Various aspects of reproductive control in freshwater fishes

Male and female guppies (Poecilia reticulata) were treated with either methyl testosterone, cyproterone acetate (CA) (an antiandrogen) or a combination of the two steroids to determine effects on behavior, coloration, and gametogenesis. The antiandrogen did not fully block the effects of endogenous or exogenous testosterone in enhancing male sexual behavior and coloration in males or females. An inverse relationship existed between the numbers of cysts in the early stages of spermatogenesis and the number of spermatophores for males receiving any of the steroids. Only one female in the testosterone and combined steroid treatments had embryos present in its ovaries.

CA was administered orally to juvenile Japanese medaka (Oryzias latipes) in two experiments to determine the possible role of the antiandrogen on sex determination. Sex ratios were determined in the mature fish by dimorphic coloration, secondary sex characters, and histological examination. Sex reversal was not established in the fish.

The effects of population density and nutrition on growth and fecundity of fathead minnows (Pimephales promelas) were studied. Length and weight of the fish increased with increased food availability. Perhaps a direct relationship of egg size with food ration also existed. Egg size was found exponentially related to growth of fish. Increased population density appeared to limit growth and perhaps gamete development regardless of food abundance. Additionally, water volume appears to limit the number (tolerance density) of fish which can be supported within a specific area. Increased food increased weight per fish but not number of fish in a particular volume of water.