ROAM ALONE: Big bull dolphin are solitary fish and nonstop eating machines.
Photo: Bill Roecker
Four years ago, with little fanfare, marine biologist Don Hammond launched the Dolphin Tagging Research Project for the South Carolina Department of Natural Resources. At the time, Hammond, his colleagues and ¿shing buddies believed they knew a lot about these amazingly popular pelagics. They were badly mistaken. Since the launch of the program, more than 700 anglers in 300 boats have tagged nearly 5,000 ¿sh, and Hammond has had a dolphin epiphany.
“Frankly, we’ve had to throw out almost everything we knew about dolphin and start from scratch,” Hammond says. “It seems the more you know about this great gamefish, the more you don’t know. The more answers we get about dolphin, the more questions we have.”
Nevertheless, information from Hammond’s landmark tagging study has been a revelation to both scientists and anglers who have examined the data. Here are nine dolphin facts you may never have heard before. But be prepared-they might just change the way you fish.
TALE OF THE TAPE: Dolphin are among the world’s fastest-growing fish, though mortality is high.
Photo: Bill Roecker
Die Young, Live Large
Dolphin are one of the world’s fastest-growing fish. Juveniles can grow from one to nearly three inches per week, and wild dolphin (with access to an abundant food supply) have been documented to grow to 40 pounds within the first 12 months of life. But they rarely live that long. In fact, most dolphin die in their first year, and none has ever been recorded to live beyond four years of age, including the world-record fish of 87 pounds. Two-year-old (and older) dolphin make up less than four percent of fish caught.
With such a fast growth rate and short life span, dolphin have evolved to spawn young and virtually year-round. They reach sexual maturity at 4 1/2 months old, when they are between 14 and 22 inches in length.
“It seems like the fastest-growing marine fish are also the highly migratory ones,” Hammond says. “This would include wahoo, tuna and marlin, but also dolphin. Dolphin grow fast, and die young, with an extreme mortality rate. I believe this is partly because dolphin are similar to menhaden. Everything eats dolphin. Their mortality rate is 99.7 percent within their first 12 months of life.”
It’s been documented that dolphin are a food source for heavyweight ocean fish, like sharks, marlin, tuna and even false killer whales.
They Are What They Eat
If ever you’ve experienced a schoolie feeding frenzy, it will come as no surprise that dolphin are eating machines, chowing down almost continuously and capable of consuming the equivalent of up to 20 percent of their body weight per day.
“We’ve found almost everything imaginable in dolphin stomachs,” says Hammond. “We’ve discovered plastic debris like fishing floats and orange-juice bottle caps in addition to squid, filefish, sea horses, sardines, crabs, small jacks, even baby dolphin, sailfish and blue marlin.”
While dolphin are notorious for targeting flying fish, Hammond says their most prevalent food source along the U.S. coast is small bullet mackerel, which look like frigate or Atlantic mackerel and are common throughout the Gulf Stream.
“Bullet mackerel are silver-blue,” Hammond says. “That might be the reason blue-and-white trolling lures are so popular and effective.”
Dolphin In Depth
| |MAC ATTACK: Bullet mackerel are a dolphin’s favorite meal. Photo: Boyceimage.com| As revealed for the first time in the May 2006 SWS (“The Dolphin Doctors,” page 85), dolphin spend more time in deep water than previously believed. Hammond’s team learned of this tendency after fitting a 25-pound bull with a satellite telemetry device. The fish was tracked for ten days, covering only 75 miles, before the device floated to the surface without the dolphin. Hammond believes the fish was likely killed by another ocean predator like a shark while swimming at 100 feet down.
“Dolphin definitely spend more time at the surface during daylight hours than at night,” Hammond explains. “They linger at the surface for a long duration in the day, up to 15 continuous hours, according to our findings. Our tagged dolphin would make up to 25 trips per day to the surface, and those trips averaged 22 minutes each. But at night, dolphin spend more time deep. The average nighttime depth was 70 feet but they will dive down to 243 feet.”
PREDATOR AND PREY: False killer whales hunt down a dolphin.
Photo: David B. Fleetham/Seapics.comr
Feel The Heat
| |To date, nearly 5,000 stufy dolphin have been tagged. Photo: Don Hammond| Dolphin are a warm-water pelagic species, and tagged fish prove this fact. They prefer water temperatures from 74 to 77 degrees, but tolerate temperatures in the range of 65 to 90 degrees.
