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The various reviews of marine fisheries in the
waters off mainland Southeast Asia identify a wide variety of serious
threats. These can be categorized mainly as excess fishing effort
and habitat destruction. Excess fishing effort, unregulated foreign
fishing activity and associated declines in abundance of target
species is thought to be a serious problem for most marine fisheries
in these waters. Habitat destruction is another threat to coastal-marine
resources. For example, the use of trawling gear is believed to
have negative impacts on the sea floor ecology. Not much information
is available regarding this important topic.
Recent satellite imagery can help to shed some light on the extent
and severity of trawling in the offshore and near-shore zone of
the Mekong delta. On February 6, 2007, the European Space Agency’s
Envisat satellite passed over the southern tip of mainland Southeast
Asia and the MERIS sensor imaged the Mekong Delta and the surrounding
coastal waters of the South China Sea and the Gulf of Thailand.
A digitally enhanced MERIS image of the area around Phu Quoc Island
(upper left) provides clear evidence of numerous linear traces of
mud clouds associated with the tracks of various offshore fishing
trawlers that are active in these relatively shallow and sediment-rich
waters. The red circles highlight some of these areas of seabed
disturbance. The light-coloured traces correspond to plumes of stirred-up
sediment that is disturbed by the heavy steel cables, trawl doors
and other heavy trawl net equipment scraping over the seafloor.
The entire Envisat MERIS image covers a swathe
more than 1000 km wide (lower left) showing many terrestrial and
marine features, including the dry season crop land (brown), the
fertile flood plain of the Mekong river and forested mountains (green),
and the coastal waters (blue).
These mud trails can be seen on a variety of satellite
imagery and Google Earth tour has been put together by Kyle Van
Houtan. If you have Google
Earth installed on your computer and a good Internet connection,
you can see some other mudtrails. For further information, click
here.
Leading scientists have claimed that ‘bottom trawling and
use of other mobile fishing gear have effects on the seabed that
resemble forest clear-cutting, a terrestrial disturbance recognized
as a major threat to biological diversity and economic sustainability’.
Although benthic structures are clearly much smaller than those
in forests, their complexity is equally essential for biodiversity.
Researchers are starting to use satellite images from many offshore
and near-shore areas around the globe to show fleets of trawlers
leaving plumes of mud behind them like contrails. Such imagery can
focus wider attention on trawling damage, and on the possible uses
of satellites to monitor fishing.
Credits:
ESA (MERIS image)
BBC News (Bottom trawling graphic)
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