Mahi Mahi: The rabbits of the ocean
Dolphinfish (mahi mahi) are highly migratory and found worldwide in tropical and warm temperate seas, inhabiting the surface waters.
Mahi are the rabbits of the ocean and grow incredibly fast. They may turn out to be the fastest-growing wild fish known to man. In captivity, these fish have been shown to grow 1.3 to 2.7 inches a week, up to four feet and 40 pounds in a year.
In the wild, at five months, they can reach 5-6 pounds and are mature. Mahi are sexually productive at an early age. All mahi can reproduce by three to five months of age or 22 inches in length.
At one year, most exceed 20 pounds. Three-year-olds usually exceed 30 pounds. Few fish live beyond the fourth year.
Once the fish reach maturity, they spawn every four to six weeks and broadcast about 400,000 eggs.
The eggs, which are about the size of the head of a pin, hatch in about 60 hours. The little fish start growing immediately at a rate of one to one-and-a-half inches a month. This fast growth rate keeps them eating constantly, and they are very aggressive biters most of the time.
Mahi are opportunistic feeders, foraging in the entire water column. They eat everything from paper nautilus to their number one dietary preference, flying fish (malolo).
They also feed both at night and during the day. Mahi consume, on average, 5.6 percent of their body weight daily. Small fish were the most voracious, eating up to 19.8 percent of their weight, while the stomach content of large bulls often contained as much as 10 percent of the fish’s body weight.
Eighty-seven percent of all mahi die during their first year. The 10 percent that make it to the second year can reach 40-50 pounds. The remaining 3 percent comprise the rare population of 60- to 70-pounders. Scientists say that few, if any, mahi make it past five years. The all-tackle world record 87-pounder, caught in the Pacific, was probably four years old.
The spawning season for mahi is long, and multiple spawnings per year are common in both males and females. Mahi spawn in pairs, rather than communally, with spawning occurring year-round In Hawaiian waters.
Peak spawning occurs in April-May coincident with peak catch rates and increased condition. Scientists suggest that the spring peak in catch rate and the summer decline in catch and condition could be related to spawning aggregation near the Hawaiian Islands.
In the summer, the density of mahi near the main Hawaiian Islands is low, perhaps because they migrate offshore to avoid their main predator, blue marlin, which increase in numbers in nearshore waters to reproduce.
Juvenile mahi migrate northward in summer up to 600 miles from the main Hawaiian Islands to the southern boundary of the subtropical convergence front where productivity is high. While offshore during the summer, mahi fatten up and improve their condition during a non-reproductive period.
When they migrate southward back to the Hawaiian archipelago in late September and October, their condition is at their highest level. The September-October maximum spawning condition is coincident with a rise in coastal catch rates. However, the increased condition could also suggest that fish return from more productive waters near the southern boundary of the subtropical convergence to the north.