Study of the aging of silver carp eggs (Hypophthalmichthys molitrix, Valenciennes 1844) and its impact on fish production (DEROUA fish farming station, Fkih Ben Salah, Morocco)

Abstract

In 1983, the Chinese carps were imported from Hungary in order to control the eutrophication phenomenon observed in the Moroccan dams. Then the silver carp, Hypophthalmichthys molitrix began to have an important effect on the improvement of water resources of the kingdom. This specie is able to reduce the unwanted algal biomass and transform it into a highly appreciated fish flesh as consequence of its phytoplanctonophage diet. Indeed, the fry that weight just 0.0013 g can reach up to 6 kg in their second year. Moreover, their growth performances are really significant and they can weight more than 25 kg in the first five years (Droussi, 1998). The present study seeks to determine the effect of the spawning period on eggs quality. During the breeding season of the silver carp, the quality of the eggs was assessed by monitoring the development of fertilized eggs and evaluating their quality by the determination of three viability rates: fertilization rate, embryos survival rate, and fry survival rate. This part will allow determining the period in the breeding season when the eggs record a satisfactory viability rates. The results showed that the fertilization rate (FR) varied from 60% to 72%, the embryos survival rate changed from 43% to 67% while fry survival rate recorded a variation of 80% to 99%. On the other hand, our study treats the effect of eggs retention in the females’ ovary of silver carp (in vivo aging). The purpose of this investigation is to determine how long eggs, ready to be ovulated, can remain in the ovarian cavity without losing their viability. The adopted method is based on inducing the ovulation of each female by pituitary injection. Then the extracted ova are fertilized immediately by the milt of males (at least three males). Four egg extractions were performed at increasing times after ovulation. The time interval is 30 minutes between two successive extractions. The assessment of egg quality was performed by the evaluation of the variability of three rates indicated above. The results showed that fertilization rate and embryos survival rate decreased from 62% (at the ovulation time) to 30% (for fertilized eggs at 90 minutes post-ovulation). In addition, fry survival rate varied unfavorably from 87% to 49%. Besides, the research was supplemented by biochemical analysis in order to find if there is a correlation between egg quality and biochemical composition of the ova. The results showed that no effect of aging and periods on lipids, proteins and fatty acids composition. However, the antioxidants: vitamin C and Vitamin E in eggs do not show any relationship with the weight of silver carp breeders. The vitamin E is impacted by the time of the post-ovulatory aging. Moreover, both antioxidants balance the oxidative events throughout the breeding season by their changes.