There has been a great deal of attention given to the issue of marine mammals and underwater sound. Whales and dolphins communicate and forage underwater, some species using echolocation to determine where they are, what obstacles are around them, and to find prey. As anthropogenic noise (noise created by human activities) increases there is concern that it will become too disruptive to the lives of these animals.
There is much still to be learned about how, where and when anthropogenic noise becomes a problem for marine animals. And, while cetaceans (whales and dolphins) have been the primary focus of most research, there is some evidence that fish are also affected. Most recently developmental delays and body malformations have been seen in scallops exposed to seismic pulses. This project was conducted within a laboratory however, every animal exposed to noise developed problems where none of the controls did. Thus it is highly likely that a similar situation occurs in the natural environment.
This finding has implications for the longer term survival of other marine species, particularly those seaducks who feed primarily on shellfish. The researchers suggest it is likely that species other than scallops, essentially shellfish in the area, would be similarly affected
Types of anthropogenic sound
A workshop on manmade noise states that anthropogenic sources most likely to have contributed to increased ocean noise are, in order of importance:
- Commercial shipping
- Offshore gas and oil exploration and drilling
- Naval and other sonar uses
But the list also includes:
- Pile driving
- Depth sounders, fish finders
- Bottom towed fishing gear
- Recreational vessels
- Overflying aircraft
Commercial vessel engine noise, usually a low frequency sound, may contribute to masking and habitat displacement. Intense low or mid-range sonar, used by the military and in seismic surveys for oil and gas exploration, as well as ocean floor mapping, can result in any or all of the effects listed below, as can pile driving and any type of explosion. Dredging, drilling and bottom-towed fishing gear all have impacts.
As can be seen from the list, it is not just industry and the military contributing to this problem. Recreational boats can have significant impact on nearshore species, causing masking and behavioural changes. Depth sounders, and fish finders, can have similar effects, depending on the species found in the area.
How anthropogenic sound may affect marine mammals
Not every manmade sound is a problem to marine mammals but there is concern because of the rate at which anthropogenic noise is increasing in the ocean. Scientists have determined that there are four primary ways in which manmade noise may affect marine animals:
- Masking- preventing animals from hearing important sounds including vocalisations from other members of their group
- Behaviour change—animals may stop feeding or move away from areas where they would normally be found
- Behaviourally-mediated effects—weight loss, lack of breeding success, etc. resulting from disruption of normal behaviours
- Temporary or permanent hearing loss or reduction
- Physical trauma—where explosive type sound has been used, injuries to the inner ear and kidneys have been found in some animals
The mass strandings of beaked whales in several countries, particularly Greece and the Bahamas have been tentatively linked to military exercises and melon-headed whales in Madagascar were affected by seismic surveys swimming deep into a bay and stranding. It is postulated that these species may have some physiological distinctions from other ceteceans that make them more vulnerable to certain sound frequencies.
Determining how each type of anthropogenic sound is likely to impact marine mammals
There are several aspects of sound production in the marine environment which may have an impact on whether the sound is likely to be a problem. These include:
- Where animals are in relation to the sound
- Level of sound at different depths and distances
- What sound levels the animals are exposed to
- The hearing threshold (sound frequencies heard) of the marine mammals in the area
These factors help determine whether there are likely to be significant impacts on the animals found in the area.
Protecting marine animals from excessive anthropogenic sound
In the US and the UK guidelines have been developed for commercial seismic activities, which include the need to visually monitor for the presence of cetaceans, slowly ramp up the sound, and shut down if marine mammals come into the area or if the survey vessel moves into areas where animals are present.
Knowledge of migration and other movement patterns for the species found in a particular area may be used to schedule activities in those areas when there is the lowest potential for impact. For example naval exercises in known calving can be scheduled at times when pregnant and nursing whales are not present. Adjustments to the frequencies emitted by sonar and other devices may prove helpful as well. A great deal of research is going into understanding the effects of sound in the marine environment and how to reduce the negative effects for marine animals.
D'Amico, A. et al. 2009. Beaked whale strandings and naval exercises. Aquatic Mammals 2009, 35(4) 452-472.
DeRuiter, S.L. et al. 2013. First direct measurements of behavioural responses by Cuvier's beaked whales to mid-frequencey active sonar. Biol.Lett 9:20130223. http//dx.doi.org/10.1098/rsbl.2013.0223.
de Soto, N.A. et al. 2013.Anthropogenic noise causes body malformations and delays development in marine larvae. Scientific Reports. http://www.nature.com/srep/2013/131003/srep02831/full/srep02831.html
Halvorsen, M.B. 2012. Effects of mid-frequency active sonar on hearing in fish. J Acoust Soc Am. 2012 Jan;131(1):599-607. doi: 10.1121/1.3664082
Southall, B.L., Rowles,T., Gulland,F., Baird, R. W., and Jepson, P.D .2013. Final report of the Independent Scientific Review Panel investigating potential contributing factors to a 2008 mass stranding of melon-headed whales (Peponocephala electra) in Antsohihy, Madagascar. International Whaling Commission Document.