These temperature parameters should be carefully considered by dolphin anglers. Fish will venture into cooler or warmer water, but only to feed or avoid predators. Thus, when baitfish or ideal dolphin habitat (flotsam, oil rigs, weeds, etc.) is found in choice water temperature, anglers should spend extra time working the areas in search of fish. However, if bait is discovered in only marginally ideal water temperature, don’t give up because it’s still possible that dolphin may be in the vicinity.
Dolphin schools are commonly associated with sargassum weed along the east coast of the United States. Large, surface-drifting weed beds are pushed by summer trade winds toward the U.S. mainland from the Sargasso Sea, located in the mid-Atlantic Ocean.
But this isn’t just a U.S. phenomenon. Hammond says dolphin and weeds (as well as any other flotsam) are found together regardless of the geographic location.
“Weeds hold food, and dolphin have to eat continuously, so they almost always travel with weeds in ocean currents for survival,” he explains. “Weeds are important to catching dolphin wherever they’re found-and they’re found in almost all warm ocean waters, including the Pacific and Indian Oceans.”
| |WEED EATERS: Dolphin prowl floating vegetation for food. Photo: Randy Morse| Hammond’s tag returns have shown dolphin are long-distance runners, and that they are fast. For example, one fish was caught and tagged, then was recaptured just 24 hours later. The fish had journeyed 130 miles. During another nine-day dash, fish were documented to have travelled at a sustained rate of 92 miles per day. Yet another fish migrated from Islamorada, Florida, to Oregon Inlet, North Carolina, in just over a week.
“These are not big fish,” Hammond says. “For those kinds of long-range runs, you’d think dolphin in the 20- to 30-pound range would be the norm. But most fish we tag are in the 15- to 22-inch range, weighing just two to four pounds. And yet they are like racehorses galloping the open ocean.”
The long-distance speed of dolphin is largely dependent on currents the fish follow during their lives. For example, the north-flowing Gulf Stream current is strong and fast in South Florida, while off north Florida, Georgia and the Carolinas, the current slows. Thus, dolphin tagged in the Florida Keys move fast, up to 15 miles per day, and are out of Sunshine State waters in less than a week-moving south to north.
But average movement of Carolina-tagged fish is just seven miles per day.
“Off South Carolina there is something called the Charleston Bump, which deflects the Gulf Stream, and sets up a gyre of current circulation,” Hammond says. “Dolphin get caught in this gyre and can stay for weeks.”
Scientists have long believed that there are two distinct dolphin stocks swimming in southern U.S. waters. One was found off the U.S. coast, in the Gulf of Mexico and near the northern Caribbean Islands. A second stock was believed to be composed of Caribbean fish, long thought to be larger size than dolphin along the U.S. Coast.
“But two dolphin tagged off Mexico’s Yucatan Peninsula show there is an interaction between these two stocks of fish, and that fact should be carefully considered in the management of the species,” Hammond says. “One of those tagged Yucatan dolphin was caught 38 days later off Pensacola, Florida, in the Gulf of Mexico. The second Yucatan fish was caught 52 days later off Fort Lauderdale, Florida.
“We also have a high incidence of large dolphin off South Carolina. This is a much higher occurrence of big dolphin than found in many other areas, like South Florida. This is why anglers have long believed that big South Carolina dolphin were coming from somewhere other than South Florida-the Bahamas, for example.”
The Circle of Life
| |TAGGED UP: Don Hammond prepares to release a volunteer. Photo: Don Hammond| Knowing that dolphin migrate with ocean currents, it’s easy to believe fish near the southeast coast simply travel in a south-to-north migration pattern. But that may be only part of the overall dolphin migratory picture. It’s possible that dolphin circumnavigate the Atlantic Ocean.
“For example, we tagged one dolphin in South Carolina in June,” Hammond says. “That February, 500 miles southwest of the Azores Islands in the eastern Atlantic-roughly 2,500 miles from where it was tagged-it was recovered by a Spanish longline fisherman. The fish weighed five pounds when tagged and about 40 pounds when recovered.
“This kind of travel by a dolphin seems impossible, or something totally out of the ordinary. But who is to say? We’re still learning.”
Telemetry provides valuable insights.
The Dolphin Tagging Research Project (www.dolphintagging.com) has helped document both the speed and range of dolphin. Although most dolphin tagged along the East Coast of the U.S. are recovered along the Atlantic seaboard and show a decided south-to-north movement, three amazing exceptions can be seen in the accompanying recovery map (right). Such long-distance recoveries open debate about pelagic migratory patterns. For example, the dolphin tagged on June 15, 2004, off South Carolina and recovered in the Azores may show a migration pattern that circumnavigates the Atlantic Ocean, much like bluefin tuna. Not even marine biologists dreamed that a dolphin would travel nearly 2,500 miles in eight months.