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.
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.