Shipping noise severely impacts the vocalization of pilot whales
A recent study published in the Journal of Experimental Biology from Aarhus University, Denmark, investigates the response of long-finned pilot whales (Globicephala melas) to increased underwater noise in the Strait of Gibraltar. This region is a hub of intense maritime traffic, one of the busiest in the world, hosting a small resident population of long-finned pilot whales already considered critically endangered locally.
Multiple whales within the same social groups have been tagged with synchronized acoustic and motion-recording succion cups. Researchers analyzed 1,336 vocalizations from 18 individuals to assess whether the whales compensate for increasing noise by vocalizing louder, a phenomenon known as the Lombard response.
The results showed that pilot whales increase their call output level in response to noise, but only partially compensate for increasing noise. This indicates that their vocalizations’ compensation is insufficient to completely offset the acoustic masking caused by ship noise.
Serious ecological consequences
The reduced effectiveness of their communication may lead to serious ecological consequences, as acoustic communication is essential for maintaining group cohesion, coordinating movement and supporting social and foraging behaviors. Chronic masking of their vocalizations could affect the essential communication to the population’s survival and reproductive success and weaken an already vulnerable population.
The study also revealed that Lombard response magnitude varies with call type and dive context, and how call output level changes with increasing depth. Some high-frequency calls showed greater compensation, while other types of calls showed weak or insignificant responses, likely reflecting physiological limitations in sound production. Furthermore, the results suggest that it is the constraints associated with depth and the diving context that mainly determine the baseline levels of the calls produced, rather than the ability to modulate their amplitude, in consistency with earlier studies in deep-diving toothed whales.
Silence as a last resort
It is interesting to note that relatively few calls were recorded during periods of intense ship noise, suggesting that whales may sometimes reduce or completely suspend their vocal activity under severe masking conditions, as has been observed in other species of cetaceans.
In summary, this study demonstrates that the vocal plasticity of pilot whales, as well as their compensatory mechanisms, are insufficient in a very noisy environment. These results highlight the urgent need to reduce ocean noise emissions.
Mandatory speed reductions: the only effective solution
OceanCare promotes the implementation of vessel speed reduction, quieter propeller design and other technical and operational measures. In particular, vessel speed reduction is a cost-effective measure which could become implemented immediately, while technical measures may take years to generate positive impact due to the longevity of merchant vessels. The Strait of Gibraltar is identified also as a high-risk area for collisions between vessels and whales by the legally binding intergovernmental treaty Agreement on the Conservation of Cetaceans of the Black Sea, Mediterranean Sea and contiguous Atlantic area (ACCOBAMS), among others. Reduced vessel speed would also reduce the risk of such lethal collisions.
Voluntary measures fail – binding rules work
The International Maritime Organisation (IMO) recommended back in 2007 a maximum speed of 13 knots (approx. 24 km/h) in the Strait of Gibraltar in order to reduce the risk of collisions with whales in this high-traffic area. However, analysis of maritime traffic using the AIS in the area indicates that mariners are not adhering to the recommended speed limit of 13 knots.
Various studies carried out in other parts of the world, where measures to reduce ship speed have been implemented, have shown that voluntary measures are largely ineffective, as shipping companies comply with them only to a very limited extent or not at all, whereas mandatory measures are more than 80% effective.
A level playing field for all shipping companies
This is also the case in the Particularly Sensitive Sea Area (PSSA) designated by the IMO in 2023 in the north-western Mediterranean – whose main measure is a recommendation to navigate at a speed of between 10 and 13 knots to avoid collisions with whales –, as concluded by the maritime traffic analyses commissioned by OceanCare in this area in the last years.
In other words, to ensure their effectiveness, speed reduction measures must be mandatory, and must apply to all vessels without exception. One of the major advantages of mandatory speed reduction for vessels is that it creates a level playing field for all shipping companies.
OceanCare believes it has been proven that only by implementing a strategy of mandatory speed reduction for vessels in areas of high ecological value due to the presence of large cetaceans will it be possible to achieve a significant reduction in the number of these endangered animals killed in collisions with boats.