State of World Fisheries and Aquaculture 2004 - Library

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The State of World Fisheries and Aquaculture 2004
Box 8
The deep sea and its environment
The environment inhabited by deep-sea fishes is large (comprising more
than 50 percent of the earth’s surface) and its ocean dynamics, fisheries
biology and ecosystems are poorly understood. However, over the last
two decades, studies of these regions have begun to describe their often
astonishing physical and biological nature. While many of these areas
are flat with silt and mud-covered bottoms, others are characterized
by chains of seamounts and bottom ridges and knolls. Across the
continental shelves, turbidity currents have excised submarine canyons,
whose importance as links to the deep seas is only beginning to be
understood. Other features, such as seafloor seeps and hydrothermal
vents, have produced bizarre and complex chemosynthetic communities
with highly evolved and unusual faunas.
Diverse and long-lived benthic fauna, most notably the deep-water
corals, are associated with seamounts and similar seafloor features.
Cold-water corals have longevities potentially in excess of 10 000
years. Their structure, proud of the bottom, and brittleness make them
vulnerable to destruction by trawls when inexpert skippers allow their
trawls to encounter the surface of seamounts. Also of concern is the
apparent high level of endemism of the species in those seamounts that
have been researched; thus recruitment of many species from other
seamounts may be less than would be expected.
Seamounts have oceanographic features that are important
for fisheries. First, commercially important species form spawning
aggregations in association with seamounts resulting in profitable catch
rates, while those distant from seamounts may produce far lower rates.
Second, currents flowing over seamounts bring nutrient-rich waters into
the photic zone, enhancing biological production. Then, when Taylor’s
Columns (named after the scientist who discovered these phenomena)
form over the top of the seamount, zones are created that retain fish
larvae in the region of the adult fish habitat. Further enhancement
occurs when plankton migrate into the surface layers at night and are
unable to descend when advected over seamounts, thus providing
biomass that can be “captured” by the seamount-based ecosystems.
Successful resource assessment and harvesting strategies for these resources will
need:
• Accurate catch data – complemented by log books and observer programmes,
particularly for areas where no management protocols have been agreed, or
satisfactory means have not yet been established that ensure that such information
will be made available for resource management purposes.
• Time series of abundance indices and physical parameters. Some important deepwater
fisheries developed and expired before any protocols for capturing fisheriesrelated
data came into effect. Additionally, vessels that were under no obligation to
record information needed for management may not have done so.
• Stock identity and distribution information. Deep-water fisheries, especially those
of the high seas, tend to be mobile; vessels may remain at sea for several months