Following Hypothetical Fish
This study has clearly shown that dolphin travel at a wide range of speeds in their movements northward along the U.S. Atlantic coast. Tag recoveries have shown that dolphin tagged off Florida travel northward at rates ranging from 1.9 to 130 miles per day. Since we only have the number of days at liberty and the total miles traveled, we apply a constant daily velocity for that entire movement when we calculate its speed.
Dolphin regularly exhibit impressive rates of travel in their movements up the East Coast. Of the fish tagged off Florida and later recaptured in the same year off the U.S. mainland north of their release site, 53 exhibited average rates of travel of 40 or more miles per day over liberty periods of one to 23 days. Out of these 53 fish, 22 traveled at rates of 60 or more miles per day for periods of up to ten days. These speeds have allowed a dolphin to travel from Islamorada, Florida, to Oregon Inlet, North Carolina, in nine days while another fish went from Key West, Florida, to Cape May, New Jersey, in 23 days.
Just as a segment of the tagged fish zipped along at high speeds up the coast, there were those fish that seemed to creep along at rates of less than 10 miles per day. Of the 210 recaptures used in this analysis, 53 traveled at this slower rate for periods up to 58 days, showing the diverse nature of their travel. This diversity in travel speed is likely a result of the behavior and features of the Florida and Gulf Stream currents that each fish encountered during its travel.
When examining any animal behavior, extremes are to be expected. One way to deal with such wide-ranging values is to use a weighted average (mean) to determine a “representative” travel rate. In this method you examine their rates by weighting the analysis by the number of days that each given speed was observed. This offers the best compromise in generating a representative speed.
Using this method we find that Florida fish traveled at an average rate of 19.11 miles per day. This translates to 0.8 mile per hour, which is slower than the average flow speed of the Florida and Gulf Stream currents.
Expanding this rate of travel the fish would travel northward at the rate of 134 miles per week. Using weekly travel rate, the progress of the fish up the coast can be tracked (see following figure). If the fish is tagged May 1 off Key West, Florida, in one month (June 1) would be off Savannah and in a mere 76 days from its release it will have traversed its normal U.S. Atlantic coast range, roughly 1,400 miles, and will be headed east for international waters.
Using a weighted average for the speed that dolphin exhibited during their travel northward along the East Coast suggests they cover 134 miles per week.
One quirk of the dolphin migration along the U.S. Atlantic coast that showed up early was that dolphin tagged on the west side of the Gulf Stream off South Carolina travel northward at a much slower pace than fish tagged off Florida. Fish tagged off the Palmetto state moved northward at rates ranging from 0.3 to 79.2 miles per day for periods of two to 76 days. Only four of the 59 tag recoveries used in the analysis exhibited speeds of 40 miles per day or more while 32 recoveries indicated speeds of less than ten miles per day being covered.
Examining the travel rates for the fish released off South Carolina using the weighted average method shows the fish traveling at an average speed of 7.38 miles per day. This is less than half the speed of the fish from Florida and amounts to an hourly speed of just over 0.3 mile.
Applying the weighted average speed indicated for the South Carolina dolphin to a hypothetical northerly track, we see far slower progress up the East Coast. If the fish is released off Charleston on May 1, by June 1 it has reached Beaufort Inlet, North Carolina, having moved only 229 miles (see following figure). By July 1, it is off Chincoteague Island, Virginia, and by August 1 it reaches Sandy Hook, New Jersey. This fish finally makes it to southeast of Nantucket Island, Massachusetts, by mid-August and heads for the international waters to the east. It has taken this dolphin 107 days to move 800 miles to reach roughly the same point that the Florida fish reached in 76 days.
Fish tagged on the western side of the Gulf Stream off South Carolina travel at less than half the speed, 7.38 miles per day, of dolphin tagged off Florida.
The radical change in travel speed is likely the result of the Carolina fish getting caught in one or more of the two semi-permanent gyres that occur on the western side of the Gulf Stream off the Carolinas as well as other rotating water masses on the western side that move northward at a slower rate than the main current. The Florida fish most likely are riding the eastern side of the Gulf Stream, bypassing the large rotating water masses that would slow their northward progress.
These two hypothetical movements are very useful to understanding the migration of dolphin northward along the U.S. Atlantic coast. They tell us that fish in the southern-most waters travel the full length of the coast to their northern range limit, and they tell us how long it takes individuals to make the trip. However, external streamer tags cannot tell us the route that each fish took. These route lines are based on the recoveries of numerous fish that exhibited short-distance movements before recapture. Thus the Key West to Montauk-type recovery proves they make the long trip, but it is Charleston to Wilmington-type recaptures that fill in the route.
