Effect of naval sonar exposure on killer whales and humpback whales – 3S-2024 cruise report

3S (Sea mammals and Sonar Safety) is a multidisciplinary and international collaboration studying how naval sonar affects cetaceans. One of the objectives of phase 4 of the 3S project (3S4) is to investigate if exposure to continuous active sonar (CAS) leads to different types or severity of behavioural responses than exposure to traditional pulsed active sonar (PAS) signals. Another is to investigate empirically if responses from short-duration experiments predict responses from longer-duration exposures conducted over an operationally relevant duration. The 3S-2024 trial took place off the coast of northern Norway in October–November 2024 using FFI’s research vessel H.U. Sverdrup II. The trial collected data to address these research questions. The purpose of this report is to summarize and document the data collected. The experimental design was based on long-duration CAS and PAS exposures to killer whales and humpback whales using real-time GPS location data of multiple tagged subjects. The sonar source vessel was moved to achieve repeated dose escalations over 8 hours, and responses to the first approach will be compared to subsequent approaches in the analysis. Multiple whales were tagged with suction cup attached mixed-DTAG++, which records high resolution movement and acoustic data and transfers the GPS position of the tagged whales directly to the source vessel. Behaviour was recorded for a minimum of 4 hours before exposure, during the 8-hour exposure and for a minimum of 4 hours after exposure. Wildlife Computers SPLASH10-F-333B Limpet tags, which transfer lower resolution data via the Argos satellites, were also deployed to record natural diurnal patterns of killer whales. In addition to data on animal behaviour recorded by the tags, we also collected data on the prey field in the area using echosounder and collected fish samples. Sound speed profiles were collected to understand how the sonar signals propagate in the area. During the 3S-2024 trial, 20 mixed-DTAG++ and 3 satellite Splash tags were deployed to killer whales, and 8 mixed-DTAG++ were deployed to humpback whales. Of the 28 mixed-DTAG++ deployments, 14 were baseline-only records with durations varying from 5 minutes to 29 hours. Six long-duration controlled exposure experiments (3 CAS and 3 PAS) on 14 tagged killer whales (5 focal and 2 non-focal exposed to CAS and 5 focal and 2 non-focal exposed to PAS) were conducted. Despite some effort, we were not able to conduct an exposure experiment with humpback whales this year. The Splash tags collected data over periods from 4 to more than 20 days. The data collected so far show a very clear diurnal pattern, with whales generally feeding around purse seine fishing vessels at night and resting during the day. During the 3S-2024 trial all exposures were conducted during nighttime using the SOCRATES sonar source in the 1–2 kHz band at 214 dB energy source level (re 1μPa2·s·m2). During the 3S-2023 trial, 2 CAS and 2 PAS exposures were conducted during daytime using the same source, but at 197 dB source level and in the 4–6 kHz band. We have collected a balanced dataset to address the CAS-vs-PAS question and the effect of longer duration exposures. However, we have a lot of variation in the dataset, and we need more data to generate conclusive results. We therefore recommend that a third trial is conducted in 2025 with the aim to complement the existing dataset. A video showing the activities during the 3S-2024 trial can be seen following this link.