Annual Gonadal Cycles in Teleosts: Environmental Factors and Gonadotropin Levels in Blood

Abstract

The seasonal reproductive cycle, involving recrudescence of the gonad, ovulation or spermiation, and spawning, is the most frequently cited example of an annual endocrine cycle in teleost fishes. Since this cycle is supposedly timed to be adaptive for survival of the species, in more constant climates, such as in the tropics or deep ocean, an annual reproductive cycle could not be predicted. Indeed, in a number of species of Caribbean reef fishes, a portion of the individuals were found to be in reproductive condition throughout the year (MUNRO et al., 1973). However, even in these species a peak of reproductive activity occurs in one period of the year, and in the majority there is a seasonal cycle in which reproductive activity is confined to one period of the year, generally when water temperatures are minimal. When the climatic conditions have greater extremes, as in temperate zones, the conditions appropriate for reproduction are obviously more restricted in time than in tropical climates. The reproductive cycle in this case may then be even more evident, in that spawning in many species occurs only once in a given year and individual populations may spawn en masse.