The use of natural products which have the least harmful effects on the environment has recently been taken as a novel approach against fish diseases. References on in vitro studies have demonstrated antibacterial activity of essential oils (EOs) against certain fish pathogens. The aim of this study was to evaluate the antibacterial effect of some plant essential oils against fish pathogenic bacteria in vitro conditions. Seven plant EOs: lavender (Lavandula angustifolia), clove (Eugenia caryophyllus), peppermint (Mentha piperitae), basil (Ocimum sanctum), rosemary (Rosmarinus officinalis), cinnamon (Cinnamomum zeylanicum) and black cumin (Nigella sativa) were used to identify their antibacterial properties against Yersinia ruckeri, Aeromonas hydrophila, Vibrio anguillarum, Vibrio alginolyticus, Lactococcus garvieae and Vagococcus salmoninarum at five concentrations using disc diffusion method. Especially the EOs of clove, cinnamon and rosemary showed the strongest antibacterial activities than other oils against the three most susceptible bacterial strains (Y. ruckeri, A. hydrophila and V. salmoninarum). Besides, the EOs of clove, rosemary, cinnamon and black cumin showed similar inhibition zones with OTC against A. hydrophila. The minimum inhibitory concentrations of the used EOs found between 500 and 62.5 µl mL-1. As a result, three of the EOs used in this study were effective on both Gr (-) and Gr (+) bacteria.

Antibiotics are extensively used in aquaculture to treat and prevent bacterial diseases, but their application can lead to negative side effects such as immunosuppression. Oxytetracycline (OTC) is a widely used antibiotic in aquaculture for treating bacterial diseases due to its broad‐spectrum activity and economic advantage. This study appraises the effects of OTC treatment on the immune response of European sea bass ( Dicentrarchus labrax , L. 1758) by assessing immune‐related gene expression, serological parameters, hematological parameters, and histological findings. Fish were divided into three groups: control (fed with nonsupplemented OTC), 75 mg OTC kg ⁻¹ day ⁻¹ feed, and 100 mg OTC kg ⁻¹ day ⁻¹ feed for 21 days. Blood and tissue samples were collected for analysis on the 4th, 7th, 10th, 14th, and 21st day of OTC feeding. The results showed a decrease in immune response parameters such as nitro blue tetrazolium (NBT) positive cell, total white blood cells (WBCs), serum lysozyme activity, serum Ig‐M levels, and serum complement activity for OTC‐fed fish, while immune‐related genes ( Lys-g , Ig-M , interleukin ( IL )‐ 1β , C3 , and major histocompatibility complex [ MHC ] –IIα ) were significantly downregulated in kidney and spleen tissues, especially with prolonged exposure. The study hypothesized that long‐term use of antibiotics in fish disrupts the functioning of the immune system. As a result of the study, the hypothesis was confirmed, and using OTC for longer than the normal treatment period or in high doses caused negative changes in the immune system parameters and made fish more susceptible to other pathogens.

The activity of Lentinula edodes (shiitake) medicinal mushroom extract were examined on the non-specific immune response and biometrical performance of rainbow trout (Oncorhynchus mykiss). Fish (20 g initial weight) were divided into 3 treatment groups (60 fish/group) and duplicated groups for 6 weeks having two experimental diets supplemented with 1-2% shiitake extract and a control diet. During the feeding process, immunological, biochemical, and biometrical observations were determined using the fish and blood samples taken at weeks 1, 2, 3, 4, 5, and 6, respectively. The results of immunological, biochemical, and biometrical parameters evaluation determined that the maximum influence occurs in rainbow trout fed with 2% shiitake extract. The amount of respiratory burst activity in the blood of fish in the trial groups significantly increased in each diet on the 2nd and 6th weeks compared to controls. The amount of total protein, bactericidal activity were significantly increased in fish being fed a mushroom supplemented diet. Cholesterol level decreased in fish blood, which fed with 2% shiitake extract supplemented diet at 3rd and 6th weeks. These results support the findings that the non-specific immune responses of rainbow trout was stimulated in fish by feeding shiitake medicinal mushroom extract yielding positive results in measured parameters compared to the control group also enhancing the overall growth performance of rainbow trout.

Plant-based additives or botanicals, have garnered considerable interest in the aquaculture industry for their multifaceted benefits in fish diets. In this study, a 30 days short term feeding experiment was designed to determine the effects of Myrrh (Commiphora myrrha) essential oil on hematological, biochemical parameters and tissue specific mRNA expression responses of common carp (Cyprinus carpio). Fish were experimentally infected with Aeromonas hydrophila and monitored for 20 days after been divided in four groups: a control group without C. myrrha and three experimental groups supplemented with 0.25%, 0.50%, and 1.0% C. myrrha essential oil (CMO 0.25, CMO 0.50, and CMO 1.0, respectively). Results revealed a significant increase in RBC count, hemoglobin, and hematocrit values compared to the control diet. Positive effects of C. myrra supplementation have been observed also in serum biochemistry parameters. mRNA transcripts of il-1ß, tnf-ɑ, il-8, sod, cat, gpx and gst genes significantly increased in spleen and liver tissue of fish fed with C. myrra supplemented diets. After the challenge with Aeromonas hydrophila, the survival rates were 60%, and 83.33%, respectively, suggesting enhanced fish resistance in the CMO 0.50 and CMO 1.0 groups compared to the control. The results suggest the immunomodulatory roles of C. myrrha essential oil.