II. MATERIALS ANDMETHODS
The fish farm is located on one of the islands in the central
coast of Croatia. The bay is about 1 800m long with the
maximum width of 714m and the maximum depth of 37m. It
is relatively shallow and sheltered bay, well protected from
dominating winds (NE and SW). Sediments are predominately
sandy with some isolated muddy zones (below the cages).
Currents are predominant tidily driven to NW. The shape of
the bay enables good dynamics of water masses and hence
good oxygen supply.
Pilot scale production started in 1981 and from 1983
industrial scale production varied from 150 to 250 tones per
year. The main farmed species is the sea bass (Dicentrarchus
labrax L.), which represents 90% of produced population. The
fish was reared in rectangular cages (10m * 10m * 5m) of
500m3. In 1994 four platforms were moved closed to the
entrance of the bay to obtain better rearing conditions for
yearly fish. The depth below the cages varied from 15 m to 30
m. From June 15th 1994 till December 1995 fish production
was 231 tones with a food consumption of 521 tones. The
phosphorus and nitrogen load estimation was based on the
food containing 0.9% phosphorus and 7.2% nitrogen with an
average food coefficient (gain/feed) of 2.3%. Final stocking
density in the cages was 12 kg/m3. Total mortality was 30%
over 565 days. Salinity was stable and ranged from 36 ppt to
27.5 ppt. The temperature ranged from 10°C in the winter to
23°C in the summer. The vertical amplitudes (from the surface
up to 10m depth) fluctuate from 0.2°C in the autumn to 1.3°C
in the spring. Daily differences in surface waters were from
0.1°C in September to 0.8°C in May [5].
Standard oceanographic methods for N and P analyses were
used [6]. Dissolved oxygen concentration (DO) and
temperature by using YSI dissolved oxygen-temperature
meter and salinity was recorded daily by an optical refracto meter. Standard fluorescence method with an acidification
step to estimate the concentration of chlorophyll a was used
[7].