Understanding marlin reproduction
Like most open ocean fishes, blue marlin are broadcast spawners. Females release their eggs, and males their sperm, into the open ocean, and fertilization takes place in the surrounding water.
There is no parental care of the young. The eggs float until the tiny larval fish hatch out, probably one to two days after fertilization.
Obviously, with this kind of reproduction, great numbers of eggs and sperm are needed to guarantee that some fertilization takes place, and that there are enough young produced to offset the high mortality early in life. Many eggs and larvae fall prey to tiny predators in the plankton.
The annual egg production of female blue marlin produced per year range from 30 million eggs for a female of 400 pounds to 100 million eggs for a female of 950 pounds. The larger (and older) a female is, the more eggs she can produce.
The great energy it takes to produce all these millions of eggs each season may be part of the explanation of why female blue marlin reach much greater sizes than their mates. Large size may be necessary if marlin are to “afford” reproducing in this way. The faster growth rate for the females thus helps them attain large size, so that they can reproduce enough to perpetuate the species.
The eggs of blue marlin are quite small, only .05 inch (1.25 millimeter) in diameter when ready for spawning. However, not all the eggs in the ovary are the same size, and they are not all ready for spawning at the same time.
There appears to be six batches of eggs in the ovary at the beginning of the spawning season, each batch a slightly different size and age. A female probably spawns several times in each season as the successive batches of eggs mature and become ready for release.
Spawning many times during the season is part of good strategy for reproduction… never put all your eggs in one basket, so to speak. If one batch of eggs fails, one of the others may be successful.
To ensure that the female’s millions of eggs are fertilized, male marlin must produce large numbers of sperm. Sperm count estimates range to over one billion sperm cells per cc (milliliter) of milt. The volume of milt stored in the sperm ducts varies between males.
A large male in spawning condition may produce as much as 200 billion or more sperm cells. The amount of milt released during a single spawning has not yet been determined, but billions upon billions of sperm may be needed in order to be sure that fertilization occurs.
If the male marlin’s reproductive output is so large, why aren’t males as big as females? The difference lies in the energy investment needed for the production of eggs and sperm, and in the timing of their production by the different sexes. Sperm cells are very small and are probably “cheaper” to produce than the eggs. And, while the females produce all their eggs at the very beginning of the season, males can produce sperm cells continuously throughout the length of the season, spreading their effort out.
The lower cost of reproduction for the males has been suggested as an explanation for their smaller size. Presumably, they simply don’t have to be big.
The study of marlin reproduction may provide answers to questions about the factors important in the reproductive success of marlin populations and the ability of marlin populations to replace themselves.