A really interesting study from our colleagues at the Sea Mammal Research Unit (SMRU), the Norwegian Defence Research Establishment, and TNO has explored how effective the sonar mitigation known as ‘ramp-up’ is for humpback whales.
This study, led by Dr. Paul Wensveen, now at University of Iceland, used data logging tags called DTAGs, which record sound levels and the movements of the animal, to conduct ‘controlled exposure experiments’ in waters off Svalbard and Bear Island (north of Norway). Via these experimental trials, the team explored the responses of 13 humpback whales to different levels of a 1.3-2.0 kHz sonar, as it was gradually increased to full intensity. Effectiveness of this ‘sonar ramp up’ was assessed in terms of whether risk of harm was less in whales presented with the ramp-up compared with those only experiencing a full-power sequence.
The study yielded some really interesting results, and found that while only ~50% of the animals exposed to the ‘ramp-up’ exhibited avoidance behaviour, this mitigation measure still resulted in reduced risk of hearing damage in whales exposed. The team highlighted the value of important contextual information (e.g. behavioural state), which plays a role in how animals respond (and thus in the effectiveness of the ‘ramp-up’ mitigation).
In particular, the authors note: “We found that gradually increasing the source intensity was not an effective method to reduce risk of physiological effects for humpback whales overall, because most whales did not exhibit very strong avoidance responses to the sonar signals. […] However, more detailed analyses suggested that ramp-up of sonar reduces risk more effectively in situations in which animals might be more responsive, e.g. when animals have not been exposed recently, animals are in a non-feeding state, or a small calf is present. This suggests that ramp-up will have greater benefits for species that are more behaviourally responsive to sonar than humpback whales.”
The paper of Wensveen et al. also gives empirical support to a previous paper by the same group (von Benda Beckmann et al. 2014). This paper established a theoretical model which was used to estimate effectiveness of ramp up. This model also predicted that the effectiveness would depend very much on the responsiveness of the animals.
A number of other interesting points, assumptions and caveats are presented and discussed in full in the paper and it makes for an informative and interesting read! Don’t rely on the summaries – so you can (and should!) read the full paper here (it’s free!):
Wensveen, P. J., Kvadsheim, P. H., Lam, F.-P. A., von Benda-Beckmann, A. M., Sivle, L. D., Visser, F., Curé, C., Tyack P. L. and Miller, P. J. O. (2017). Lack of behavioural responses of humpback whales (Megaptera novaeangliae) indicate limited effectiveness of sonar mitigation. J. Exp. Biol. 220, 4150-4161 (DOI: 10.1242/jeb.161232).
Photo credit: Lars Kleivane/3S project