The extracts from seaweeds, Ulva spp. and Solieria chordalis, were incorporated in the diet of red tilapia (Oreochromis sp) juveniles of 57.3 ± 2.8 g. Two thousand fish were allocated into 20 tanks of 1000 L/each with a density of 100 fish/tank using an indoor recirculating system and implemented for 30 days. Fish were fed with 5 diets including 2 control diets (one negative control and one positive control) without seaweed extract inclusion and 3 test diets containing 3 inclusion levels of seaweed extracts including 0.1% (T1), 0.2% (T2) and 0.3% (T3). All diets are commercial feed with iso nutritive values. The seaweed extracts were mixed with 20 mL fish oil/kg and coated on top of the test diets. The control diets were top-dressed with 20 mL fish oil/kg only. After 30 days of the growth phase, the fish were submitted to a bacterial challenge phase between day 31 and day 44 with 50 fish/tank and 4 replicates. The fish were injected with a Streptococcus agalactiae bacterial strain at LD50 (0.5 x 105 CFU/fish) at day 31 and recorded clinical signs for 14 days follow post challenge. Fish body weight was recorded at day 28 and 44. Blood samples were collected from the caudal vein of 3 fish/tank at day 1, 7, 14, 21, and 30 at the pre-challenge; at day 7 and 14 post-challenge with heparin as anticoagulant. Cumulative mortality was calculated. Blood immune parameters (respiratory burst) and intestinal goblet cells (mucin producing cells) were quantified.
After 4 weeks of the growth phase, fish fed the seaweed extracts at 0.2% and 0.3% had the highest average body weight (ABW) of 152.6 g and 154.1 g, respectively, compared to control diet, 128.8 g (p<0.05). After being challenged, fish fed with the seaweed extracts showed higher average body weight (157.2 g and 159g) than that of the positive control 130.6 g and like the negative control 161.3 g (p<0.05). All fish were fed with seaweed extracts at 0.1%, 0.2% and 0.3% inclusions had significantly lower cumulative mortality (32.5%; 30.5%; 25.5%) than the positive control group, 51.0% (p<0.05). The respiratory burst increased significantly (p<0.05) as increasing the seaweed extract levels of 0.1%, 0.2%, and 0.3% and higher than fed control diets. The number of blue-stained goblet cells were significant higher in all fish fed seaweed extract groups than control groups (p<0.05). Moreover, the increasing inclusion levels of seaweed extracts stimulated linearly (p<0.05) the number of goblet cells. Fish were fed the mix of seaweed extracts at 0.3% had the highest goblet number followed by 0.2% and 0.1%. These results show the efficacy of the seaweed extract mix on reinforcing intestinal and systemic health in red tilapia meanwhile assuring growth performance.