الفهرس 
Importance of water quality and mineral salt balance in two fish farming systems in the north nile delta
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Abstract
The study was conducted in two fish farming systems in earthen ponds located at (Bahr Elarab drains Elrayed area- Kafer Elshiakh government Egypt); namely, the feed fish farming and fertilizer fish farming systems. Each system was replicated in four ponds (8400m2 each). The experimental period lasted from July 2014 to June 2015, including 3 month overwintering. Nile tilapia (orechromis niloticus); mullet (Mugil cephalus) and catfish (clarias garipinus) were cultured in each pond at initial weight of 2, 30 and 5 g, respectively. Samples were collected once every month to determine water quality, phytoplankton and zooplankton abundance and fish growth performance. Sediment samples were collected at the beginning and the end of experimental period to determine sediment quality. Showed no significant differences in monthly values of water temperature between the feed fish farm and fertilizer fish farm ponds. The monthly pH values were significantly higher in the fertilizer fish farm (7.9-8.9units) compared to those of the feed fish farm (7.6-8.2 units) due to the increased photosynthetic activities in the fertilized ponds. Results clear that salinity, EC and total hardness were not affected by pond management. Pond management (feed and organic fertilizer) did not affect soil pH in both the surface and subsurface layers. While, water quality in fish farms is greatly influenced by feed management and fertilizer inputs which in turn affect sediment quality in ponds. The total production of fish within each fish farm averaged 5050 and 2957 kg/feddan for feed and fertilizer fish farms, respectively. The production costs per kilogram of fish were 8.06 and 4.53 LE in the feed and fertilizer farms, respectively. While, the total income per feddan was higher in the feed fish farm, the net income was significantly higher (P<0.05) in the fertilizer fish farm due to its lower operating costs per unit of production. The cost of fish production in the fertilizer system was significantly less than that of artificial feed system. It is concluded that the fertilizer fish ponds needed additional doses of artificial feed during the last two months to increase individual growth rates of Nile tilapia and catfish