What's the Problem with Porpoises?
Collectively, porpoises, dolphins and whales are known as
cetaceans.
Porpoises are close relatives of dolphins, although they
differ in the shape of their dorsal fins and their teeth. Porpoises also have a
more rounded body shape and lack a distinct "beak".
Porpoises
share many characteristics with other mammals; they are warm blooded, feed their
young with milk, and breathe air.
The harbour porpoise spends a large
part of its day hunting and feeding. Making dives up to five minutes long, the
harbour porpoise feeds mainly on fish near the bottom of the sea - this is where
the trouble starts.
Thousands of harbour porpoises are accidentally
caught in fishing nets each year. This current level of by-catch cannot be
sustained. If it persists, the harbour porpoise population will become
increasingly depleted and thousands more porpoises will continue to suffocate
and die in nets.
Threats to Porpoise Survival
How does IFAW's Automatic Porpoise Detector Work?
We know the problem of harbour porpoises dying in nets is serious, but it might be more serious than we think. We need more and better information on harbour porpoise populations, distribution and behaviors in order to design and implement effective methods of protecting them from harm.
An important step that can be taken immediately is for governments to establish independent systems to monitor by-catch levels on fishing boats.
Are There Practical Solutions?
Yes. Some fishing grounds are also critical porpoise habitat and could be closed for set periods of time.
Many fishers are genuinely concerned about catching marine mammals in their nets; they could change some of their fishing techniques.
Reflective fishing nets and acoustic warning devices such as "pingers" may help the porpoise detect nets, without interfering with fish. However, more studies are needed to prove whether the use of pingers causes other problems.
IFAW Research and Conservation
The elusive harbour porpoise is notoriously hard to study. There are serious gaps in our knowledge. Accurate information will help us better protect the harbour porpoise and its ecosystem.
IFAW's scientific team on board our research vessel, Song of the Whale, is helping find out more about the mysterious harbour porpoise.
IFAW researchers have developed an "automated porpoise click detector" which is used on board Song of the Whale and by other porpoise researchers. It picks up the high frequency clicks -- inaudible to human ears -- that porpoises make.
Whatever the weather, the IFAW detector can keep an "audio window" open at all times. If echolocating porpoises are in the area, the IFAW gear will pick up their clicks, even if they are not visible.
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How does IFAW's Automatic Porpoise Detector Work?
Porpoises are hard to spot visually. Sighting rates for porpoises decrease
rapidly as wave-size, or sea state, increases. Sightings are virtually
impossible over Beaufort sea state 3. Sightings are, of course, impossible at
night and in fog.
Porpoises, like most toothed whales
(odontocetes) make click-type sounds when they echolocate to find food
and orient themselves. In the case of harbour porpoises, the clicks are very
high frequency, 115 to 145 kHz, or about three octaves higher than we humans can
hear (a young adult human can generally hear up to about 20
kHz).
Porpoise clicks are also described as "narrow band."
That means that they contain energy in the 115 to 145 kHz range only. The ocean
is a noisy place, and getting noisier. There are many other sounds in the ocean
with energy at these frequencies, such as high-revving engines from power boats.
Even other animals such as shrimp make a surprising racket!
Fortunately
for us, these other sounds tend to be "broad band." That means that they
contain energy not only in our frequency band of interest, but at other
frequencies as well. Therefore, by looking for sounds with energy in the 115 to
145 kHz band -- but none at two lower "control" frequencies -- we can
differentiate porpoise clicks from the noise.
Because the frequencies at
which porpoises vocalize are well beyond the limit of human hearing, specialized
equipment must be used to detect these sounds. IFAW scientists have been
building automatic detectors for porpoises since 1993.
Incoming sounds
are first processed with a custom-built electronics module. This splits the
sounds coming up the cable from our hydrophone (underwater microphone) into
three frequency bands. The signals in each band are then digitized and analyzed
on a computer. Specially written software measures the energy in each frequency
band and makes a decision as to what type of click it is.
With the latest
equipment, two hydrophones are towed, one behind the other. By measuring the
difference in the time of arrival of clicks on each hydrophone, it is possible
to measure the bearing (direction) the clicks are coming from.
The
properties of individual detected clicks are stored on the computer's hard-drive
to give a permanent record of detections and to facilitate off-line analysis of
the data collected in the field.
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Thousands of harbour porpoises are caught in fishing nets and drowned each year. Researchers on board IFAW's Song of the Whale are studying porpoise behaviour and distribution to help develop practical methods of stopping these tragic deaths. Photo: Courtesy of Tim Lewis
IFAW researchers locate harbour porpoises using sophisticated computerized detection gear they developed. Screen-grab image courtesy of Song of the Whale research team.
