m radius on video plus an area further east in Arild as a comparison. Essentially, the entire water surface 600 m from the shore was monitored twice per week in pre-programmed transects that the drone flew ‘on autopilot’.

After reviewing the video footage carefully and performing statistical analysis on it, several patterns emerged. Firstly, there was an increase in the number of porpoises detected from June until September. Secondly, there were more porpoises seen in the footage earlier in the day compared to afternoons. Thirdly, whenever there were more motor boats in an area, there were fewer porpoises. Lastly, we found that higher wave intensity also had a negative effect on the number of porpoises that were seen.

The seasonal patterns (and possibly daily activity pattern) could be related to the presence of prey. In late August, blue-fin tuna (Thynnus thynnus) appeared in the area, which may have been attracted by the same food source as porpoises. Additionally, it could be that Kullaberg acts as a mating ground since the mating season starts in August. This is also supported by observations in the field of mating behaviours such as acrobatic jumps out of the water. Another problem was the ability to detect calves – their small body sizes may have been more difficult to see on the drone, which was flown at a 66 m altitude at 10 m per second. Calves also were not detected until late August and a calf proportion of 6 % was calculated for Kullaberg, which is average. Calves grow quickly and their larger size in August may have made it easier to see them. Further, calves often swim beneath the mother, which makes it impossible to see them when looking straight down at the water surface. The finding that motor boats have a negative effect on the number of porpoises indicates that the presence of motor boats changes the way the porpoises behave – either they tend to come to the surface less or they leave the areas whenever there are more motor boats. This is most likely related to the noise disturbance, since physical disturbance by sailing boats and other large objects in the water do not seem to have the same effects.

When comparing the number of porpoises at Kullaberg to large-scale studies using small airplanes and boats, it seems that Kullaberg is comparatively high in porpoise density, so the hypotheses on Kullaberg as a crucial habitat find support. Interestingly, 64 % of porpoises were found outside the 300 m marine reserve, where policies such as boat speed limits do not apply. This result shows that in order to protect this local population, expanding the marine reserve and implementing more restrictions on motor boats could be useful measures. However, we should conduct more research in order to find out the exact ways in which porpoises are impacted and respond to motor boats and other disturbances. This is an important step towards exploring why harbour porpoises are declining and what kind of new research methods there are and how we can improve them.

Exploring drones further as a monitoring tool for marine mammals is essential in this context, as they can not only provide information on the number of porpoises present, but also behavioural responses that can teach us what to do to reduce disturbance. For this, our method could be combined with ‘focal follows’ involving active follows of specific individual porpoises with the drone to investigate their behaviour. Our result of wave action negatively impacting the number of porpoises in an area could also point to a possible method bias, meaning that because of the nature of the method the waves make it more difficult to detect porpoises. This also needs further studying as it is important to have the same probability of seeing porpoises during different flights. Otherwise, results could be misleading. To do this, we could test different versions of our methods such as different flight altitude, speed, paths and sampling conditions. This may also improve the detectability of smaller animals such as porpoise calves.

Master’s Degree Project in Biology, Conservation Biology, 60 credits, 2024
Department of Biology, Lund University
Advisor: Johanna Stedt, Per Carlsson
Division of Aquatic Ecology

• author: Hartmann, Dinah Josefine
• supervisor: Johanna Stedt; Per Carlsson
• organization: Degree Projects in Biology
• course: BION03 20231
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9153945
• date added to LUP: 2024-05-27 11:53:54
• date last changed: 2024-05-27 11:53:54