For definitions and information on acoustic parameters of military sonar, please see www.marinemammalriskmitigation.com
Military sonar has the potential to mask vocalisations of some species of marine mammals. Masking occurs when noise interferes with, or obscures, bioacoustic signals, and thereby hinders the detection, discrimination, or recognition abilities necessary for effective communication, navigation, or foraging (Trickey et al. 2010). Some marine mammals have developed techniques to counteract the effects of masking. Right whales (Eubalaena sp.), for example, have been shown to alter the frequency, amplitude, and rate of calling in the presence of noise occupying their baseline acoustic niche (Parks et al. 2011).
HUMPBACK WHALES AND MASKING FROM MILITARY SONAR
Baleen whales are known to communicate over large distances, and the introduction of Low Frequency Active Sonar (LFAS) (operating between 600 and 1,500 Hz, www.marinemammalriskmitigation.com) by the military puts sound directly into the same frequency band as baleen whale vocalisations. Male humpback whales (Megaptera novaeangliae), for example, sing a song to attract a mate, and if this song cannot be heard it limits their chance to reproduce. Miller et al. (2000) found that humpback whale songs were longer when a LFAS recording was played compared to before or after playback. All themes (verses) in the song were still sung in the correct order, only the length of the song increased. Song length returned to pre-exposure levels when playback ceased, suggesting no long-term effects from sonar.
PILOT AND FALSE KILLER WHALES AND MASKING FROM MILITARY SONAR
Rendell and Gordon (1999) studied vocal response of long-finned pilot whales (Globicephala melas) to military Mid Frequency Active Sonar (MFAS) in the Ligurian Sea in 1994. They found the whales whistled more often in response to MFAS and returned to normal levels when transmission stopped.
During a study performed in the Bahamas in 2007 and 2008, DeRuiter et al. (2013), found that false killer whales (Pseudorca crassidens) also whistled more frequently in response to simulated MFAS compared to before or after exposure; however, the same trend was not observed for short-finned pilot whales (Globicephala macrorhynchus) in the same study.
It is important to recognise, that it is unknown whether the increase in whistling had any effect on the animals, either positive or negative.
SPERM WHALES AND MASKING FROM MILITARY SONAR
Due to the nature of masking effects, it is sometimes difficult to be certain whether an animal has stopped vocalising, or if the vocalisation itself has been masked, and therefore not detected by the recording device. Some work has been undertaken in this field, notably by Watkins et al. (1985), who observed sperm whales (Physeter macrocephalus) appearing to cease vocalisations during MFAS use. Authors accepted there was a possibility that some vocalisations were masked, as some vocalisations were of the same frequency as the MFAS used; however, this was deemed unlikely because sperm whale vocalisations have longer durations than the MFAS signal used, therefore some of the clicks should still be detected. More recent observations from Watkins et al. (1993) showed that sperm whales ceased vocalising in response to submarine sonar signals. Two sperm whales were being tracked both visually and acoustically until a high level sonar pulse was detected, and both whales submerged immediately and disappeared, and were not seen again that day.
More research is needed in this area, but due to the secrecy of military sonar and its use, it is a difficult area to investigate, with most recent studies relying on the use of simulated sonar sounds to assess effects.