The Secret Lives of Dolphin
The Secret Lives of Dolphin
Author: Albia Dugger
There are two species of dolphin, the common dolphin (Coryphaena
hippurus) found throughout the world in warm offshore waters, and
the pompano dolphin (Coryphaena equiselis), which is smaller with a
proportionally deeper body, smaller pectoral fins and shorter
dorsal fin.
Brilliantly colored and powerful acrobats, common
dolphin are one of the few offshore species in which males and
females appear clearly different. Round-headed females grow no more
than 40 pounds, while bulls, with their high, squared-off
foreheads, reach weights of 90 pounds or more. (Pompano dolphin
rarely reach 10 pounds.)
Water temperature controls the range and migrations of common
dolphin.
Most abundant in surface temperatures of 79 to 82 degrees,
common dolphin generally avoid waters where surface temperatures
drop below 70. Pompano dolphin prefer surface temperatures above 75
degrees. In all oceans, dolphin may be found during any season of
the year in waters that provide their favorite temperatures.
Since the pompano dolphin is more oceanic and commonly caught by
U.
S. sport fishermen only in the waters off Hawaii, we will
concentrate on the common species, simply known as dolphin. In the
western Atlantic, dolphin have been found as far north as George's
Bank, Nova Scotia, and as far south as Rio de Janeiro, Brazil. In
the eastern Pacific, they range from Peru to Oregon.
Dolphin living above the equator may lead separate lives from
those found below the equator. Dolphin tagging has shed little
light on the subject, since most tagged dolphin have been recovered
soon after being tagged. It seems, however, that dolphin in the
northern hemisphere generally migrate north from the equator during
the spring and summer then return south (toward the equator) in the
fall, while the southern population makes the opposite migration in
mirror image.
Surface Navigators
The most surface-loving of all large, oceanic game fish, dolphin
stay between the surface and 100-foot depths.
They're particularly
attracted to objects floating on or near the surface, including
boards, sea turtles, whale sharks, logs and patches of sea weed.
Grant Beardsley of the National Marine Fisheries Service once
observed large schools of young dolphin following sargassum
concentrations from the Florida Keys to Palm Beach, Florida - a
distance of 265 miles.
These floating objects serve many purposes, primarily providing
food such as the invertebrates and small fish found in seaweed
mats. They also shelter young fish from predators and provide
adults with navigational reference points.
Other objects may simply act as schooling companions. Dolphin
have been known to follow sailboats or towed vessels for hundreds
of miles. At certain times of year, they're known to follow rays
and cobia.
A Sizable Advantage
The key to dolphin behavior, says Capt.
Jim Sharpe of the
charter boat Sea Boots, lies in the size of the fish, which in turn
relates to its age and sex. Once dolphin pass the juvenile stage,
Sharpe divides them into three size groups, each with its own
feeding and schooling characteristics. Though local names for the
different size groups vary, their behavior remains the same
worldwide.
Chickens: Young dolphin from 2 to 8 pounds, known as "peanuts"
(when particularly tiny) and "chickens" in the Florida Keys, form
large feeding schools of 100 to 200 fish, both male and female
together.
The large size of the school helps the young fish find
food and protects the school from larger predators, which tend to
pick off individuals from the outside of the pack. When a predator
appears, the alarm quickly spreads through the school, allowing the
majority to escape.
More than other size groups, peanuts and chickens tend to orient
to floating objects ranging from seaweed to debris. Small females
particularly associate with tide lines where sargassum accumulates,
although males of the same size tend to be found in the open ocean
along with larger males and females.
Feeding schools become common in south Florida around May, says
Sharpe, before the young fish begin their summer migration north
along the East Coast. At this time, they're easy to catch with any
bait or gear.
Schoolies: When dolphin reach about 8 pounds, they begin to
mature and form adult spawning schools. Fish in this size group can
grow to more than 20 pounds in one summer.
Studies age these fish
at around 90 to 180 days old. Spawning schools can be found moving
north from the east coast of Florida to the Carolinas in mid- and
late summer.
During the summer, males and females in this group pair off to
spawn. Females in the wild spawn roughly three times per year,
depending on food availability and temperature conditions.
When they aren't spawning, these young-adult fish segregate by
size and sex. "It's not unusual to find schools of female dolphin 8
to 22 pounds during the off-spawning cycle," says Sharpe. "These
fish concentrate around tidal rips and weed lines along the
continental shelf, where they're caught by trolling small baits or
lures."
Wolf Packs: Large, mature dolphin, ranging in size from 20 to
more than 45 pounds, form hunting pods, or "wolf packs," made up of
two to six fish, usually one or two bulls and several cows.
The
bulls in the group are usually larger than the cows.
Big dolphin in small packs prefer unobstructed, open ocean to
weed lines and debris, and often strike baits trolled for billfish
or tuna. "These fish are extremely agile, strong and able to escape
most predators with ease," says Sharpe, "with the exception of blue
marlin."
Sometimes large dolphin roaming in wolf packs gang up to feed on
schools of chicken dolphin around weed lines, finding them easy
prey.
"In late August along the Florida Keys, I've seen schools of
10 to 20 dolphin ranging from 20 to 35 pounds feeding under weed
lines or flotsam," says Sharpe.
Studies of sex ratios in schools of different-size fish support
the idea that dolphin change schooling and feeding patterns. In
non-spawning periods, researchers found that schools of dolphin
tended to be either predominantly males or female, but during
spawning periods, the ratio of the sexes in dolphin schools was
more equal.
Feeding Time
Studies of dolphin stomach contents reveal that they'll feed on
any living organism available.
Though visual feeders, dolphin also
use the vibration-detecting organs of the lateral line to home in
on prey. Male dolphin feed more voraciously than females of the
same length, which may be due to males' greater weight and activity
for their length.
Dolphins' diets change as they grow and are able to capture and
swallow larger prey. Dolphin larvae feed mainly on tiny copepods
and fish larvae.
Dolphin between 14 and 41 inches eat mostly fish
in the 1- to 6-inch range.
Different foods are available in different areas, and studies
have shown that dolphin eat them all. Off North Carolina, flying
fish dominated dolphin diets, while no flying fish were found in
stomachs of dolphin caught farther south in the Gulf Stream.
Sargassum was often found in the stomachs of small female dolphin,
which seek the floating mats as shelter and feed on the small fish
and crustaceans that live there.
Dolphin feed mostly during daylight hours, but also feed
actively during brightly moonlit nights. This accounts for the fact
that although dolphin are surface feeders, deepwater animals that
rise nearer the surface at night are sometimes found in dolphin
stomachs.
All tropical species that live near the surface in open ocean,
including dolphins, have very high metabolic rates and require lots
of food. In feeding experiments with captive dolphin in North
Carolina, the fish ate well and grew rapidly at temperatures from
75 to 85 degrees.
Feeding decreased below 73 degrees and ceased
entirely at 65 degrees. In a similar experiment in Japan,
one-year-old dolphin at optimum temperature grew at a rate of 4
inches per month.
Based on studies of dolphin scales, Beardsley believes that
dolphin grow rapidly but live only four to five years. Japanese
researchers reported that dolphin reach 15 inches, fork length, in
their first year, 27 inches the second year, 35 inches by age
three, 42 inches by age four, and 48 inches in their fifth
year.
Summing up, Beardsley says, "Common dolphin have an extended
spawning season, a very early age at maturity, a voracious
appetite, a cosmopolitan diet and a very short life span." To sport
fishermen, this translates into a fast-growing, hard-fighting,
actively feeding game fish. The fact that it's also one of the most
common, colorful, acrobatic and tasty of fishes is merely a
bonus.
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