Mass budget of the sea bass (Dicentrarchus labrax) farming losses
was examined. Experiments were
carried out in April, September and November 2001 in tanks of 8.5 m3
containing sea bass of 1, 31 and 53 g, respectively. Samples were taken at
the input and discharge points of the tank at hourly intervals over 24 h.
They were later analysed for nutrient concentration, as well as particulate
organic carbon and nitrogen, chlorophyll a (Chl a) and total bacterial
abundance. Despite the high water supply (exchange rate: 50% h−1),
NH4 concentration was significantly higher in discharge water than in
input water in all three experiments independent of fish size. The same was
found to hold true for PO4 for part of the day (April, November) or for
the entire day (September). NO3 and SiO2 ions did not show any
differences during April. Particulate organic carbon (POC) and nitrogen
(PON) were higher in the discharge water in all seasons. Chl a concentration
was higher in the discharge water during the entire day in September whereas
bacteria presented higher counts in the output water only during part of the
day in April. It was found that 5−7% of the nitrogen supplied is released
into the water column as fine particulate material in the form of PON and
21−29% as NH4. A small proportion (13−16%) of supplied phosphorus
(P) was released as PO4. The results provided in the present paper are
useful in assessing environmental changes in water quality in the vicinity
of fish farms in the Mediterranean.