Why do marlin numbers fluctuate by season?
As one might suspect of highly mobile creatures like blue marlin, the picture of their population in any one fishing ground is not static. The dynamic nature of the blue marlin’s lifestyle is reflected by seasonal changes in number of fish caught, as well as by changes in the size composition and sex ratio of the population.
The large summer catches are dominated by males, and these smaller animals outnumber the larger females by from two to six times. Late fall catches show more equal numbers of males and females, and small spring catches show the larger females two to seven times more numerous than males.
Evidence from several different kinds of fisheries, including sportfishing and longlining, indicates that the unequal representation of the sexes on the grounds does not appear to be due to differences in feeding habits of the sexes or to the kinds of fishing gear used. Then why the change? There is a very strong relationship between the sex ratio and the reproductive condition of the fish.
Samples with the highest male to female ratios contained the highest percentage of reproductive individuals. For example, in August, 85 percent of the males and 50 percent of the females were in spawning conditions. Where the sex ratio was nearly equal (fall) or females outnumbered males (spring), few of the fish showed signs of reproductive activity.
Similar changes in catch, sex ratio and size composition by season have been observed in both Pacific and Atlantic populations of blue marlin, as well as in striped and black marlin. In these studies, the large influx of males corresponded to the beginning of the fishing season.
Females, in smaller numbers, dominated the off-season fishing grounds during the non-reproductive season. Some researchers have recommended using the sex ratio change as an indication of the timing of the reproductive season.
But why should there be so many males present during the reproductive season? In Hawaii and elsewhere, anglers and boat captains report that during the summer months, they frequently see a single large marlin (presumably female) escorted by several smaller fish (presumably males).
Perhaps the presence of several males is needed to ensure high fertilization success of the female’s millions of eggs. In Hawaii, the summer sex ratio reflects the influx of large numbers of males, perhaps to a “spawning ground” close to the islands, to which large females are attracted when ready to spawn.
Some longtime Hawaiian fishermen even suggest that there are distinct areas where marlin are frequently seen engaged in possible spawning behavior.