Abstract
Anthropogenic underwater sound in the environment might potentially affect the behavior of marine mammals enough to have an impact on their reproduction and survival. Diving behavior of four killer whales (Orcinus orca), seven long-finned pilot whales (Globicephala melas), and four sperm whales (Physeter macrocephalus) was studied during controlled exposures to naval sonar [low frequency active sonar (LFAS): 1–2 kHz and mid frequency active sonar (MFAS): 6–7 kHz] during three field seasons (2006–2009). Diving behavior was monitored before, during and after sonar exposure using an archival tag placed on the animal with suction cups. The tag recorded the animal's vertical movement, and additional data on horizontal movement and vocalizations were used to determine behavioral modes. Killer whales that were conducting deep dives at sonar onset changed abruptly to shallow diving (ShD) during LFAS, while killer whales conducting deep dives at the onset of MFAS did not alter dive mode. When in ShD mode at sonar onset, killer whales did not change their diving behavior. Pilot and sperm whales performed normal deep dives (NDD) during MFAS exposure. During LFAS exposures, long-finned pilot whales mostly performed fewer deep dives and some sperm whales performed shallower and shorter dives. Acoustic recording data presented previously indicates that deep diving (DD) is associated with feeding. Therefore, the observed changes in dive behavior of the three species could potentially reduce the foraging efficiency of the affected animals.
Highlights
Our understanding of the effects of military sonars on marine mammals has increased in the past decades (Richardson et al, 1995; Nowacek et al, 2007), but large gaps of knowledge still exist (Nowacek et al, 2007)
It has been suggested that these strandings are associated with a change in dive behavior which leads to development of tissue nitrogen gas bubbles and symptoms related to decompression sickness (DCS) (Jepson et al, 2003)
Long-finned pilot whales spent the majority of their time in the upper 50 m (Figure 1B), separated by bouts of multiple deep foraging dives to 300–600 m (Figure 3)
Summary
Our understanding of the effects of military sonars on marine mammals has increased in the past decades (Richardson et al, 1995; Nowacek et al, 2007), but large gaps of knowledge still exist (Nowacek et al, 2007). Cetaceans often are reported to respond to anthropogenic noise with avoidance (e.g., Morton and Symonds, 2002; Olesiuk et al, 2002; Kastelein et al, 2008a,b; Tyack, 2008), either by horizontally swimming away from the sound source (Nowacek et al, 2004; Lusseau, 2009; Miller et al, 2011) or vertically, by a change in diving behavior (Miller et al, 2009) Such behavioral responses may protect the animals from direct physical injuries such as hearing impairment, but are likely to involve costs of leaving preferred habitat, costs of increased energy of locomotion as well as reduced feeding or higher risks of predation, etc. A change in diving behavior may have potential consequences involving ecological effects such as reduced foraging efficiency as well as potential physiological consequences such as DCS (Kvadsheim et al, 2012)
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