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By Tom Norris - CSI Fellow
 Just
as we rely on light to see our world through vision, marine mammals rely
on sound to sense their environment and communicate with others
animals. In their underwater world, light is of very limited use because
it does not propagate very far except in a few clear underwater
environments. Sound, however, propagates quite effectively underwater.
As a result, sound is the primary sensory modality for most, if not all
marine mammals. In fact, this group of mammals has evolved specialized
anatomical and behavioral mechanisms that allow them to use sound to
communicate, hunt, navigate, and sense their environment. For example,
Blue whales’ calls are known to travel, and are likely detected by other
whales, at distances of over 1,000 kilometers (600 miles). Sperm whales
and most dolphins actively produce loud clicking sounds when they
echolocate. Echolocation allows many species of marine mammals to
search for and capture prey in an otherwise dark and inhospitable
environment. Finally, sounds produced by other species including
predators (such as killer whales), naturally produced “ambient noise”
and other underwater sounds provide information that may be important
for the survival of marine mammals. As one might expect, disruption of
sound producing behaviors, masking of important sounds and even stress
caused by loud non-natural sounds can adversely affect marine mammals.
Scientists
have recently become concerned that rapidly growing sources of
human-made noise in the oceans may be negatively impacting the survival
of marine mammals. Loud sources of man-made noise are produced from a
variety of sources such as military sonar, large ships, oil exploration
and drilling activities, nearshore underwater construction, and even
small motor boats. The concern is that noise may be disrupting
important behaviors, masking important signals, and/or causing animals
to move out of important breeding and feeding areas. In the most
extreme cases, death of whales, dolphins, and other marine life may
result. For example, in 2001, 16 whales and a dolphin were live-stranded
on the northern islands of the Bahamas shortly after a major US Navy
exercise was conducted nearby. Most of these whales belonged to a
mysterious group of deep diving odontocetes known as “beaked whales.”
Six of the stranded whales and the dolphin are known to have died.
Necropsies indicated that they had severe hemorrhaging most notably in
various regions of the head. Although initially speculated to be the
direct result of the sonar transmissions, it is now believed that these
acute injuries were caused by a physiological phenomena related to “the
bends.” The bends are a life-threatening situation encountered by diver
who surfaces too rapidly resulting in nitrogen gas bubbles forming in
the blood and tissue. It has now been accepted by many respected marine
scientists that mid-frequency sonar transmissions produced by U.S. Navy
ships participating in anti-submarine exercises nearby were the cause of
these live-strandings. After much investigation, it is now believed
that mid-frequency Navy sonar disrupted the natural diving behavior of
these deep diving species resulting in a condition similar to the bends,
which ultimately caused the stranding and eventually deaths of many of
these animals. The exact sequence of events resulting in this type of
response is still poorly understood, but intense man-made sonar sounds
(such as mid-frequency sonar used by many navy vessels) is now believed
to have been the culprit in several other documented stranding
incidents.
There are
growing fears that some regions of the ocean have become so noisy, that
important behaviors such as foraging, mating and even normal breathing
and diving patterns of whales, dolphins and other marine mammals are
being disrupted. Other, less observable effects may include the
displacement of animals from important (e.g. critical) habitat,
increased levels of stress, difficulty detecting predators and prey, and
difficulty communicating with con-specifics.
Most
of the research efforts to date have been focused on three types of
man-made noise: Loud low-frequency sounds (e.g. from oceanographic and
navy sources), mid-frequency navy “operational” sonar, and more
recently, seismic “air-guns” used to survey for oil. In the most
extreme cases, these sounds can damage the hearing of marine mammals,
but perhaps more importantly, they can affect the behavior of large
numbers, sometimes even entire populations of marine mammals. The
severity of the effects of most of these sounds remains unknown for many
species and populations of marine mammals, but is now being studied at
many locations around the world.
Other sources
include commercial shipping which is the most pervasive and largest
contributor of ocean noise energy in the world’s oceans. These sounds
traverse great expanses of the ocean, and because there are very few
areas without shipping, affect most of the world’s oceans.
To get a
handle on how much noise is too much, and what effects result from
exposure to excessive noise, scientists are conducting a variety of
studies. These range from controlled studies of hearing in captive
marine mammals (for the few species that can be kept in captivity) to
“playback” experiments, usually using sounds at reduced levels to
examine how it affects their behavior in the wild, to studies comparing
present day noise with historical levels. A variety of new technologies
have been developed, including miniaturized computer-based tags that
record sounds, locations, orientations, and movements while they are
attached to animals. Other scientists use arrays of hydrophones
(underwater microphones) to “passively” listen to and track animals’
sounds. Or sometimes animals are observed from shore with high-power
binoculars and surveyor’s instruments in order to track their movements
and monitor their behaviors at the surface. Some scientists have even
used planes and blimps to follow whales from the sky. Continued studies
and improved technology will provide answers to many of the unanswered
questions surrounding this topic area.
For now, the
debate between scientists and government officials over exactly what
types and how much noise is harmful to marine mammals continues. And
some may say that there are more questions than answers at this point.
For example, do loud sounds that drive away whales from a feeding area
constitute a minor inconvenience or real threat to their survival? It's
an important but difficult question to answer. The U.S. Marine Mammal
Act states that marine mammals cannot be harassed or harmed without
first obtaining a permit. If scientists determine that whales, dolphins,
and seals are impacted negatively by certain man-made noises further
action may be warranted to protect and manage them.
Please turn
up your volume, and compare the sounds of beluga whales and an ice
breaker. Click on
beluga calls to hear their vocalizations in Beaufort Sea. Click on
ice breaker to hear propeller-cavitation noise.
 (I
CAN GIVE YOU SOME SEISMIC SOUNDS FROM THE GULF OF MEXICO ALONG WITH
SPERM WHALE CLICKS) – THERE SHOULD ALSO BE NAVY MIDFREQUENCY SONAR ON
THE WEB. |
