- For years, apparent increases in illness among marine
creatures, from whales to coral, have left marine scientists with the
uneasy
suspicion that the seas are increasingly plagued by disease. Now, US
researchers
have uncovered the first good evidence that they are right.
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- In 1998, a dozen of the world's top experts on diseases
of marine animals warned that sea creatures seemed to be getting sick more
often, with more diseases.
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- New viruses had appeared in whales and seals, while
corals
were dying of fungal and algal infections. Pilchards succumbed to viruses
and an aggressive parasite expanded its range to attack commercial oysters,
scallops and clams. In the Caribbean, some unknown bacteria wiped out what
had been the dominant sea urchin.
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- But there was no way to tell if the apparent increase
was simply due to more scientists paying more attention to marine disease.
There was no baseline, as no one had ever measured disease incidence in
any of these species decades ago.
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- Now, Jessica Ward, at Cornell University in Ithaca, New
York, has shed important new light on the problem by looking at how the
number of reports of marine diseases in nine different groups of marine
creatures has changed in the scientific literature since 1970.
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- "We wanted to find out if something was actually
happening," Ward told New Scientist. "For most groups of
organisms,
we found that yes, there is something going on out there. Now we hope more
people will try and figure out where it is coming from."
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- True incidence
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- Ward, with Kevin Lafferty, of the University of
California
in Santa Barbara, first tested whether changing numbers of scientific
reports
of rabies in US raccoons matched the true incidence of the disease, which
is known independently. They matched, suggesting more scientific reports
really do mean more disease.
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- The pair further tested the relationship by removing
the most prolific laboratory from the publications they collected for each
group of marine creatures - just in case increased reporting reflected
only one scientist's funding success. This did not change any apparent
disease trends. Neither did taking out multiple papers on one well-reported
disease event, such as the Caribbean urchin die-off.
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- So using scientific reports as a measure, Ward and
Lafferty
found that disease has increased in turtles, corals, marine mammals,
urchins,
and molluscs such as oysters.
-
- Illness seems to have remained steady in the shark and
shrimp families, and in seagrasses. Surprisingly, disease reports have
diminished for fish.
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- Easy prey
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- There are numerous possible reasons for rising disease.
One, Ward suggests, is increasing sea surface temperatures due to global
warming. This can cause corals to bleach, making them easier prey for
infections.
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- Warming has also led to the northward spread of the
oyster
parasite Perkinsus. And warming is thought to accelerate the growth of
tumours in turtles caused by a herpes virus.
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- Another possible factor is that human over-fishing has
destabilised marine ecosystems. For example, when the urchins in the
Caribbean
died, corals were overwhelmed by the algae the urchins used to eat.
"Normally
fish would have eaten the algae instead, but they weren't there,"
says Ward.
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- Other suggested causes include:
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- - new pathogens from domestic animals, such as dog
distemper
virus and the parasite Toxoplasma
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- - bioaccumulation of toxins weakening marine mammals'
immunity
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- - new species carried across oceans in ships' ballast
tanks introducing new diseases
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- In the face of all this, the apparent health of fish
is intriguing. Ward says this could be because the fish are simply fewer
in number. Many pathogens die out among animals that are not packed densely
enough to pass the infection on. But it is also possible, she says, that
the frequency of disease is just as bad or worse - but fewer fish mean
fewer observations, and fewer reports.
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- Journal reference: PLoS Biology (vol 2, p 542)
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http://www.newscientist.com/news/news.jsp?id=ns99994897
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