The Beneficial Effects of Using Some Natural Products for Prevention and Treatment of Saprolegnniosis in Grass Carp Eggs and Fingerlings

Saprolegniosis is a major concern in freshwater aquaculture, particularly hatcheries since it causes high mortality and economic losses. Various chemical compounds are used to treat the disease, but they are not recommended due to their hazard effects on fish, humans, and ecosystems. The current study assessed the potential anti-oomycetes activity of crude and aqueous extracts of clove (Syzygium aromaticum) and thyme (Thymus vulgaris) against Saprolegnia australis (S. australis) and Achylia bisexualis (A. bisexualis) in vitro and in vivo. Both herb aqueous stock solutions were eventually prepared using the decoction method. The minimal inhibitory concentrations (MICs) of both herbs' aqueous extracts were determined using the agar plate diffusion method. The ability of both herb aqueous extracts to kill the selected water molds was tested at various time points of exposure. Based on in vitro results, two in vivo field trials were planned to assess the efficacy of crude clove and thyme, first on the fecundity and hatchability of fertilized grass carp eggs. The second trial was designed to assess the antifungal potential against fungal infection during artificial egg incubation. Additionally, two experiments were planned to determine the toxicity parameters of each crude herb in grass carp fingerlings. The second experiment assessed the anti-saprolegniosis activity of selected crude herbs in artificially infected fingerlings. The results showed that S. australis and A. bisexualis were significantly inhibited at a 10% concentration in clove (0.8g/ 100mL) and thyme (3.2g/ 100mL) aqueous solutions. However, both herb aqueous extracts had fungicidal properties and killed both fungi at a 50% concentration of their stock solutions (4 and 16g/ 100mL of clove and thyme, respectively). Incubated eggs exposed to concentrations of 2 and 8g/ L clove and thyme, respectively, exhibited high fecundity and hatchability rates. Furthermore, clove and thyme at concentrations of 2 and 4g/ L could protect incubated eggs and fry from attack by S. australis and A. bisexualis, respectively. Clove and thyme at concentrations of 0.2– 0.25 and 0.5– 0.8g/ L provided a safe indefinite bath, but at a concentration of 0.5 and 1g/ L caused anesthesia after 24 hours of exposure. To successfully treat the infected Ctenopharyngodon idella (C. idella) fingerlings, the most effective doses were 0.25- 0.5g/ L for clove and 0.5g/ L for thyme. Finally, clove and thyme in both crude and aqueous forms could introduce safe, low-cost, environmentally friendly, and easy applications into freshwater hatcheries.