The effect of water quality on the growth of Sparus aurata in marine fish ponds

The growth rate of S. aurata in sea cages in Eilat under presumed optimal feeding and water quality conditions was determined in detail. The individual mean annual growth of 245 g fish^-1 year^-1 for fish grown in sea cages at a density of 120 fish/m³ was compared with growth rates from previous studies on this species from Eilat and elsewhere. In order to study the effect of water quality and, in particular, sediment condition on the growth rate of S. aurata, the growth rate of fish in three ponds was determined by monthly weighings. The experiment was repeated over 2 consecutive years. One pond was dried for 41-43 days before filling with water. In the second, aged sediment was allowed to remain in the undried pond, and in the third, the fish were prevented from interacting with the sediment by being placed in cages in the pond. Mullet were stocked in each pond at a rate of 6000/ha. For both experiments, no significant difference (P≤0.05) was found between growth rates of S. aurata stocked at 32 000 fish/ha in the three ponds, and those stocked in the sea cages. During the spring/summer period, when there were daily oxygen variations from 30% to 290%, ammonia values up to 40 μM and significant accumulations of ammonia and hydrogen sulphide (hundreds of μM) in the sediment, there was no observed inhibition of growth. It is suggested that for most of the time blooms of phytoplankton keep the water conditions close to that necessary for optimum growth. Plankton crashes which occur on an approximately monthly frequency are of too short duration to affect overall fish growth rate. However, under certain circumstances mass mortalities occurred which resulted in a reduced annual production of fish.