For the estimation of apparent digestibility coefficient (ADC) of the
experimental diets, chromic oxide (Cr2O3) (Sigma-Aldrich, St. Louis,
USA) was included in the diets as an inert indicator at a concentration
of 1.0%. The digestibility trial was conducted in fiberglass fecal collection
tanks of 300 L capacity designed according to Cho et al. (1982). New sets
of red sea bream and olive flounder with mean body weight of ~50 g
were stocked into each tank at a density of 40 fish per tank and each
group of fish was fed one of the test diets. The tanks were supplied
with cartridge-filtered seawater at a flow rate of 1 L min−1 and aeration
to maintain enough dissolved oxygen. The digestibility trial consisted of
three periods of 10 days. In each 10-day period, the fish were allowed to
become acclimatized to the feed for the first three days and feces were
collected over the next 7 days. Then, diets were randomly changed
between tanks and the procedure was repeated for two more times
giving a total of three fecal samples for each diet. All feces collected
from each tank in each period were pooled and frozen at −20 °C for
analysis. After each feeding, the tanks and the settling columns were
thoroughly cleaned to eliminate all feed wastes and fecal residues. Chromium
oxide content of diet and feces samples was analyzed by the
method described by Divakaran et al. (2002). The apparent digestibility
coefficient of the experimental diets was calculated through the following
formula: