Some environmental factors affecting the reproduction of Aphanius dispar (Rüppell, 1828)

In order to determine the suitability of the cyprinodont fish Aphanius dispar as a biological control agent for mosquito larvae, the effects of temperature, salinity, feeding rate and photoperiod on its reproductive condition were studied. The factors examined were the gonado-somatic index (GSI) and four distinct stages of cell maturation in the ovaries (oogonia, primary oocytes, secondary oocytes and mature ova). The fish were found to be tolerant to a wide range of the examined factors. GSI's were not affected by ambient temperatures over the range of 18°C to 37°C. However, oocyte maturation stages increased as temperatures rose from 18°C to 27°C, with mature ova first being found at the latter temperature. No significant differences among maturation stages were found between 27°C-37°C. At salinities ranging from of 0 ppt to 56 ppt, no significant differences were found between mean GSI's, except for the two extremes of the test range. The ovaries of fish kept in distilled water contained primordial germ cells only. In feeding experiments, significant differences were found only between the ovaries (GSI's and oocyte maturation stages) of fish deprived of food and those fed at ≥ 1% body weight per day. Regarding photoperiod, a decrease in day length from 14L:10D (the longest day of the year) to 10L:14D (the shortest) caused a decrease in oocyte maturation stages. Fish at the intermediate level (12L:12D) still had ovaries suitable for spawning, whereas those at 10L:14D did not. The results of this study show that limiting levels for adequate reproductive condition were found for some of the factors investigated, however, ranges of the factors above these levels were relatively large. That Aphanius dispar's reproductive condition may be controlled in an artificial environment is essential for it to be mass produced, and a prerequisite for its use in the biological control of mosquito larvae.