Effect of naval sonar exposure on killer whales and humpback whales – 3S-2025 cruise report

3S (Sea mammals and Sonar Safety) is a multidisciplinary and international collaboration that studies 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 different severity of behavioural responses compared to 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-2025 trial was the third and last planned trial under 3S4 where we collected data to address these research questions. The trial took place off the coast of northern Norway in October 2025 using FFI’s research vessel H.U. Sverdrup II. 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 a mixed-DTAG++ attached with suctions cups, recording high resolution movement, camera and acoustic data. The tag transfers the GPS position of the tagged whales directly to the source vessel and via satellite. 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-2025 trial, 18 mixed-DTAG++ and 2 satellite Splash10 tags were deployed to killer whales, and 7 mixed-DTAG++ were deployed to humpback whales. We conducted 4 controlled exposure experiments (CEE), 2 CAS and 2 PAS, on 10 animals (7 killer whales and 3 humpback whales). The CEEs were conducted during both daytime resting (3 experiments) and nighttime feeding behaviour (1 experiment). All CEEs were long duration exposures using the SOCRATES LFAS source transmitting 1.3–2.0 kHz hyperbolic up-sweeps at max energy source level (ESL20s) of 214 dB re 1 μPa2·s·m2 for both CAS and PAS. However, 5 of the 10 tags detached before the start of the second approach. This means that all 10 sonar exposure datasets are useful for the CAS-vs-PAS comparison, but only 5 are useful for the long-duration question. The combination of premature tag release during CEEs, rough weather offshore and fewer animals in the fjords might have been a limiting factor for our data collection. Despite this we were able to reach the target number of tag deployments and CEEs, and complemented the 3S4 dataset by filling important gaps. In total during the three 3S4 trials (3S-2023, 3S-2024 and 3S-2025), we have collected a balanced dataset to address the questions of responses to continuous and pulsed sonar and whether the animals habituate or sensitize to longer duration exposures.