Identification and pathogenicity study of emerging fish pathogens Acinetobacter junii and Acinetobacter pittii recovered from a disease outbreak in Labeo catla (Hamilton, 1822) and Hypophthalmichthys molitrix (Valenciennes, 1844) of freshwater wetland in West Bengal,India

The disease outbreaks in aquaculture system of wetlands are the major cause of fish mortality. Among various bacterial septicaemic diseases, fish mortality caused by Acinetobacter spp. is recently reported in different fish species. Fish disease outbreak was investigated in a wetland of West Bengal, India to identify the aetiological factors involved. The moribund fish were examined and subjected to bacterial isolation. Two bacterial causative agents were identified as Acinetobacter junii and Acinetobacter pittii by biochemical characterization and 16S rRNA gene amplification. Both the isolates were oxidase‐negative, nitrate‐negative, catalase‐positive and indole‐negative. The molecular identification using 16S rRNA gene sequencing and phylogenetic tree analysis further confirmed the two Acinetobacter spp. with 97%–99% similarity. The antibiotic resistance patterns of these two bacteria revealed that both of them were resistant to β‐lactam, cefalexin, cephalothin, amoxyclav, cefuroxime, cefadroxil, clindamycin, vancomycin and penicillin. In addition, A. pittii was also resistant to other antibiotics of cephams group such as ceftazidime and cefotaxime. In the challenge experiment, both A. junii and A. pittii were found to be pathogenic with LD₅₀ of 1.24 × 10⁵ and 1.88 × 10⁷ cfu/fish respectively. Histopathological examination of gill, liver and kidney revealed prominent changes supporting bacterial septicaemia. The investigation reports for the first time on concurrent infection by A. junii and multidrug‐resistant (MDR)‐A. pittii as emerging fish pathogens to cause severe mortality in Labeo catla and Hypophthalmichthys molitrix in a freshwater wetland.     

Fishmeal alternative from renewable CO2 for rainbow trout feed

The rapid global growth in fish farming and limited supply of fish meal (FM) has consequently reduced FM inclusion levels in compound feeds leading to a higher reliance on alternative protein sources. Sasya is a single cell protein (SCP) product that has a similar amino acid profile as FM. The aim of this study was to evaluate the effects of replacing FM with SCP on in vivo digestibility, growth, feed efficiency, whole‐body proximate/amino acid composition and gene expression levels of various hepatic enzymes in rainbow trout (Oncorhynchus mykiss). Three isonitrogenous (470 g/kg crude protein) and isolipidic (18 g/kg crude lipid) diets were formulated as follows: Diet 1: control (30% FM); Diet 2:24% FM + 6% SCP and Diet 3:18% FM + 12% SCP. Each diet was hand‐fed to triplicate tanks containing 30 rainbow trout fingerlings (4.99 ± 0.20 g) for 9 weeks. Apparent digestibility coefficients of SCP for dry matter, crude protein, lipid and energy were 60, 80, 93 and 74% respectively. Growth performance (final weight: 69–71 g), feed conversion ratios (0.91–0.94) as well as whole‐body protein and amino acid composition were unaffected by diets. However, Diet 3 significantly increased whole‐body crude fat and energy. Fish fed the SCP‐based diets had significantly higher expression for carnitine palmitoyltransferase‐1b (CPT1b), fatty acid delta 6 desaturase (FADS6) and fatty acid elongase 5 compared to the control. Overall, the quality of the SCP was similar as FM. Therefore, this product could enlarge the portfolio of alternative protein sources that can be used in fish diets and thus open a new market opportunity for use of a new feed resource in the feed industry.     

Effect of dietary Curcuma longa on enzymatic and immunological profiles of rohu,Labeo rohita (Ham.), infected with Aeromonas hydrophila

The aim of this study was to evaluate the dietary dosages of turmeric that enhance immune response and disease resistance against the opportunistic pathogens Aeromonas hydrophila in Labeo rohita fingerlings. Hence, four different dosages of turmeric at 0.1, 0.5, 1.0 and 5.0 g kg^?1 feed were given to the fingerlings of L. rohita for 60 days at 4% body weight. At every 20‐day interval, different biochemical, haematological, enzymatic and immunological parameters of fish were evaluated. After 60 days, fish were challenged with A. hydrophila. The mortality (%) was recorded on the tenth day post challenge. Most of the immune parameters including lysozyme activity, superoxide anion production and serum bactericidal activity were significantly (P<0.05) higher on 60 days of feeding of 1.0 g of turmeric per kg of feed. Challenge study indicated 100% and 89% survivability in the group of fish fed with 5.0 and 1.0 g of turmeric per kg of feed respectively. Feeding of turmeric might have maintained long‐term protection in fish by elevating the nonspecific immune system such as Nitroblue tetrazolium (NBT), lysozyme and serum bactericidal activity. The result showed that turmeric at a dose of 1.0 g kg^?1 feed for 60 days provided the greatest protection to pathogen challenge.     

