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Major problem faced by aquaculture industry is the need to obtain a balance between rapid fish growth and optimum use of feed. Since the feed cost accounts ~60% of the total farm production costs (Tan & Dominy, 1997), the economic viability of the culture operation depends on the feed. The global aquaculture feed industries are heavily dependent on fishmeal protein because of its balanced amino acid and fatty acid composition and palatability. However, limited supply and high price of fish meal together stimulated several studies to substitute fish meal partially or completely with alternative protein sources (Fournier, Huelvan, & Desbruyeres, 2004;

ABSTRACT:Investigations on the ovarian cycle of fish species that inhabit Hungarian rivers are necessitated by both environmental and economic reasons. The objective of our research was to explore new fundamental knowledge concerning the ovarian cycle of the white bream (Blicca bjoerkna, Linnaeus, 1758), barbel (Barbus barbus, Linnaeus, 1758), orfe (Leuciscus idus, Linnaeus, 1758) and nase (Chondrostoma nasus, Linnaeus, 1758). Histological investigation of ovaries and determination of proportions of oocytes in different stages of development is an appropriate method for the description of spawning characteristics of these species. Our results show that the GSI value for all four investigated species starts to increase at the end of summer and reaches its maximum before spawning. In the barbel and white bream, the presence of oocytes in the stage of cortical alveoli and the heterogeneous size of oocytes in the stage of vitellogenesis in the pre-spawning period indicate that barbel and white bream are multiple spawners. In contrast, in the orfe and nase, the absence of oocytes in the stage of cortical alveoli and the homogeneous size of cells in the stage of vitellogenesis indicate that orfe and nase are single spawners.