Geoscience Reference
In-Depth Information
For the entire North Atlantic, large fishes have declined to a mere 11 percent of their numbers in 1900. The
fate of large sharks in the Northwest Atlantic was even more disturbing, as they have been reduced to a mere 2
percent of their former abundance in the past 50 years.
Declines have occurred with astonishing speed; community biomass was reduced by 80 percent within fif-
teen years after industrialized fishing began in any given sector of the sea. At this rate it is possible for a large
fish to go extinct, or virtually so, before anyone can make a move to protect it or even notice that it is in peril.
A case in point is the barndoor skate.
As the name might imply, the barndoor skate was the largest skate in the Northwest Atlantic and once was
found on the continental shelf from the Grand Banks of Newfoundland to North Carolina. Henry Bigelow de-
scribes it in his encyclopedic The Fishes of the Gulf of Maine as “growing to a length of 5 feet,” and possibly
reaching 6 feet, “though there is no definite record of one that large.” He says that it was “plentiful off the out-
er Nova Scotia shore” and abundant on Georges Bank, where an experimental trawl would routinely dredge up
twenty of them. It has been estimated that there were 600,000 individuals in the 1950s, a number that shrank to
200,000 in the 1960s, and plummeted to a mere 500 individuals in the 1970s. Recent surveys, however, indic-
ate that barndoor skates have survived in small numbers only in the very deep waters off Newfoundland, as
well as on Georges Bank and Browns Bank, probably because of the faster growth rate there (due to warmer
water temperatures) and the seasonal closure of these latter banks to trawling. The authors warn, however, that
if a fish as conspicuous as the barndoor skate can nearly disappear, virtually unnoticed, in an area regularly
surveyed, then the fate of less well known species might well be worse.
BARNDOOR SKATE
The demise of these top predators is cause for alarm in itself, but more so because each loss has top-down
cascading effects on the marine food web. The classic example comes from the Pacific, where overexploitation
of sea otters in Alaska led to population explosions of their sea urchin prey, which in turn caused an overgraz-
ing of kelp forests. Likewise, the recent collapse of groundfish stocks in the Northwest Atlantic due to over-
fishing cascaded through the food web, demonstrating once again that all parts of the marine ecosystem are
connected, even on the high seas. On the Scotian Shelf, plankton-eating fishes such as herring, capelin, and
sand lance—once the prey of cod and other bottom-dwelling fishes—burgeoned in numbers when released
from predation pressure, and the Newfoundland cod collapse led to a similar dramatic increase of shrimp and
crab and possibly lobsters as well. In fact, Worm and Myers showed that on both sides of the North Atlantic,
