Causes of fish mortalities in semi-intensively operated seawater ponds in Eilat, Israel

Seawater fish ponds were operated for the semi-intensive culture of gilthead seabream and grey mullet in Eilat, Israel from 1975 to 1982. In that period there were mortalities, involving the deaths of 12-2194 individual fish, on 18 occasions out of a total of 46 pond-years. The mortalities occurred principally in mid-summer. They were associated with low oxygen at dawn (six occasions) or high oxygen in mid-afternoon (12 occasions). Subsequent detailed water quality measurements in 1983-1984 suggest the actual cause of death was low oxygen combined with high ammonia, and high oxygen (> 300% saturation) together with high temperature (≳ 33°C). The incidence of mortality correlated with ponds in which the sediment had not been dried the previous year. Possible reasons for the association of mass mortalities with aged sediment are given.