## 3 Results and Discussion
### Relative Percent Survival  
No mortality was observed up to 24 h after exposing fish to _E. tarda_ infection. Fish treated with varied concentrations of methanolic extract of _C. aerea_ exhibited higher survival (\(p<0.05\)) compared with the control. The highest relative survival (83.5%) was shown in the T2 group, which received 50 mg/kg of algal extract, while it was lowest in the controls  
Figure 6: Effect of _C. aerea_ extract on the reactive oxygen species production by peripheral blood leukocytes in _L. rohita_ (values are mean \(\pm\) SE). Mean values with different superscript within a column for a parameter are significantly different (\(p<0.05\)). Figure 7: Effect of _C. aerea_ extract on the reactive nitrogen species production by peripheral blood leukocytes in _L. rohita_ (values are mean \(\pm\) SE). Mean values with different superscript within a column for a parameter are significantly different (\(p<0.05\)). (Figure 8). This may have resulted from an increase in non-specific immune responses. In several previous reports, various herbal medicinal plant extracts (_Cyanodon dactyllon, Tinospora cordifolia, Aegle marmelos, Eclipta alba_, and _Picorhiza kurooa_) exhibited a positive influence on survival rate (74%), and were associated with a decrease in viral load in shrimps infected with the white spot syndrome virus [57]. The results of the study demonstrated that the RPS was strongly correlated with functional assays as well as hematological parameters. An inverse relationship was found between the amount of extract administered and the mortality rate. In another similar study, methanolic extract of _C. antenna_ significantly increased survival percentage in _L. rohita_ after induction of _E. tarda_ infection [21]. It has been found that various doses of fucoidan-rich extract of a seaweed (_Sargassum wightii_, a brown algae) has also reduced mortality in infected _L. rohita_ compared to controls [39]. The low mortality rate in fish with algal extract treatment may also be attributed to antimicrobial properties. It has also been reported that intraperitoneal injections of protein-bound polysaccharides from _Coriolus versicolor_ reduced mortality rate in _Oreochromis niloticus_ against _E. tarda_ infection [58]. Screening of antimicrobial activities of extracts from various algal species including _Cmuosporabic analculata, Ulva fasciata, Cheilosporum spectabile_, _Bryopsisplumosa_, _Gratelcupia flicina_, _C. antenna_, _Hypneapannosa_, _Centroceras clavulatusum, Portieria hornemannii_, _Gracilaria corticata_, _Sargassum wightii_, _A. orientalis_, _Stocheospermam marginatum_, and _Padinate trastromatica_ has been performed.It has been demonstrated that antimicrobial compounds extracted from fresh algae using methanol: toluene (3:1) solvents are more effective due to maintenance of lipophilic nature over wide temperature range (30-60 \({}^{\circ}\)C), whereas the extracts from dry algal species showed decreased level of bioactive compounds. Several of the extracts from algae like _Acrosiphonia orientalis_ exhibited antibacterial properties against 70% of the microbes analyzed. The extracts from several algal species like _Stocheospermam marginatum_ and _Gracilaria corticata_ were found to be more effective against Gram-negative bacteria compared to Gram-positive bacteria [59]. The findings of the current study suggest a novel insight into the immunostimulation capacity of the methanolic extract of _C. aerea_ in _L. rohita_ to reduce mortality caused by bacterial infection. Thus, the health of the fish can be improved by the use of ethnoveterinary therapeutic agents like algal extracts which boost the non-specific immune response. The immunostimulants obtained from algal extracts may replace the use of antibiotics in aquaculture. However, as a future perspective, antimicrobial activity of _C. aerea_ extract against _E. tarda_ needs to be assessed in vitro. Figure 8: Effect of methanol extract of _C. aerea_ algae administered on the percentage mortality in _L. rohita_ challenged with virulent _E.