Sex ratios and sexual differences in size among mesopelagic fishes from the central Pacific Ocean

Abstract

Population sex ratio, size at maturity and maximum size of both sexes were determined for 22 species of mesopelagic fishes in order to document the occurence and consider the ecological significance of deviations from even sex ratio and equal-sized sexes. Specimens were collected in oblique trawls near Hawaii between August 1977 and October 1978. Eight species (mostly small myctophids) appeared to have even sex ratios and no sexual differences in size. In some other myctophids and most stomiatoids, females were either more abundant or larger than males, and consequently the biomass of mature females in the population was greater than that of mature males. Such differences mean that there must be sexual differences in ages at maturity, growth rates, or mortality rates, but adequate data on these are not available. The adaptive value of such sexual differences may be to maximize eggproducing biomass and minimize intraspecific competition in a situation where food resources are low; however, equally plausible alternate hypotheses need to be tested. Sex ratio favored males in two melamphaid species, and the relative biomass of males in the populations was greater than in comparable myctophids or stomiatoids. Although this pattern means decreased egg production per population biomass, it also means increased probability of females encountering males. This increase may be particularly important to the melamphaid species because they have no luminous organs or other sexual dimorphisms to facilitate communication.