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October 26, 2001

The Secret Lives of Dolphin

Understanding life history and habits makes catching dolphin easier.

Feeding Time
Studies of dolphin stomach contents reveal that they'll feed on any living organism available. Though visual feeders, dolphin also use the vibration-detecting organs of the lateral line to home in on prey. Male dolphin feed more voraciously than females of the same length, which may be due to males' greater weight and activity for their length.

Dolphins' diets change as they grow and are able to capture and swallow larger prey. Dolphin larvae feed mainly on tiny copepods and fish larvae. Dolphin between 14 and 41 inches eat mostly fish in the 1- to 6-inch range.

Different foods are available in different areas, and studies have shown that dolphin eat them all. Off North Carolina, flying fish dominated dolphin diets, while no flying fish were found in stomachs of dolphin caught farther south in the Gulf Stream. Sargassum was often found in the stomachs of small female dolphin, which seek the floating mats as shelter and feed on the small fish and crustaceans that live there.

Dolphin feed mostly during daylight hours, but also feed actively during brightly moonlit nights. This accounts for the fact that although dolphin are surface feeders, deepwater animals that rise nearer the surface at night are sometimes found in dolphin stomachs.

All tropical species that live near the surface in open ocean, including dolphins, have very high metabolic rates and require lots of food. In feeding experiments with captive dolphin in North Carolina, the fish ate well and grew rapidly at temperatures from 75 to 85 degrees. Feeding decreased below 73 degrees and ceased entirely at 65 degrees. In a similar experiment in Japan, one-year-old dolphin at optimum temperature grew at a rate of 4 inches per month.

Based on studies of dolphin scales, Beardsley believes that dolphin grow rapidly but live only four to five years. Japanese researchers reported that dolphin reach 15 inches, fork length, in their first year, 27 inches the second year, 35 inches by age three, 42 inches by age four, and 48 inches in their fifth year.

Summing up, Beardsley says, "Common dolphin have an extended spawning season, a very early age at maturity, a voracious appetite, a cosmopolitan diet and a very short life span." To sport fishermen, this translates into a fast-growing, hard-fighting, actively feeding game fish. The fact that it's also one of the most common, colorful, acrobatic and tasty of fishes is merely a bonus.