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Detection of Vessel Passes in Acoustic Tags on Short-Finned Pilot Whales from the Canary Islands

By Marín, O., Rubio, C., Arranz, P., Johnson, M. and Aguilar de Soto, N. (University of La Laguna)

We are excited to announce the latest findings from our ongoing research project at the University of La Laguna in the Canary Islands, Spain. After analyzing the data obtained from 21 deployed DTAGs over 117.5 hours on individuals of short-finned pilot whales (Globicephala macrorhynchus), we present the preliminary results of our ship passage analysis within the Special Area of Conservation (SAC) Teno-Rasca and its surroundings off the island of Tenerife.

Figure 1: Tagging maneuver on a pilot whale. The DTAG is attached to a tagging pole. © Universidad de La Laguna.

Firstly, we conducted a pilot study involving four (N = 4) individuals. This is the first pilot study using acoustic data gathered with onboard tags attached to deep-diving echolocating whales to assess the exposure of these whales to ship noise. The study presented instances of short-finned pilot whales encountering daily exposure to noisy boat passages. We devised a supervised detector for vessel passages, capable of identifying up to 11 passages within 50.5 hours of recording (see Figure 2), within the Third Octave Level (~decidecade level) of 5Hz. During these events, sound levels reached 105 and 115 dB re. 1 μPa, within and outside the SAC, respectively. Remarkably, this method proved effective despite the fluking noise produced by the whales and the broadband echolocating clicks in the four recordings. The findings of this pilot study were showcased at the OCEANOISE Conference held in May 2023, in Barcelona, Spain.

Figure 2: Dive profile of short-fin pilot whales tagged during different field trips in 2019 and 2022 in Tenerife (Canary Islands) Vessel noise events were identified by visual inspection of long-time spectrograms in MATLAB (Mathworks). Each vessel noise event is color-coded by the noise intensity in dB relative to 1 μPa in the Third Octave Level (~decidecade level) of 5Hz.

Subsequently, this analysis was enhanced and expanded with data from additional whales (N = 18) in collaboration with the Institute of Terrestrial and Aquatic Wildlife Research (ITAW) in Hannover. Additionally, the maximum depth of each whale was determined, including 3 adults (880 m), 5 juveniles (957 m), 2 calves (98 m), and 8 indeterminates (1086 m), along with the GPS position of each individual. This time, to detect the highest number of vessel passages, a different approach was taken. Instead of attempting to detect the low-frequency components of ship-produced noise, efforts were directed toward isolating the high-frequency components produced by cavitation. Thus, a threshold was set at 16 kHz (see Figure 3), as it was observed to be the TOL with the highest number of high-frequency events detected, despite the fluking noise produced by the whales, traduced as “jerk”. Similarly, it was established that events falling outside the 95% distribution (i.e., outliers) were produced by elements unrelated to the activity of the animal itself, setting a threshold at 84 dB re. 1 μPa @ 16 kHz (Fig. 2). In total, 370  number of events were recorded, of which 24 were attributed to vessel passages. These vessel events were further classified as rotating machine noise (46%), whiney whoose (29%), and Loyd’s mirror (25%). The median exposition rate was 0.25% for all the whales, with a maximum of 13.25%.

Figure 3: Distribution of the third-octave levels in the 16 kHz 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 at 84 dB re 1 μPa for detecting vessels. All data points above the red dashed line are the 370 events classified manually. 

This successful preliminary study will now be expanded, delving deeper into the potential impacts of ship noise on the behavior and physiology of the whales. This includes examining alterations in activity levels, respiration rates, and foraging behaviors, as well as potential reductions in effective communication and echolocation ranges. Such data hold significant relevance for managing acoustic pollution in marine-protected areas, particularly in regions like the Canary Islands, where whales face frequent daily encounters with whale-watching boats and ferries.


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