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The impact of trawling on marine ecosystems may be larger than
we realize. Certainly, the footprint of global trawling is bigger
than many people suspect, according to a recent analysis of the
world's trawling grounds.
Trawling is the dragging of weighted nets across the sea floor to
catch shrimp and bottom-dwelling fish such as flounder. Trawling
grounds are areas of the ocean where commercial trawling—legal
or illegal—is prevalent.
As part of a global study of ecosystem conditions, researchers attempted
to map the seafloor areas that trawlers sweep—a task made difficult
by the fact that no global database for ocean trawling exists. Only
24 countries—representing some 40 percent of the world's continental
shelf area—provided sufficient data to map trawling grounds
in their waters. (See Figure 1.) The map analysis concluded that
trawling grounds in these countries encompass 8.8 million km² (Burke
et al. 2001:22-23). Extrapolating from these figures suggests that
the world's trawling grounds could total approximately 20 million
km², nearly two and one-half times the size of Brazil (UNDP, UNEP,
WB, and WRI 2000:80).
Trawling is a major source of pressure on the biodiversity
of coastal ecosystems. Modern techniques are capable not only of
rapidly depleting targeted fish stocks, but also of damaging or
destroying nontarget species, including corals and sponges, that
may take years to recolonize. The thick natural carpet of bottom-dwelling
plants and animals is important for the survival of the fry of groundfish
such as cod, which find protection there. Thus, destruction of seafloor
habitats is one of the principal factors in the decline of fishing
stocks in heavily trawled areas (Engel and Kvitek 1998:1204-1205).
In fact, some marine ecologists have compared the effects of trawling
on the biology of the seafloor to the effects of clearcutting on
forest ecosystems (Watling and Norse 1998:1180-1194).
Increasing use of new deep-water trawling equipment means that damage
is no longer restricted to shallow shelf areas. More and more, fishers
are trawling beyond the continental shelf, expanding into areas
that previously were not regularly swept by trawlers. Harvesters
are trawling at depths up to 400 m and, in some places, more than
1,500 m (Burke et al. 2001:23). Because deep-living species tend
to grow more slowly than shallow-water species, the long-term impact
of trawling is magnified as trawl depths increase (Watling and Norse
1998:1190). |
