Dolphin Preliminary Movement Patterns

Preliminary Movement Patterns for Western North Atlantic Dolphinfish
(Published May 2010)

 

Studying the movements of fish requires that a large number of individuals to be tagged and recovered to define normal travel for the species. When the fish is a highly migratory species known to occur over vast areas of an ocean, then animals need to be tagged throughout its known range to get a comprehensive understanding of the routes used and to establish the relationship between widely separated groups. Understanding movement patterns is critical to fishery managers in their efforts to properly manage a stock of fish.

The Dolphin Tagging Study began in 2002 as a tagging effort off South Carolina. As the breadth of the movements by East Coast dolphinfish began to materialize, the program quickly grew to encompass the dolphin's range throughout the Western North Atlantic Ocean. The program has been extremely successful, having more than 11,000 fish marked with the majority released along the U.S. Atlantic coast. Tagging activity has not been uniformly distributed along the East Coast. More than 60 percent of the fish tagged have been off south Florida and its Keys, Stuart to Key West. The second largest proportion of tags, 22 percent, has been deployed off South Carolina. Anglers in Bahamian waters have generated the third highest proportion of dolphin tagged, 8 percent. The majority of the 350-plus tag recoveries have come from fish tagged in these three areas, allowing for the first look at dolphin dispersal patterns from these locations.

  • 76 percent of fish tagged off Florida and later recaptured are recovered before they leave Florida.
  • Tagged fish recovered before leaving Florida traveled northward at an average speed of 8.1 miles per day.
  • Florida tags have been recovered along the U.S. East Coast from Brunswick, Georgia, to Montauk, New York.
  • Fish recovered from North Carolina into the Mid-Atlantic Bight average traveling 21.6 miles per day.
  • Fastest movement by a dolphinfish was a Florida fish that traveled 130 miles from one day to the next.
  • A fish traveled from Islamorada, Florida, to Oregon Inlet, North Carolina, 835 miles, in nine days, averaging 93 miles per day.
  • A fish traveled from Key West, Florida, to Cape May, New Jersey, 1,123 miles, in 23 days averaging 49 miles per day.
  • Numerous fish have traveled their full East Coast range within a two-month period.
  • More than 90 percent of all Florida tag recoveries exhibited a net northerly movement even during winter months.
  • Heavy fishing pressure off Florida's southeast coast removes a large proportion of dolphin present in the western side of the Gulf Stream and appears to push most of the remaining fish to the east side of the Gulf Stream where they remain during their movement to North Carolina.
  • Eighty-eight percent of the fish tagged off South Carolina that were later recovered exhibited a net northward movement.
  • More than 60 percent of the recoveries of South Carolina dolphin come from North Carolina.
  • South Carolina dolphin traveled to North Carolina and the Mid-Atlantic Bight at a much slower pace than Florida fish, moving an average of 7.6 miles per day.
  • The slower speed of travel likely results from fish entering an intermittent gyre off South Carolina where they could swim in circles for weeks or even months.
  • The impact of this gyre can be seen in the recovery of a dolphin tagged off Charleston, South Carolina, that was recovered 70 days later only 19 miles from its release site. The fish was tagged and recovered on the west side of the gyre.
  • Florida fish traveling the eastern side of the Gulf Stream are able to by-pass these gyres allowing them to sustain a faster movement rate.
  • Fish tagged in the Bahamas have been recovered from Ft. Pierce, Florida to Montauk, New York, and even out in the North Atlantic Ocean.
  • Tag recoveries from Florida and Georgia suggest that dolphin coming from the east side of the Bahamas Bank cross the Gulf Stream to its western side as they move northward along the U.S. coast.
  • One fish averaged traveling at 25.8 miles per day moving from Elbow Cay, Bahamas, to a point 350 miles east southeast of Nantucket Island, Massachusetts, in 45 days.
  • Dolphin from the Bahamas and recovered off the U.S. Atlantic coast averaged traveling northward at 15.4 miles per day, suggesting that some fish may travel the east side of the stream.
  • These recoveries have shown that fish in this area of the Western North Atlantic cross to the U.S. Atlantic coast during spring and summer, serving to replenish the dolphin population moving up the coast from the Straits of Florida.
  • The recovery of a fish tagged off Marathon, Florida, in June and recovered the next spring off Long Island, Bahamas, shows that fish will move from U.S. waters to those in the eastern Bahamas.
  • Fish tagged in the Tongue of the Ocean in the central Bahamas Bank have shown a tendency to linger in this area, being recovered there as much as 51 days later.
  • Tag recoveries suggest there may only be one stock of dolphinfish in the western North Atlantic Ocean, as opposed to three currently recognized by fishery managers.



