2. Material and methods
This study was performed at the experimental facilities of the Marine Zoological Station, Faculty of Sciences, Porto University, with European sea bass (Dicentrarchus labrax) obtained from a commercial fish farm.
After a period of quarantine and adaptation to the facilities, groups of 30 fish with an average body weight of 50 g were randomly assigned to 12 tanks of 55 l water capacity in a thermoregulated recirculation water system. Tanks were designed according to Cho et al. (1982) and to each tank was adjusted a faeces settling column. During the trial water temperature averaged 21 ± 0.5 °C, salinity 33.4 ± 1.1‰ and total phosphate was kept lower than 5 μg/l.
Diets used in this trial are presented in Table 1. One basal diet (diet D1) was formulated to contain a low level of P (0.18%) essentially from corn gluten and included 1% chromium oxide as an external inert marker. Eight test diets were formulated to contain the basal diet and different fish meals (diets D2 to D6) or inorganic P supplements (diets D7 to D9) in such a proportion that total P content was 0.5%. Three other test diets were formulated to include the basal diet and inorganic P supplements at a final dietary inclusion level of 0.3%. Dietary ingredients were thoroughly mixed and dry pelleted in a laboratory pellet mill (California Pellet Mill) through a 2 mm dye.