Knowledge of the age and growth rates of billfish is essential for sensible management of our fishery. The continued viability of billfish stocks is of substantial importance to sport and commercial fishermen alike. Their high value in the marketplace and economic significance to the sportfishing industry have made billfish one of the most valued oceanic fish.
The most reliable means of estimating age and growth rates of fishes is by examination of certain bony structures, which have growth patterns similar to the rings in the trunk of a tree. Some of the bony structures that have been examined from billfish are the otholiths (ear-bones), vertebrae, dorsal and anal fin spines. Another way is by measurement of the eye-fork length growth patterns through tag-recapture studies.
What do we know about age and growth of blue marlin? Based on growth age data, Pacific blue marlin males appear to have a longevity of at least 18 years, and females appear to live until at least 26 years of age. The largest female sampled (1,649 pounds) was one of the oldest at 26 years. There are records of larger Pacific blue marlin, including "Choy's Monster" at 1,805 pounds, and reports of commercial longline-caught fish exceeding 2,000 pounds, so females probably live to exceed an age of 30 years. A large male sampled (306 pounds) was only nine years old. The oldest male (18) was only of an average size, so it is probable that males may attain an age of 25 years or more.
Blue marlin are not hermaphroditic. The dramatic difference in size between males and females is known as a sexual size dimorphism. This size dimorphism is not due to a sex change from male to female, but just a differential growth pattern. There was a pronounced difference in growth between male and female marlin.
So while male and female marlin cannot be distinguished from their external features, size can be used to predict the sex of the larger fish. However, there are problems involved with trying to estimate the age of a marlin merely from its size. Age was quite variable with fish size.
For example, a 250- to 300-pound female could range from 7-16 years of age, and a male in the 200- to 250-pound category could range from 6-18 years. The huge group of 150- to 200-pound males and the group of 250- to 300-pound females probably represent fish in the same age group: 6-8 years old.
Nearly all male Pacific blue marlin documented have weighed less than 300 pounds, with 85 percent of the males weighing between 100 to 200 pounds. Males appear to grow steadily to an average size measurement of a jaw-tail fork length of about 7'9" total length and about 165 pounds. They reach this size at about six years, after which their growth rapidly levels off.
Large males do occur, with ones weighing 306, 311, 315, 324, 334, 342, 348, and 375 pounds verified in Hawaiian waters since studies began in 1982. Males as large as 350 pounds have been reported from the Atlantic.
Of the hundreds of Pacific blue marlin that I have sexed in the last 30 years that were 300 pounds or more, I have only found two that were male, a 324- and a 334-pounder.
Females dominate the larger size classes and reach weights much greater than the males. In females that were documented, 80 percent weighed between 250 to 600 pounds, and they can reach weights of well over 1,000 pounds. A rule of thumb I have found is 99 percent of blue marlin over 300 pounds will be female. I have identified small female marlin on a regular basis, depending on the time of year and spawning cycle.
Growth of female marlin is more rapid during their earlier years, and then steadier for a greater portion of their life span; however, this growth is quite variable from fish to fish. Females probably continue to grow, especially in weight and girth, for the remainder of their life span.
Important to this research are the tag-recapture studies needed to help to validate the true meaning of these growth patterns. When a tagged fish is recaptured, scientists calculate its growth from the time it was released until the time it was recovered. The longer the time between tagging and recovery, and the more accurate the initial weight estimate, the better. Age validation work is difficult in large fish like the blue marlin, but it can be accomplished through tag and release programs.
As the International Game Fish Association representative for Maui County, I handle the Tag & Release program, distributing FREE tags and recovering recaptured tags and tag cards, which are sent to the National Marine Fisheries Service's Southwest Fisheries Science Center in La Jolla, California.
For information on the IGFA, the Tag and Release program or tags, contact Donnell A. Tate at firstname.lastname@example.org or (808) 298-5674.