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Abstract

The paper presents water quality studies carried out in a deep (8 m) dual purpose reservoir in the Jordan Valley during the 1991 fish culture season. In Israeli dual purpose reservoirs for fish culture and field crop irrigation, two processes overlap: (1) an increase of loading through the culture season due to fish growth and the corresponding increase of feed inputs, and (2) a decrease of the water level during the dry summer due to irrigation. These two trends strongly affect the water quality in which fish grow, and hence the success or failure of the fish culture. The water quality changes in time (through months and daily) and space (vertically and horizontally) are analyzed in relation to biological transformations, hydrological processes and reservoir management. Results are compared to shallow (4 m) dual purpose reservoirs located in the Coastal area of the country, where the seasonal stratification observed in deep reservoirs does not develop. It is concluded that fish culture management in deep reservoirs where seasonal stratification develops requires special care and a follow up of stratification breaking, and that seasonal stratification should be avoided when designing new reservoirs which include fish farming among their purposes.