In Dead Water: Merging of climate change with - UNEP

Among the most destructive fishing methods in the World
is bottom trawling (Thrush and Dayton, 2002; Pusceddu et
al., 2005; Tillin et al., 2006; de Juan et al., 2007, Hixon et al.,
2007). Large nets, kept open and weighted down by heavy
‘doors’ and metal rollers, are dragged by a trawler across the
sea bed. This virtually plows and levels the seafloor, picking
up fish and shrimps but also catching, crushing and destroying
other marine life.
The North Sea and Grand Banks have been major sites of
bottom trawling, with some traditional and easily accessible
areas being trawled multiple times per year. Indeed, landings
data collated for round- and flatfish caught in the northern,
central and southern North Sea from 1906 to 2000 as proxies
for total otter and beam trawl effort, respectively, indicate
that the southern and much of the central North Sea were
fished intensively throughout the 20th century, whilst the
northern North Sea was less exploited, especially in earlier
decades. The fisheries efforts intensified markedly from the
1960s onwards. Biogeographical changes from the beginning
to the end of the century occurred in 27 of 48 taxa. In 14 taxa,
spatial presence was reduced by 50% or more, most notably
in the southern and central North Sea; often these were longlived,
slow-growing species with vulnerable shells or tests. By
contrast, 12 taxa doubled their spatial presence throughout
the North Sea. Most biogeographical changes had happened
by the 1980s. Given that other important environmental
changes, including eutrophication and climate change, have
gained importance mainly from the 1980s onwards, the study
concluded that the changes in epibenthos observed since the
beginning of the 20th century have resulted primarily from
intensified fisheries (Callaway et al., 2007). Whereas trawling
in shallow coastal waters is often carried out by smaller vessels,
deep-water and high sea bottom trawling requires large
and powerful ships. Such fleets are mostly based in industrialised
countries, but fish intensively and for months at a time
across the World’s oceans. Often these distant water fishing
fleets are fuelled and kept afloat (literally) by subsidies and
incentives, without which their operation would hardly be
economically viable.