For a definition of military sonar, including source levels and frequencies, please see www.marinemammalriskmitigation.com.
MID FREQUENCY ACTIVE SONAR AND BEAKED WHALES
Tyack et al. (2011) investigated the behavioural response of Blainville’s beaked whales (Mesoplodon densirostris) to Mid Frequency Active Sonar (MFAS) from naval exercises (www.plosone.org). Using the US navy’s hydrophone array in the Bahamas, they found that beaked whales stopped echolocating (see www.echolocationclickdetectors.co.uk for a definition of echolocation) during their deep foraging dives, and moved away from the sound source. This was apparent by a lack of detections in the centre of the range and an increase in detections at the edge. When the naval exercise stopped, beaked whales moved back gradually into the centre of the range over the course of two to three days. One whale was tagged with a satellite transmitter to monitor its movements. During the sonar exercise, the whale moved several tens of kilometres further away from the centre of the range, returning only slowly to the centre over two to three days following cessation of the exercise.
Tyack et al (2011) also tagged six beaked whales with devices that recorded sound, movement, and orientation. The whales’ responses to playback of simulated MFAS, recorded killer whale (Orcinus orca) calls, and a signal with similar bandwidth and timing to sonar were analysed. Beaked whales responded to each of these sounds by ceasing to echolocate and by undertaking an unusually long and slow ascent, with the longest and slowest ascent after detecting a killer whale call. Data collected from these investigations show a significant change in behaviour from the baseline data, as the whales echolocated for shorter durations, with fewer clicks, a slower ascent rate, and an increased interval between dives. Results show that the nature and ‘biological interpretation’ of a call, can also be a determining factor in an animal’s response.
McCarthy et al. (2011) (www.creem.st-and.ac.uk) performed a similar study to Tyack et al. (2011) above. By measuring duration and location of vocalisations produced by Blainville’s beaked whales before, during, and after naval exercises using MFAS, the authors were able to quantify whale foraging behaviour. In both years of the study, more vocalisations occurred before sonar transmission than during. In 2007, the number of vocalisations increased significantly after sonar transmission ceased, but did not return to pre-exposure levels. In 2008 the number of vocalisations did not increase significantly after sonar transmission for 65 hours, whereas between 65 and 108 hours after transmission, number of vocalisations was significantly higher than before, during, and immediately after. Also, in both years no detections were recorded in the centre of the range (where the military operations occurred) when the sonar was active. These results suggest that the MFAS caused whales to cease echolocating and leave the area during sonar operations, and they only returned gradually after sonar activity ended.
In a study by Croll et al. (2001), vocal behaviour and distribution of blue (Balaenoptera musculus) and fin (Balaenoptera physalus) whales was examined before, during and after broadcast periods of LFAS in the southern California Bight (http://bio.research.ucsc.edu). Vocalisations were recorded by acoustic monitoring via two US Navy hydrophone arrays (SOund SUrveillance System, SOSUS; http://en.wikipedia.org) in the area. Playback of LFAS consisted of a 42 second transmission every 6–10 minutes, during daylight hours only, for approximately two weeks. The rate of whale detections was too low to facilitate statistical comparison. There was no substantial change in vocalisations at the southern SOSUS array throughout the duration of the study. There was, however, an increase in vocal activity at the northern SOSUS array after LFAS had completed transmitting. During this study, behaviour and distribution of the whales appeared to be associated more strongly to the changes in prey abundance than the LFAS transmissions and the authors found no obvious responses from the whales to LFAS.
Goldbogen et al. (2013) performed experiments using simulated MFAS to test the response of tagged blue whales in the Southern Californian Bight. Authors found that whales on deep foraging dives were affected most commonly by MFAS, with deep dives being terminated and whales swimming away from the sound source. Some surface feeding individuals showed no response to MFAS playback, suggesting that it might be a combination of the whale’s behavioural state and received sound level that influences a behavioural response. This study shows that blue whales can be affected by MFAS at relatively low received sound levels, as the source level was several orders of magnitude below some military systems. This response to MFAS may negatively impacts their feeding performance and, if subjected to long durations of MFAS use, has the potential to negatively affect the health of the population.
KILLER WHALES AND MFAS
A study by Miller et al. (2014) analysed avoidance behaviour of tagged killer whales (Orcinus orca) to military sonar, using 1–2 kHz or 6–7 kHz MFAS signals. The tags recorded sound, movement, and orientation data. Authors found that in six of the eight sessions, avoidance behaviour was observed with the killer whales ceasing their foraging activity and actively swimming away from the sonar source. Avoidance behaviour was preceded by an increase in the number of calls immediately after a sonar signal was produced. Their study found that avoidance behaviour did not consistently differ based on sonar frequency; instead a mean received Sound Pressure Level (SPL) of 142 ± 15 dB re 1 µPa for either frequency band appeared to cause the response. The authors also noted that whilst the killer whales were feeding on herring in some of the trials, herring schools did not respond to sonar at higher received levels, so it is unlikely that the whales were simply following herring.
It is clear that far more research is needed in this field to better understand effects military sonar have on marine mammals. With more information, better policies, guidelines (www.marinemammalmitigation.co.uk) can be implemented governing the use of sonar, thus provide more protection for marine mammals and clearer information to the military.