Summary

While dolphin are caught off the southeastern U.S. coast throughout the year, there is a definite seasonal occurrence. Data indicate that the majority of these highly migratory fish are constantly moving northward along the U.S. Atlantic coast regardless of season. Dolphin begin increasing in numbers in the Straits of Florida off the Keys in late March and April, building in numbers into late May or June when they normally peak in abundance. Off South Carolina, dolphin typically reach their peak abundance from May to early June. Peak abundance off North Carolina commonly occurs in June, while the peak abundance in the Mid Atlantic Bight normally occurs from late July to early September.

These fish are entering the Straits of Florida from the west by way of the Loop Current in the Gulf of Mexico and from the east traveling the Old Bahamas Channel. These fish move northward along the east coast of Florida where heavy fishing pressure on the western side of the Stream harvests a large proportion of the fish. This fishing activity appears to push surviving fish to the eastern side of the Gulf Stream where they remain as they travel northward to North Carolina and the Mid-Atlantic Bight.

Dolphin moving northward from the Straits of Florida are joined by fish traveling from the eastern side of the Bahamas Bank. Rounding the northern tip of the Bahamas Bank, eastern Bahamas fish begin entering and even crossing to the western side of the Gulf Stream at a point east of Fort Pierce Inlet, Florida. Fish from the eastern Bahamas serve to replenish the dolphin stock on the western side of the Gulf Stream for their northward journey along the U.S. Atlantic coast. Fish coming out of the Bahamas could be the primary source for dolphin caught off the Carolinas.

Fish tagged off South Carolina on the western side of the Gulf Stream have exhibited a much slower rate of northward travel than their Florida counterparts. The reason may be the presence of two intermittent gyres found on the western wall of the Gulf Stream off the Carolinas. Dolphin could enter these large rotating water masses and swim in circles for days, weeks or even months before exiting to continue their northward trek. Fish moving up the eastern side of the Stream would by-pass these obstacles and not be slowed in their northward movement.

It is off North Carolina that dolphin coming out of Florida are once again caught in large numbers by recreational anglers, likely because the Gulf Stream comes much closer to landfall in this area. It is unknown what portion of the dolphin population departing North Carolina continues northward into the Mid-Atlantic Bight and what segment turns more eastward, moving out into the open North Atlantic.

Dolphin probably use a large number of routes to move south when they depart the U.S. territorial waters. These routes could be as long as a circum-North Atlantic trek or as short as a circle around the Grand Bahamas Bank. The route chosen will likely involve an ocean current that helps them along, even if it is only an intermittent current, and it will likely contain a good abundance of baitfish to sustain them. Dolphin tagged off the U.S. East Coast have traveled to South of the Azores Islands in the Eastern North Atlantic, Puerto Rico, Antigua, St. Kitts, Venezuela, Mexico and Cuba. Such widely dispersed recoveries question the validity of a three-stock theory currently held by fishery managers for dolphinfish in the Western Central North Atlantic Ocean.

Recoveries have also shown that dolphin will return to U.S. territorial waters. Such return visits have occurred within the same year, but most have spanned a winter. Typically, the fish is tagged during the spring or summer of one year and recovered in the next year. This does open the possibility that if fishermen release more small fish, they could be rewarded the next year with more big fish.

By Donald l. Hammond, Director, Dolphinfish Research Program, Charleston, South Carolina
NOTE: Article prepared May 2010. Information presented here is based on a dynamic data base that is continually being updated, which can lead to new information altering our understanding of dolphinfish movements.