This brings us to a new research study the DRP needs to undertake. That is to document routes used by individual dolphin. While I feel confident in the routes drawn for movements along the U.S. coastline, it is the routes used when they move out into the Atlantic to return to warm southerly water of the Caribbean and western north Atlantic that we can only speculate. The current route being used takes the fish out to the Mid-Atlantic Ridge riding the Gulf Stream, then south down the ridge on the Azores current to the North Equatorial current that it rides westward to the Caribbean Islands. This route has been selected because an East Coast fish has been recovered near the Azores Islands and recent satellite tag tracks for white marlin, a species that shares many highly migratory characteristics with dolphin, have been shown to use the Mid-Atlantic Ridge.
Yes, it is a long route, roughly 6,000 miles but it fits the time frame. This distance meshes with the travel rate observed for the Florida fish, 19 miles per day. To complete this amazing trek in one year, a dolphin would have to average 16.5 miles per day, which falls within the observed travel rate.
The new research effort will focus on documenting the travel routes used by individual fish to generate a composite pattern of their movements in the western north Atlantic. This study will use a different style satellite tag that is far less expensive, about one-third the cost, that is more streamlined in design, and that provides data on daily positioning and temperature. Through the use of these instruments the study hopes to identify the route(s) that dolphin use to return south for the winter when they leave U.S. waters.
Earn Some Great Year–end Prizes
Anglers are reminded that in addition to earning program T-shirts for tagging fish, they can win some valuable prizes at the end of the year. The eligible categories in the year-end awards program have been increased from four to six divisions, featuring three new categories. By expanding the awards and making them regional, the DRP hopes to inspire fishermen in these areas of special interest to tag more dolphin.
Since the inception of the Dolphin Tagging Study in 2002, year-end prizes have been offered as a way to reward the top participants and to spark a competitive spirit among the fishermen. These awards have served their purpose well, inspiring anglers to work a little harder to tag more fish than they normally would.
A top-quality Star standup trolling rod and TLD30 2-speed reel has been the featured award for the top tagger in each category. This outfit is valued at more than $500. The TLD reels have been generously provided by Haddrell’s Point Tackle and Supply of Mt. Pleasant and Charleston, South Carolina. Star Rods, a division of Sea Striker Tackle of Morehead City, North Carolina, has graciously provided the rods.
Thanks to the generosity of Costa Del Mar of Daytona Beach, Florida, a second-place award was offered for the first time in 2011. As manufacturer of the official sunglasses for the Dolphinfish Research Program, Costa Del Mar put together a gift packet consisting of the recipient’s choice of a pair of Costa’s sunglasses, one of their fabulous backpacks, a Costa T-shirt, a Costa hat and a sunglasses’ lanyard. This packet is worth more than $400.
The new divisions for 2012 will consist of top private boat from all areas, top charter boat from all areas, plus the boat tagging the most fish in each of four regions: the Mid-Atlantic Bight, South Atlantic Bight, the tropics (Bahamas, Caribbean and western north Atlantic), and the Gulf of Mexico. This is the first year that boats fishing three different regions are eligible to win a rod and reel or Costa gift package without competing against the south Florida anglers.
Regions will be as follows: Mid-Atlantic Bight (MAB), U.S. Atlantic territorial waters north of 36.5o N latitude; South Atlantic Bight (SAB), U.S. Atlantic territorial waters from 24.0o N latitude to 36.5o N latitude; Gulf of Mexico (GOM), all waters in the GOM; Tropics, all waters of the Bahamas and Caribbean Sea including contiguous waters of the north Atlantic adjacent to the islands.
Winners will be determined by the total number of dolphin a boat crew tags in a region. Boats must tag a minimum of 20 dolphin that year in the area they win in to qualify for that area’s year-end award The region a fish is tagged in will be determined by the latitude and longitude given as the release site. A boat can only win one year-end award each year.
A diver had a chance-encounter with a large bull dolphin off the coast of Brazil and was quick enough to get this rare picture of a free-swimming trophy dolphin. (Photo by Sergio)
For More Information, Contact
Don Hammond
Cooperative Science Services, LLC
961 Anchor Rd., Charleston, SC 29412-4902
Telephone – FAX (843) 795-7524
Email CSSLLC@bellsouth.net
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