Burgeoning population growth, increasing affluence, and changing dietary habits of people have caused a phenomenal rise in fish demand over the years in India. Diversification of aquaculture species aimed at increasing yield and providing a wider choice of fish protein to the consumers has led to the introduction of several more affordable species into the culture system. Of these, few fish species are yet to get government nod for culture, but these have increasingly been adopted by the farmers. Red-bellied pacu, Piaractus brachypomus is one of such kinds which has increased its spread in culture system to more than 2500 ha ever since its introduction in 2003-2004. The present attempt envisaged a systematic study to find the status of this species in the culture system, especially in the three States, i.e. Andhra Pradesh, Kerala, and West Bengal where its adoption has increased considerably. A total of 351 pacu farms were surveyed using a structured open-ended questionnaire during the period 2011-2015 in these three States. The survey revealed a gradual increase in culture area and production of pacu during the five years. There was a proportionately higher increase in the sale price of pacu compared to other concurrent major cultured species such as the Indian major carps and striped catfish. The majority of the farmers (87%) practicing pacu farming in the three studied states opined in favour of the culture of pacu due to its overall profitability. The findings of this study will be helpful for making informed decisions by the policymakers for the expansion of its culture in the country.

In recent years, infection with Shewanella putrefaciens has emerged as a serious problem in freshwater aquaculture. In the present study, we report large‐scale mortality in farmed tilapia Oreochromis niloticus from Hisar, North India. The diseased tilapia exhibited lethargy, swimming near the water surface, fin rot, hemorrhages on the operculum, and approximately 40% mortality. Gram‐negative, motile, catalase, and oxidase‐positive bacilli were isolated from the kidney of affected tilapia and confirmed as S. putrefaciens on the basis of biochemical tests and 16S rRNA gene sequencing. The antimicrobial sensitivity test revealed that S. putrefaciens strains were sensitive to quinolone, nitrofuran, tetracycline, aminoglycoside, cephalosporin, aminocyclitol, sulphonamide, and macrolide antibiotics, but resistant to beta‐lactam and glycopeptide antibiotics. The histopathological examination revealed degenerative changes in the kidney, liver, spleen, and intestine suggesting systemic infection. In experimental infection studies, the disease was reproduced in naive tilapia, and the clinical signs resembled those observed in natural cases. Further more, S. putrefaciens was re‐isolated from the kidney of challenged tilapia, fulfilling Koch's postulates. Therefore, the mortality in tilapia was attributed to S. putrefaciens infection.

Infection with Aphanomyces invadans is one of the most destructive diseases of freshwater fishes. Indian major carps, the dominant cultured species in the Indian sub-continent are highly susceptible to this disease. Till date, there is no effective treatment for control of this disease and immunization can be one of the strategies to reduce disease-related losses. In the present study, inactivated germinated zoospores of A. invadans were evaluated as antigen in conjunction with and without adjuvant Montanide™ ISA 763 A VG, for assessing their efficacy in rendering protection against A. invadans infection. For the experiment, rohu Labeo rohita, (n = 160, 74 ± 12 g) were divided into 4 groups (C, A, G and GA) with 40 fish in each group. The fish in groups i.e., C, A, G and GA were injected intraperitoneally with PBS, adjuvant emulsified with PBS, inactivated germinated zoospores, and inactivated germinated zoospores emulsified with adjuvant, respectively. After 21 days of immunization, the fish were given a booster dose as above. After 7 days of the booster dose, the fish were challenged with zoospores of A. invadans to determine the relative percent survival (RPS). The results revealed that all the fish in C, A and G group succumbed to infection (0% RPS), although there was delayed mortality in fish from A and G groups in comparison to the C group. However, the fish in GA group showed significantly higher (P < 0.05) protection (66.7% RPS). In addition, following booster immunization (before challenge), the antibody level in the GA group was significantly higher (P < 0.05) than the control group. In western blotting, sera from G and GA groups showed reactivity with peptides of about 54 KDa. Following challenge (on 14th day), the antibody level as well as total antiprotease activity in fish of all the groups was significantly decreased in comparison to pre-challenge, except fish in GA group not exhibiting any gross lesions. In addition, sera of surviving fish of GA group showed significant inhibition of germination of zoospores and germlings growth in comparison to other groups (P < 0.05). Further, histopathological examination of the muscle tissue revealed that, in fish of GA group without any gross lesions, there were well developed granulomas and extensive mononuclear cell infiltration restricted to the site of injection, whereas in other groups, there was extensive myonecrosis with proliferating hyphae. These preliminary findings indicate that inactivated germinated zoospores of A. invadans in combination with adjuvant could stimulate good immune response and confer remarkable protection in rohu.

Inactivation of quorum sensing (QS) signal molecules, such as acylhomoserine lactones (AHLs) of pathogenic bacteria, has been proposed as a novel method to combat bacterial diseases in aquaculture. Despite the importance of micro-algae for aquaculture, AHL degradation by bacteria associated with micro-algal cultures has thus far not been investigated. In this study, we isolated Pseudomonas sp. NFMI-T and Bacillus sp. NFMI-C from open cultures of the micro-algae Tetraselmis suecica and Chaetoceros muelleri, respectively. An AHL degradation assay showed that either monocultures or co-cultures of the isolates were able to degrade the AHL N-hexanoyl-l-homoserine lactone. In contrast, only Bacillus sp. NFMI-C was able to inactivate N-hydroxybutanoyl-l-homoserine lactone, the AHL produced by Vibrio campbellii. The isolated bacteria were able to persist for up to 3 weeks in conventionalized micro-algal cultures, indicating that they were able to establish and maintain themselves within open algal cultures. Using gnotobiotic algal cultures, we found that the isolates did not affect growth of the micro-algae from which they were isolated, whereas a mixture of both isolates increased the growth of Tetraselmis and decreased the growth of Chaetoceros. Finally, addition of Bacillus sp. NFMI-C to the rearing water of giant river prawn (Macrobrachium rosenbergii) larvae significantly improved survival of the larvae when challenged with pathogenic V. campbellii, whereas it had no effect on larval growth.