Dolphin, the Perfect Gamefish?

In today's world of bio-genetically engineered food products, science strives to produce perfect plants and animals. These altered organisms typically feature higher growth rates, higher reproductive capacity and improved food quality. If science applied these qualities to a fish, they would attempt to recreate the dolphin fish, Coryphaena hippurus.
    The common dolphin, frequently called mahi mahi or dorado, is likely the perfect game fish. Ranging worldwide in warm temperate and tropical oceans, this species is recognized as a premier game and food fish wherever it is found. The dolphin is revered among anglers for its aggressive strikes, long, fast runs, stunning aerial acrobatics and its vibrant, neon colors. Historically harvested almost wholly by
recreational fishermen around the world, only in recent decades did the fish catch the attention of commercial anglers. Today the species is a shared stock between recreational and commercial interests, both vying for their share of this true seafood delicacy.
    Ranging from Massachusetts to the Florida Straights, throughout the Gulf of Mexico and the Caribbean down to Brazil, dolphin are commonly found in the warm oceanic waters of the Western Atlantic where surface temperatures exceed 70F. Pelagic by nature, they live their life in the uppermost portion of the water column. These fish are commonly found off the east coast of the U.S. in schools aggregated under large mats of Sargassum weed that drift in from the Sargasso Sea. School size decreases as the fish grows. Later, large males, commonly called bulls, take on a solitary, nomadic life.  Off South Carolina, dolphin are present virtually year round in the warm waters of the Gulf Stream 60 to 75 miles offshore, but are at peak abundance April through July. As shallower Continental Shelf waters warm in the summer, the fish move closer to shore, occasionally being found within ten miles of the coast.
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  The species matures at an early age, estimated as early as 3 months. Dolphin are known to be sexually mature as small as 14 inches in fork length, with all fish reaching sexually maturity by the time they are 22 inches in fork length. The presence of very young dolphin almost year-round in the Florida Current, coupled with almost continuous spawning observed in some captive fish, may indicate these fish spawn continuously throughout their life. A Florida study calculated that a female 39 inches in fork length produces roughly 555,000 eggs per spawn.
    With such rapid growth, it is only natural that dolphin possess a voracious appetite and are aggressive predators.  They are opportunistic feeders, eating whatever is abundant at the time, even each other. Off North Carolina squid, crabs, small jacks and small triggerfish, which are the most abundant members of the Sargasso weed community, comprise the bulk of their diet.
    However, off Barbados, flying fish and its cousin the flying gurnard are more prevalent in the area and make up the bulk of dolphin's diets, with squid and small triggerfish playing a smaller role. Dolphin held in captivity have consumed over 20 percent of their body weight in food per day.
Dolphin may very likely be the fastest growing wild fish known to man. This species has been reported to grow at the phenomenal rates of 1.3 to 2.7 inches in length per week.   Studies have shown dolphin are capable of reaching a body length of four feet and a weight of 40 pounds in less than 12 months. Males were found to grow at a faster rate than females, weigh as much as 20 percent more per given length and reach a larger size at equal ages. Females seldom exceed 40 pounds while males routinely reach 60 pounds with the current world record being an 88-pound bull caught in 1998 in the Bahamas.  Dolphin is one of the few species of fish where the sexes can be identified by external features. Males have a pronounced blunt head shape while the females are gracefully rounded.
    Dolphin are known to spawn off South Carolina as evidenced by the collection of early juvenile fish and adult fish in spawning
condition in our offshore waters from spring through fall. They are described as batch spawners, reproducing multiple times during the year. Spawning takes place in the warm, open waters of the Gulf Stream where the females release their eggs into the water for the males to fertilize. Young dolphin possess an innate instinct to associate with Sargassum and other objects floating in the ocean for protection from predators.
Dolphin Length - Weight Table
Weights observed for size classes of dolphin when sexes are combined.N=552   
Weight shown in pounds and size classes in inches fork length. 
(Hammond, D. L. 1998. SC Recreational dolphin fishery. SC Dept. Nat. Res. Special Rpt. 14p.)

