Trawling of ocean floor takes heavy toll on marine habitats Relevancy:
100; ( USA Today ) Tim Friend; 12-15-1998 Size: 8K
Picture a large blimp dragging a heavy net over the ground,
knocking down all of the trees in a forest to catch birds for people to eat.
This is a strange scenario, and it likely would never be
allowed. But scientists say it is similar to what occurs when commercial fishing
vessels engaged in the widespread practice of bottom trawling drag their nets
across the ocean floor.
Bottom trawling is done by thousands of fishing vessels
worldwide to catch marine animals that live near, on or under the sea floor,
such as cod, flounder, grouper, shrimp, scallops and mussels. Nets are dragged
at all depths, from 10 feet to 6,000, near coastlines, in bays and along
continental shelves and their slopes. More than 80% of fish are caught along the
continental shelves, and a little more than half of the catch is bottom species.
Bottom trawling has been done for more than 100 years and has
increased greatly since the 1970s.
A series of studies today in the December issue of
Conservation Biology documents for the first time the types and extent of sea
floor damage caused by bottom trawling.
According to the teams of scientists that conducted the
studies, the destruction is similar to that caused by clear-cutting of old
forests but on a far greater scale. They calculate that the area of seabed
trawled each year is 150 times greater than the area of forest cut and is
roughly the size of the 48 contiguous states.
''When I make a dive to the bottom of the Gulf of Maine in a
research submersible, I can tell immediately whether an area has been recently
trawled for fish or dredged for scallops,'' says Les Watling, professor of
oceanography at the University of Maine. ''After trawling, the sponges and
mussels, the tube-dwelling worms and the amphipod crustaceans that live on the
sea floor are almost all gone. Boulders formerly covered with marine animals are
almost lifeless from being rolled around by nets or dredges. Nothing humans do
to the sea has more impact.' '
The studies by nine academic groups were released Monday at a
news conference in Washington, D.C., held by the Marine Conservation Biology
Institute (MCBI) and the American Oceans Campaign, which
was founded by actor Ted Danson.
They suggest that 50% of the world's continental shelf area is
disturbed by bottom trawling. As the nets are dragged across the ocean floor,
mud is suspended in the water and dispersed elsewhere by currents, ''altering
the structure of the sediment while destroying the burrows and tubes of the
animals that live there,'' Watling says.
David Nitchman, spokesman for the National Fisheries
Institute, which represents about 1,000 companies involved in the fish and
seafood industry, says bottom trawling is necessary to meet the public's demand
for seafood.
The nation's appetite for seafood has increased dramatically
the past 20 years as people have moved away from beef to fish at the
recommendation of public health officials and doctors.
Compounding the problem are dwindling fish populations near
shore, forcing fishing vessels to scour the bottom in ever deeper waters. New
technologies also have made it possible for commercial fishing operations to
trawl over areas once considered inaccessible because structures on the bottom,
such as coral and boulders, snagged and damaged nets. Now, most nets are
equipped with heavy rollers that allow the nets to move over the coral and rocks
without snagging, says Elliot Norse, president of the Marine Conservation
Biology Institute.
These jagged bottoms are a rich habitat for thousands of
varieties of marine plants and animals. The research documents that bottom
trawling scours the seabed clean of the structural complexity of rocky reefs,
boulders, cobbles and gravels. Structural complexity of the seabed is key to the
survival of many species.
The apparently smooth floor of the deeper ocean also is much
more complex than most people realize and is teeming with life. Many mud-
dwelling organisms build small calcium structures protruding out of the mud to
capture oxygen in the sea water. Bottom trawling destroys these structures and
kills their builders. Crabs and many other organisms create burrows and tubes to
hide from predators and capture food.
''Scientists find that bottom trawling is the largest
disturbance to the world's sea floor and possibly the largest human-caused
disturbance to the biosphere,'' Norse says. ''Until these studies were released,
we had no idea that this was happening to this extent.''
Fishing vessels used for bottom trawling vary in length from
30 to 400 feet. The most common type of net used on the bottom is the otter
trawl. The largest of the otter trawls is several hundred feet across. The nets
are held open by heavy steel plates called otter boards. The boards can weigh up
to 5 tons each and create significant damage as they are dragged along the ocean
floor sediments, says Peter Auster of the National Undersea Research Center at
the University of Connecticut at Avery Point, Groton.
The bottom leading edges of the nets are fitted with metal
bobbins that weigh from tens to hundreds of pounds each. Many nets also have
wheels or rollers to help nets over boulders and coral.
Some nets also have tickler chains, which are designed to stir
up the bottom to force fish or shrimp out of the sediment and into the water
column so they can be caught.
While they are highly effective at catching a wide variety of
marine animals, they also damage the sea floor, destroy marine habitats and
alter the normal exchange of chemicals and gases between the seabed and the
water, Norse says.
A study by Norse and Watling concludes that the otter trawl
crushes, buries and exposes bottom-living marine life to predators.
Many of the structures on the bottom are slow-growing and
long-lived. Some sponges live 50 years. North Pacific geoduck clams and Atlantic
quahog clams live to 150 and 220 years, respectively, Watling says. Some
ocean-bottom species can live 500 to 1,500 years.
When these communities or colonies are disrupted, they can
take decades or hundreds of years to recover, Norse says. Because the
deep-dwelling species are adapted to very stable environments, they might never
recover.
Jonna Engel and Rikk Kvitek of the Moss Landing Marine
Laboratories and California State University-Monterey Bay compared physical and
biological differences between areas where trawling was light and heavy.
Using photography, video and bottom samples of sea floor in
Monterey Bay, they determined that bottom trawling reduced the structural
complexity of the sea floor, reduced biodiversity and increased the number of
predators of the species that lost their habitats.
''These differences are consistent with the action of rubber
bobbins, steel weights, tickler chains and heavy otter boards, all of which dig
into the substrate to varying degrees, scraping the sediment surface and
potentially crushing'' organisms that live in the sediments, Engel and Kvitek
say.
With his colleagues, Peter Schwinghamer of the Department of
Fisheries and Oceans at the Northwest Atlantic Fisheries Center in St. John' s,
Newfoundland, studied the effects on a sandy bottom environment. They found that
tracks made by otter doors were faintly visible after one year. The amount of
organic material in the sediment was reduced, and the surface was smoothed over
and absent of structures. Unlike the deeper muddy sediment, the sandy areas were
able to recover after about one year.
''These new studies are the most significant scientific
findings about human disturbance of ecological systems since scientists revealed
the widespread destruction of tropical forests some two decades ago, '' says
Gary Meffe, editor of Conservation Biology.
Copyright 1998, USA Today, a division of Gannett Co., Inc.
Tim Friend, Trawling of ocean floor takes heavy toll on
marine habitats. , USA Today, 12-15-1998, pp 06D.
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