It was observed that a period of intense boating activity caused the killer whales to adjust their call duration levels to compensate for the background noise. This clearly indicates that anthropogenic noise levels directly interfere with the routine life of the killer whales, which are dependent on vocal communication for successful hunting and survival. [Andrew et.al. 2004] It is well-known that anthropogenic sounds can even have fatal consequences as evidenced by the recent mass strandings of beaked whales that coincided with the mid frequency sonar exercises by the navy. A recent research by (Holt et.al, 2009) focused on the effects of anthropogenic sounds on the vocal behavior of killer whales. The resident killer whales of the waters of the Puget Sound, Seattle, were the subjects of this study. The southern resident killer whales in three pods (J, K, and L) frequently forage along the inland waters. It was observed that on an average around 20 vessels surround these pods. This may increase up to 50 vessels on Sundays and weekends. As Erbe 2002 showed, the frequencies of noise emitted by these vessels closely overlap that of the noise generated by the whales. This study analyzed the impact of these vessel noises and how the SRKW adapt their vocal communication to this background noise. This research was conducted between August 23rd and Sep 4, 2007. Calibrated omnidirectional hydrophones were used to obtain both background noises as well as Call data. Call source levels were calculated using the formula SL = RL + 20 log R. where R. is the range of the call. A significant correlation was observed in call source levels with respect to the background noise levels. (p< 0.001, Radj 2 =0.25, n=274).

This research found that the whales exhibited the 'Lombard effect' as a compensative behavior for the increasing background noise. An increase of 1 decibel in the source level was observed for an increase in 1 decibel of the background noise. Earlier a research by Schiefele et.al (2005) also reported such an increase in the source call levels in response to the increased noise level caused by vessels in the St. Lawrence river beluga. This way of increasing the amplitude of their calls as an adaptive response clearly suggests the importance of vocal communication for the survival of killer whales and the potential danger of the ever-increasing anthropogenic interference in the lives of the killer whales. [Holt et.al, 2009]

Another research by Andrew et.al (2004) studied three ecotypes of killer whales in the northeastern Pacific Ocean. It was observed that the different ecotypes exhibited different feeding choice and variations in their frequency range of their pulsed calls. While the offshore ecotype produced high frequency noises which is thought to be an adaptive response to overcome the potential masking effect of offshore low frequency winds. Similarly the resident ecotype used considerably higher frequencies compared to the transient ecotype suggesting an adaptive mechanism to evade eavesdropping by the salmon, which has low frequency hearing. [Andrew et.al (2004)]

Acoustic Pollution and Whale Displacement

Some studies have focused on the effects of acoustic pollution on whale habitats. This study by Morton et.al (2002) observed the population density of killer whales in the Johnstone Strait and the adjacent Broughton Archipelago from 1985 till 2000. The study region was constantly monitored using seven remote hydrophones and from strategically located observation vessels. All the hydrophones were connected to a radio transmitter that facilitated the study of the signals at the research center. An average of 1000 hours of recording was available every year. Similarly a 32 square miles region of the Archipelago was acoustically monitored. Acoustic harassment devices were deployed in strategic locations along the archipelago and their output constantly monitored by portable hydrophones. Between the years of 1994...

It was found that there was no significant change in the whale sightings along the strait (mean=192.40, s.d.=8.26, n=5) while it was considerably reduced along the archipelago where the AHD was in operation mean=9.80, s.d.=4.97, n=5) clearly indicating the negative effects of the aquatic harassment devices. Though the exact cause for the displacement of the whales is not ascertained it is believed that the interference of the high amplitude signals from the acoustic harassment devices could have caused a masking effect on the killer whales communication system disrupting their routines. [Morton et.al (2002)]
Conclusion

Killer whales communicate among themselves and their environment using a wide variety of sounds produced from their nasal sacs. These distinct noises known as dialects are almost unique for a particular pod of the killer whales. Using the whistles, low-pulsed noises and echolocation, Killer whales have developed effective foraging, hunting and social skills that are vital for their survival. From the literature that we have reviewed it is clear that environmental pollution and marine acoustic pollution can have serious implications for the lives of killer whales. The growing aquaculture industry and the use of aquatic harassment devices to enhance fishing profits across the oceans poses a serious threat to the whales and some other marine mammals. It is also clear that there is going to be a steady increase in marine noise pollution due to the growth in marine traffic and increase in oceanographic experiments in the future.

The consequences of such uncontrolled development for the killer whales and other marine species are certainly not good. With positive evidence from many researches indicating the importance of vocal communication for the survival of killer whales and other marine mammals it is imperative upon us to take steps to ensure that their natural survival mechanisms are not trodden over by man made interferences. Creating such awareness is the responsibility of every marine biologist as maintenance of the ocean ecosystem is critical for our own environmental balance.

Bibliography

Whale Songs, 'Killer Whale', Accessed 15th March 2009, available at http://www.whalesongs.org/cetacean/killer_whale/home.html

SeaWorld, ' Killer Whales: Communication and Echo Location," Accessed 15th March 2009, available at http://www.seaworld.org/animal-info/info-books/killer-whale/communication.htm

Wilfredo Santiago Benitez, 'Echolocation and strategy used by Southern resident Killer Whales (Orcinus orca) during foraging', 2005, Accessed 15th March 2009, available at http://beamreach.org/051/papers/wilfredo.pdf

Volcker B. Deecke, John KB Ford & Peter JB Slater, 'The Vocal Behavior of Mammal eating killer Whales: Communicating with Costly Calls ' Animal Behavior, 2005, 69, 395-405, http://www.behaecol.amu.edu.pl/files/the_vocal_behav_of_mammal-eating_killer_whales.pdf

Scheifele, P.M., Andrew, S., Cooper, R.A., Darre, S., Musiek, F.E., and Max, L.

2005). "Indication of a Lombard vocal response in the St. Lawrence River beluga," J. Acoust. Soc. Am. 117, 1486-1492.

Marla M. Holt & Dawn P. Noren, ' Speaking Up: Killer Whales Increase their call amplitude in response to vessel noise', J. Acoust. Soc. Am. 125 (1), January 2009

Andrew D. Foote Richard W. Osborne & a. Rus Hoelzel, 'Environment: Whale-call response to masking boat noise', Nature 428, 910 (29 April 2004)

8) Andrew D. Foote, Jefferey a Nystuen, 'Variation in call pitch among killer whale ecotypes', J. Acoust. Soc. Am. Volume 123, Issue 3, pp. 1747-1752 (March 2008)

Alexandra B. Morton and Helena K. Symonds, 'Displacement of Orcinus orca (L.) by high amplitude sound in British Columbia, Canada, ICES Journal of Marine Science, 59: 71-80. 2002

Whitlow W.L AU, 'Backscatter measurements of three species of salmon using simulated killer whale echolocation signals. (a) J. Acoust. Soc. Am. Volume 124, Issue 4, pp. 2482-2482 (October 2008)