Although we are not seeing much of Bob, the off shore volcanic vent, south the smallest Canary island, El Hierro, we do have some idea of what the sea bed there looks like. Survey ships still monitor the area, testing the water properties as it is not certain if the area is safe for humans to enter the waters.The ideal way to see the sate of the ocean floor is of course by using an ROV, Remotely Operated Vehicle.
These are expensive pieces of equipment and conditions near a volcanic vent could damage them. So the usual way to study the topography of an area like this is to use seismic air guns as seen in the clip below.
To a biologist and anyone concerned with animal welfare this method of mapping raises the question of how the explosions affect the marine animals, not just the large dolphins, but also small creatures that are important in the ecosystem.
The sea is not a silent place. Despite the human concept of a still, blue environment filled only with the sound of the escaping air bubbles of a diver’s breathing apparatus the sea is an incredibly noisy place. There are the sounds of the tides over reefs of rocks and corals. The sound of rain on the surface. The movements and sounds made by animals and seaweeds and of course in seas around volcanic islands like El Hierro there are geologic sounds.
These sounds are the background noise to marine animals as the noises of the countryside are background to a human’s auditory senses.
The importance of the auditory senses to marine animals.
Sound gives a marine inhabitant a 3D picture of it’s environment. Fish have incredibly sensitive hearing.Several studies have shown that fish can determine the range and direction of underwater sound at frequencies ranging from 0.1-1.0 kHz. Most fish have two “inner ears” composed of tightly packed hair cells. More about the construction of these can be found here.
Marine mammals also have internal ears that are designed to pick up love density sound waves (infra sound)
More information here.
Sound is also used as communication, in mating behaviours and for finding food. We are all aware of the “songs” of whales and dolphins but many animals make sounds we humans can hear. The most amazing one is of the Pistol shrimp. This is an excellent video and explanation how this tiny creature creates and uses noise .
Just like humans any strange noises produce altered behaviour in fish from being startled to stress and even death. Man made noise and it’s effects.
Human activity on oceans and shores have introduced alien noise to the marine habitat. Fish and mammals have such sensitive hearing over long distances that they can move away in advance on the approach of a boat with it’s engine running. The sounds of a ships engine may be uncomfortable for some fish but for dolphins, boats often appear to be a source of entertainment and they will happily approach and swim alongside. Hence it can be seen that not all marine creature react to noise in the same way.
There is such a huge diversity of animal life in the oceans , the detailed study of their physiology and behaviour is still very much in it’s infancy.
Like any Biological study the sheer number of species, each with it’s complex behaviours and habitats makes accurate observations and experiments incredibly difficult. Laboratory experiments on the whole will not give accurate results as any natural marine environment is very difficult to replicate.
However some work has been carried out on the effects of seismic air guns.
Seismic air guns are used to do underwater geological surveys. These project sound down toward the water into the rocks below. The air guns are towed by a moving boat. Long strings of hydrophones pulled behind the air gun array detect the reflected signals. This data provides information about the geology below the sea bottom.
The peak source level of a seismic air gun array can exceed 250 dB re 1 μPa, and
although the bulk of the energy is projected downward, there is considerable lateral energy as well. There has been quite a lot of work carried out on the effects of Air Guns. McCauley et al. (2003) demonstrate that shots from a single seismic air gun can cause some damage to the sensory hair cells of the saccule of the ear of the pink snapper (Pagrus auratus, Forster 1801).
It was not accurately found what physiological damage was incurred but the observed fish suffered no mortalities and continued feeding despite some sensory loss.
Skalski et al. (1992) showed a 52% decrease in rock fishhh (Sebastes sp.) catch when the area of catch was exposed to a single air gun emission resulting in a received level of sound at 186–191 dB re 1 μPa (mean peak level) (see also Pearson et al. 1987, 1992) These investigators also found that fishes would show a startle response to received sounds as low as 160 dB, but this level sound did not appear to elicit a decline in catch. The basis for the decrease in catch is not clear, and it should be noted that, for the most part, there was no actual visual observation of the behaviour of the fish during air gun exposure. It is known that, like humans, when exposed to explosive or frequent loud noise, fish suffer permanent hearing loss.
Although these behavioural studies suggest that there might be some changes in behaviour associated with seismic air gun activity, a study by Wardle et al. (2001) that actually observed behaviour on a reef off Scotland as an air gun was fired at a level that was measured to be 210 dB re 1 µPa at 16 m from the source and 195 dB re 1 µPa at 109 m from the source found results to the contrary. The investigators found that several species of fish showed virtually no response to the air gun emission other than perhaps a transient startle response that did not change in any way the pattern of movement of the fish.
The effect on marine mammals is a cause for concern. There have been suspicions that Air guns could possibly interfere with the navigation organs causing beaching. As a result more studies are being undertaken such as this four year study of Humpbacked whales.
How does all this affect El Hierro marine inhabitants?
El Hierro has a very special and well protected Marine Conservation area that is a magnet for professional and amateur marine biologists.
The eruption has caused huge worry for all those whose livelihoods and studies are centred in the waters off La Restinga.
The good news is that the volcanic activity has increased the fertility of the habitats by introducing beneficial nutrients, so encouraging the growth of algae, plants and micro-organisms. This of course will provide food for animals. So once the emissions stop the area will return back to a thriving marine community in a relatively short period of time.
The use of air guns in the area has been minimal. Maybe some fish will have impaired hearing and so be more at risk of becoming prey and maybe less healthy if noise is the primary sense used in hunting.
However I am of the opinion that most fish will be avoiding the area affected by the emissions anyway. This will be because of chemical and gaseous changes in the water. Any animals that can move away will have done so and have set up new territories or feeding grounds away from the polluted area. As the air guns were being used for only a short time over the polluted area then it is probable that it has caused very little detrimental effect, if any at all.
McCauley RD, Fewtrell J, Popper AN (2003). High-intensity anthropogenic sound damages fish ears.
Journal of the Acoustical Society of America 113,
Popper AN, Fewtrell J, Smith ME, McCauley RD (2004). Anthropogenic sound: effects on the behavior andphysiology of fishes. Marine Technology Society
Journal 37 Song J, Mann DA, Cot, PA, Hanna BW, Popper AN (2008). The inner ears of northern Canadian freshwaterfishes following exposure to seismic air gun sounds.
Journal of the Acoustical Society of America 124,
In these Blog Posts I am keeping the contents as non-technical and jargon free as possible. For most people biology study ended after high school so I am aiming to provide an article that is easy to understand and informative for everyone and one which will encourage the reader to question and seek more information. I am also aiming to demonstrate how the different sciences do not stand in isolation. Each science is interwoven and affects another……… The human lifespan is frustrating as it is far to short to get to grips with the vastness of knowledge.