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April 12, 2004
Holy
Mackerel!
Today's dive in Portimão Canyon off the
southern coast of Portugal took divers to the continental shelf
and the slope of the canyon wall. The Delta submersible
touched ground on the continental shelf, which is the region
of the continent that is
under the sea, where the water was about 120 meters (393 feet)
deep.
 The
coastal seas above the continental shelf are important because
they provide sources of food and energy, are valuable for
business, shipping, and recreation and are greatly impacted by
human activities such as fishing, boating, pollution, and
development.
The
continental slope was rich with crinoids, or
Feathered sea stars
(Antedon sp.), as well as large
Pink shrimp (Parapenaeus
longirostris). This abundance of life exists because
the area is not heavily fished with trawl nets, which disturb
the bottom, digging up the organisms that are attached to the
sediment.
At about
150 meters (492 feet), a school of mackerel (Scomber sp.) joined the dive by surrounding the submersible.
The lights from the sub attracted the zooplankton, which the fish feed
on, so they got to enjoy an easy lunch. At one point, the
fish had been swimming with the current, but turned around and
swam against it so that the current could sweep their
prey into their mouths.
Going
deeper to 260 meters (853 feet) on a plateau at the head of the
canyon, trawl marks were visible, as well as beer cans and
bottles, plastic bags, and other trash. Marine animals
were nowhere near as plentiful as on the slope, which is not
heavily fished.
A brief
period of excitement about the discovery of a piece of debris
from a potential archeological site quickly faded as the
explorers realized that the debris was a metal cable from a
modern trawl net.
Tomorrow we will be diving in the deep axis (the middle) of the
canyon looking for archeological targets. The objectives of these dives
are
to search for future archeological study sites to help
understand Portuguese heritage.
Cool Fact for
Today:
"Marine snow" is made up of particles of organic matter (like plankton)
that are suspended in the water column and look just like an
underwater snowstorm. Unfortunately the "snow" can reduce
the visibility during a dive.
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