Exposure of short-finned pilot whales (Globicephala macrorhynchus) to high-amplitude vessel noise in the Teno-Rasca Special Area of Conservation (Tenerife, Spain)

Rubio, C., Aguilar de Soto, N., Nachtsheim, D.A., Schnitzler, J., Arranz, P., Johnson, M., Marín, O. - BIOECOMAC Lab (University of La Laguna)

Figure 1: Distribution of the decidecade band levels in the 16 kHz decidecade band (dB re. 1 μPa RMS) for each pilot whale. The boxplots nestled inside the violins display the median and interquartile range of the distributions. The red dashed line represents the threshold of 84 dB re. 1 μPa for detecting vessel passes. All data points above the red dashed line are the 370 events classified manually.

Over the past year, the University of La Laguna team has studied the exposure of short-finned pilot whales (Globicephala macrorhynchus) to noise from maritime traffic in southwest Tenerife, specifically in the Teno-Rasca Special Area of Conservation (SAC), through on-animal acoustic biologging devices (DTAGs). This research, conducted in collaboration with the Institute for Terrestrial and Aquatic Wildlife Research at the University of Veterinary Medicine Hannover (TiHo), applied an innovative methodology to semi-automatically detect vessel passages which has already been successfully applied to other marine mammal species.

A total of 21 short-finned pilot whales were tagged, of which 18 were included in the present data analysis. The tagged animals were classified in-situ into four categories: calves (11%), indeterminates (44%), juveniles (28%), and adults (17%). The study collected 120.0 hours of acoustic data, with each tag remaining attached for an average of 6.7 hours. Short-finned pilot whales are highly active cetaceans exhibiting sprinting behavior, generating strong water flows over the tags. This activity can cause tags to detach if they are not perfectly secured, presenting additional challenges during data collection. Consequently, careful sensor calibration is essential to ensure data accuracy and reliability.

Acoustic data were processed, employing a systematic methodology tailored for analyzing sound recordings from DTAG devices, as tags capture all sounds the animals are exposed to, including water flow noise, whale vocalizations, and anthropogenic noise. To find vessel passes, the applied methodology proposes identifying the optimal decidecade band for analyzing vessel noise in this species and establishing an appropriate decidecade band level threshold to detect peaks associated with vessel passages. The 16 kHz decidecade band was chosen to work with, as it showed the lowest correlation between sound and whale activity among the analyzed decidecade bands. A threshold of 84 dB re. 1 μPa RMS (@16 kHz) was set to detect vessel passes (see Figure 1).

A total of 370 events were manually classified into seven categories: vessels, possible vessels, other anthropogenic noises such as human voices, weather phenomena like rain, device noise caused by bubbles striking the hydrophone, whale communication sounds, and unknown sources. Out of these events, 24 were confirmed as vessel passages.

Initial results indicate that 22 of the 24 vessel passages occurred within or near the Teno-Rasca SAC, with sound levels reaching up to 110 dB re. 1 μPa. The geographic position of the remaining two vessels cannot be verified due to insufficient GPS data. This noteworthy finding highlights that, despite the presence of an environmental protection and conservation designation in the study area, the resident population of short-finned pilot whales is exposed to high-amplitude noise events (see Figure 2).

Figure 2: Geographical position of 22 out of 24 vessel noise events detected according to GPS information for each individual, along with their maximum decidecade band level, shortened as DBL (dB re. 1 μPa RMS @16kHz). The map was created using a combination of MATLAB data over Google Maps (2024).

One calf, for example, spent 13% of its monitored time exposed to a vessel passage. This is likely due to their shallower diving patterns, with a maximum recorded depth of 166 m, compared to adults, which can dive to depths exceeding 1000 m. Interestingly, none of the tagged adults recorded any vessel passages, likely due to their deeper diving behavior. Most vessel noise events occurred during shallow dives (less than 200 m) or when the whales were at the surface (see Figure 3). This pattern underscores the vulnerability of younger individuals and shallow-diving behaviors to anthropogenic noise, which could have ecological implications and may require specific conservation efforts.

Figure 3: Example of three dive profiles of short-finned pilot whales, with pw21_214a being a calf, pw22_124b an indetermined, and pw22_125a a juvenile. The x-axis represents depth in meters, and the y-axis represents time expressed in date and time format according to the tag-on and tag-off times of each DTAG. Vessel passes are represented with their duration and decidecade band levels throughout the event. The color bar on the right shows the color associated with each decidecade band level, abbreviated as DBL. The dashed line in the plots shows the shallow dive limit, set at 200 m.

These findings highlight the pressing need to better understand how vessel noise impacts the behavior and well-being of this unique resident population of short-finned pilot whales. Further research is necessary to investigate the behavioral responses of whales to vessel noise, including changes in diving patterns and vocalization rates, and to assess the cumulative impact of repeated noise exposure over time.

The high amplitude noise events recorded in the Teno-Rasca SAC emphasize the urgency of implementing marine traffic regulations to protect not only short-finned pilot whales but also other cetacean species inhabiting the Canary Islands. As top predators, short-finned pilot whales play a fundamental role in maintaining the stability of marine ecosystems. Their conservation is intrinsically linked to the health of the broader marine environment, and protecting these marine mammals directly supports the preservation of the entire ecosystem they inhabit.