Sources of water to the seawater well which supplies the fish ponds in Eilat Implications for fishpond management

The sources of water to the well supplying the fishponds in Eilat were determined using a simple mass balance approach, in which the concentration of various components in the source waters were measured and a series of simultaneous equations solved to determine the hydrological budget. It was calculated that 80% of the water supplied to the well was recycled pond water, 10% was seawater and 10% was sewage effluent. On the basis of major ion determinations, it was shown that there could be no significant contribution from natural groundwater. This method of water supply has both disadvantages and benefits. The fishpond system was operated to produce gilthead seabream and grey mullet in the pond and oysters on the pond effluent. The major disadvantage was that the excess nutrients supplied in the inflow may have limited the total biomass of fish which could be grown in the ponds by making them more susceptible to mass mortalities due to both high oxygen or high ammonia/low oxygen concentrations. The benefits include the high productivity of diatoms due to the nutrients supplied which enabled there to be a successful oyster culture. In addition, 95-97% of the nitrogen supplied in the sources was as ammonium which is toxic to fish. After passage through the aerobic groundwater, 97% of the dissolved nitrogen was in the form of nitrate. At the same time, the coliform counts were reduced to undetectable levels. The major ion composition of the pond was enriched 25% relative to seawater because the evaporated seawater was replaced by sewage effluent. Despite this unusual ionic composition fish, oysters and prawns were grown successfully in these ponds.