Geoscience Reference
In-Depth Information
ATLANTIC COASTAL
waters, from Long Island southward, are home to bottlenose dolphins, relatively small,
toothed cetaceans which frequently can be seen from shore performing their aquatic acrobatics. The coastal
bottlenose populations are separate from offshore stocks, which are generally larger and prefer deeper waters.
The coastal population consists of three principal groups. A northern migratory stock moves northward in
the summer, as far as Long Island, then retreats to the Cape Hatteras region for the winter. The southern mi-
gratory stock summers in nearshore North Carolina waters, then moves along the south Atlantic coast in
winter. Five coastal resident stocks consist of animals that do not migrate but instead occupy estuarine habitats
along the coast, such as Pamlico Sound, year-round. However, some of these estuarine stocks move onto the
coast during the winter and overlap with the northern migratory animals.
The two coastal stocks seem to be separated on the basis of water temperature. They travel in small groups,
or pods, of approximately ten animals. Total population numbers are not known; estimates vary from twelve
thousand to as few as two thousand. Annual mortality due to encounters with fishing gear is at least 10 percent
in the northern and southern migratory stocks. From 2002 to 2006, 1,570 bottlenose dolphins were stranded
along the Atlantic coast from New York to Florida. The cause was not always apparent, but 32 percent of the
dolphins showed evidence of entanglement in fisheries gear.
In the summer of 1987, an unprecedented number of dead and dying dolphins began washing up on the
coast of New Jersey, and by the spring of 1988, a total of 750 were accounted for along the coast south to Flor-
ida. It is estimated that more than half of the population of Atlantic bottlenose dolphins may have died during
this period. Studies of the die-off, organized by the National Oceanic and Atmospheric Administration
(
NOAA
), indicated that the dolphins had died by eating fish tainted with a naturally occurring toxin caused by
“red-tide algae.”
Red tides occur when environmental factors—including the right water temperatures, nutrient supply, and
sufficient sunlight—combine to allow an explosive growth of dinoflagellates, single-celled plankton that, in
sufficient quantities, literally turn the waters red.
One of the planktonic organisms in red tides is a species called Ptychodiscus brevis, which produces a neur-
otoxin known as brevetoxin. This toxin is believed to have killed the dolphins when the red tide organisms
were swept into the Atlantic from the Gulf of Mexico and ingested by planktivorous fishes such as menhaden.
When the dolphins consumed the fish, they were fatally poisoned, or became sick and succumbed to bacterial
infections. Autopsies of the carcasses also yielded very high concentrations of persistent organic pollutants
such as polychlorinated biphenyls (
PCBS
). The levels ranged from 13 to 620 parts per million (
PPM
), with one
animal registering 6,800. Currently products are required to be labeled as toxic waste if they contain 5 ppm. It
is not known whether these levels of contaminants contributed to the deaths of the dolphins, but animal studies
have indicated that
PCBS
can compromise immune systems.