      22                           8                     21-23              3.4             4.3                  3.9 
      23.7                        8                     23-25              4.0              5.3                 4.6 
      25.9                       10                    25-27              4.5               7                   5.6 
      28.5                        7                     27-29                6               8                   6.8 
      30.3                       11                    29-31              7.3             11.3                8.8 
      32.1                       11                    31-33                8              12.6               10.1 
      34.1                       40                    33-35               5.3            14.5                11.1 
      36.2                      101                   35-37              10.3            20.7               14.2 
      38.1                        97                   37-39              11.5            22.5               16 
      40                          38                    39-41              13.3            23.3               19 
      42.1                       53                    41-43              11.3           27.8                 21.4 
      44.1                       23                    43-45              20              32                   26.5 
      45.9                       33                    45-47              20.3           40                    31.6 
      48.1                       38                    47-49              27.3           50.3                 37.8 
      50                          33                    49-51              35.3           51.6                 43.7 
      52                          18                    51-53              33.6           54                    48.4 
      54.1                         9                    53-55              50.3           63                    57.7 
      55.7                         9                    55-57              58.1           67.9                 63.9 
      57.4                         4                    57-59              60.6           74.4                 67.4 
       0                             0                    59-61                0               0                      0 
       0                             0                    61-63                0               0                      0 
      64                            1                    63-65                73.2
Any animal that lives life in this "fast lane" is bound to have a down side. Dolphin serves as an important forage species for many of the ocean's apex predators such as marlins and sharks. Research indicates that the annual production of juvenile dolphin suffers a total annual mortality level of 98 to 99.7 percent. This would translate into only 0.3 to 2.0 percent of all juvenile fish produced in a given year to survive a full 12 months. Aging studies conducted on fish harvested in North Carolina, South Florida and Barbados in the Southern Caribbean found that fish less than 1 year of age made up 80, 74 and 100 percent respectively of all fish landed by fishermen. These are very high percentages considering that these are very biased fisheries, generally selecting older, larger fish.   Two year-old fish are exceptionally old for the species and have been shown to make up less than 3 percent of the catch. The oldest individual reported was only four years old.  For comparison, this is the age at which red drum just begin to mature sexually.
The species is considered highly migratory and is known to travel as far as 800 miles in ten days. Recoveries of fish tagged off the South Carolina coast shows that when leaving, they move north to North Carolina venturing even as far north as Long Island, NY. Currently, science theorizes that the stock of fish found off the U.S. east coast is shared with the Bahamas, Northern Caribbean Islands and possibly even Bermuda.   The Atlantic stock may even intermix with dolphin from the Gulf of Mexico. This is largely speculation since no scientific data has been collected to confirm this theory. An attempt to use molecular genetics to identify the various stocks of dolphin present in the Western North Atlantic failed to show any differences among fish from the Southern Caribbean, Gulf of Mexico, Bahamas or U.S. east coast.
    The Marine Resources Division of the Department of Natural Resources initiated a three-year study in 2001 to address questions about the geographic range of the U.S. east coast dolphin stock. The research will utilize uniquely numbered streamer tags to mark individual fish in order to track their movements, migrations and distribution patterns. The study is divided into two phases, one where department biologists will tag fish and the other will utilize volunteer anglers from New York to Key West to the Bahamas to tag dolphin along the full length of their east coast range.  
    Through the recapture of these fish, science will begin to map movements of individual fish. Because fish can be individualistic in behavior, it will require the recapture of many fish to establish patterns of distribution or migrations. Tag recovery rates for highly migratory oceanic fish is commonly only 1 to 2 percent. The recapture of tagged dolphin is complicated by their extremely high annual mortality, highly migratory nature and overall short life expectancy. This means that each tag recovery is extremely valuable and very important to the success of the program.

Donald L. Hammond
South Carolina Department of Natural Resources
Marine Resources Division
Dolphin Life History & Length-Weight Table
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