Page 459 - Environment: The Science Behind the Stories

Total, all dolphins
Northeastern offshore spotted dolphins
Eastern spinner dolphins

Bycatch reduced by Marine
Dolphin bycatch (in thousands) 400 Increase in bycatch due
Mammal Protection Act
600

to arrival of new fleets

Bycatch reduced by
”dolphin-safe“ ecolabeling
200

0 (a) Before trawling at Georges Bank
1960 1970 1980 1990 2000
Year
(a) Reduction in dolphin bycatch

Dolphin population (in thousands) 3000 Eastern spinner dolphins
4000
Northeastern offshore spotted dolphins

2000

1000

1980
1960
1970
Year 1990 2000 (b) After trawling at Georges Bank
(b) Lack of recovery by dolphin populations
Figure 16.22 Bottom-trawling causes severe structural
Figure 16.21 Bycatch has severely decreased dolphin damage to reefs and benthic habitats, and it can decimate
populations. Bycatch of dolphins by tuna fleets (a) declined underwater communities and ecosystems. A photo of an
first as a result of regulations following the 1972 U.S. Marine untrawled location (a) on the seafloor of Georges Bank shows a
Mammal Protection Act and later, when “dolphin-safe” ecolabeling vibrant and diverse benthic community. A photo of the same site
encouraged fleets to adopt methods that reduced bycatch. after trawling (b) shows a flattened expanse of sea bottom with
However, despite this success, populations of the two dolphin only scarce biological diversity and productivity.
species most affected (b) have not rebounded, possibly because
there are fewer fish for them to eat. Data from (a) National Oceanic and important part of restoring Grand Banks populations; bycatch
Atmospheric Administration, www.noaa.gov, and (b) Wade, P.R., et al., 2007.
Depletion of spotted and spinner dolphins in the eastern tropical Pacific: Modeling of cod while fishing for other species in the Grand Banks rose
hypotheses for their lack of recovery. Mar. Ecol. Prog. Ser. 343: 1–14. from 600 metric tons in 2006 to 1100 metric tons in 2009,
slowing the recovery of cod populations.
Bottom-trawling not only results in bycatch but also can
destroy entire ecosystems. The weighted nets crush organisms
in their path and leave long swaths of damaged sea bottom. Modern fishing fleets deplete marine
Bottom-trawling is especially destructive to structurally com- life rapidly
plex areas, such as reefs, that provide shelter and habitat for
animals. In recent years, underwater photography has begun We can see the effects of large-scale industrialized fishing in
to reveal the extent of structural and ecological disturbance the catch records of groundfish from the Northwest Atlantic.
done by bottom-trawling (Figure 16.22). Bottom-trawling is Although cod had been harvested since the 1500s on the Grand
often likened to clear-cutting (pp. 335–336) and strip mining Banks, catches more than doubled once immense industrial
(p. 657). In heavily fished areas, the bottom may be damaged trawlers from Europe, Japan, and the United States appeared
multiple times. At Georges Bank, it is estimated that the aver- in the 1960s (Figure 16.23a). These record-high catches lasted
age expanse of ocean floor has been trawled three times. Bot- only a decade; the industrialized approach removed so many
tom-trawling here is known to destroy young cod as bycatch, fish that the stock has not recovered. Likewise, on Georges
and this is thought to be a main reason why the Georges Bank Bank, cod catches rose greatly in the 1960s, remained high for
458 cod stock is not recovering. Reducing bycatch is also an 30 years, and then collapsed (Figure 16.23b).

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