Effect of sub-lethal nitrite on selected haematological parameters in fingerling Catla catla (Hamilton)

A sub‐lethal nitrite toxicity trial was conducted using static conditions for a period of 96 h with fingerlings of Catla catla (21.4±3.6 g). Fingerlings were exposed to five concentrations of nitrite, that is, 1, 2, 4, 8 and 10.4 mg L^?1 and a nitrite‐free control to study changes in haematological parameters. Nitrite caused an increase in immature erythrocyte population (7–24%) in lower concentrations (0–4 mg L^?1) at 6 h while they were absent in higher concentrations. The total erythrocyte count was reduced at 6 h followed by an increase at 12 h with further reduction up to 96 h in all concentrations of nitrite. The 96‐h exposure resulted in 21.2–31.8% reduction in erythrocyte population in 1–10.4 mg L^?1 nitrite. The haemoglobin content decreased progressively with increasing nitrite concentrations as well as exposure periods. Total leukocyte count decreased initially at 6 h in all treatments followed by an increase after 12 h, signifying development of a protective response of the body to nitrite stress. Blood glucose decreased initially up to 24 h followed by an increase through 96 h. Serum protein level decreased continuously with increasing exposure period. The study revealed that exposure to nitrite caused changes in almost all the haematological parameters in the fingerlings depending on the concentration as well as exposure period. Nitrite being one of the important inorganic nutrients often recorded at higher levels in intensively cultured ponds, the present study highlights its adverse impact on fish and stressed the need for the management of this nutrient in culture ponds.     

Sex change in coral reef fish

Gonadal differentiation can take many forms in fish, ranging from gonochorism, where individuals directly develop as male or female and finally possess only testis or ovaries at sexual maturation, to hermaphroditism where the same individuals can produce mature male and female gametes at some time in their lives. Hermaphrodite fish are, thus, an excellent model for studying the plasticity of sex determination and differentiation in vertebrates. We have shown that sex steroids play a principal role in sex differentiation and sex change in fish. Our laboratory implements several fish models that undergo sex change from female to male or male to female or in both directions. In this review, we will briefly discuss recent advances in our understanding of the mechanism of sex change in coral reef fish.     

Cage‐pond integration of African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus) with carps

Cage‐pond integration system is a new model for enhancing productivity of pond aquaculture system. A field trial was conducted using African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus) in cages and carps in earthen ponds. There were four treatments replicated five times: (1) carps in ponds without cage, (2) tilapia at 30 fish m^?3 in cage and carps in open pond, (3) catfish at 100 fish m^?3 in cage and carps in open pond, (4) tilapia and catfish at 30 and 100 fish m^?3, respectively, in separate cages and carps in open pond. The carps were stocked at 1 fish m^?2. The cage occupied about 3% of the pond area. The caged tilapia and catfish were fed and the control ponds were fertilized. Results showed that the combined extrapolated net yield was significantly higher (P < 0.05) in the catfish, tilapia and carps integration system (9.4 ± 1.6 t ha^?1 year^?1) than in the carp polyculture (3.3 ± 0.7 t ha^?1 year^?1). The net return from the tilapia and carps (6860 US$ ha^?1 year^?1) and catfish, tilapia and carps integration systems (6668 US$ ha^?1 year^?1) was significantly higher than in the carp polyculture (1709 US$ ha^?1 year^?1) (P < 0.05). This experiment demonstrated that the cage‐pond integration of African catfish and Nile tilapia with carps is the best technology to increase production; whereas integration of tilapia and carp for profitability.     

Changes in soil and water quality at sediment–water interface of Penaeus vannamei culture pond at varying salinities

The health of the pond environment in shrimp farming is important for sustainable and profitable aquaculture, in which sediment–water interface is the most important influencing area. With this objective, the key parameters of water and sediment at the interface was studied in shrimp ponds with varying salinities and compared with the surface water and soil to understand the variations and the underlying causes. Total ammonia nitrogen and total alkalinity were higher in water at the interface as compared to surface water, whereas pH, nitrite, oxide forms of nitrogen (NO₃), phosphorus (PO₄) and oxidizing bacteria were observed to be lower. There was no significant difference with respect to macrominerals such as sodium, potassium, calcium and magnesium between water at the interface and surface. These changes are profoundly influenced by the uppermost 1.0‐cm thick sediment layer, where the maximum transition of nutrients takes place with the water column. The outcome of the study highlights the effectiveness of water sampling for metabolites at the interface, which reflects at an early stage any potential deterioration of pond environment, which will help the farmer to initiate timely mitigation measures.     

Fecundity and egg dimension of freshwater prawn Macrobrachium assamense peninsulare (Tiwari, 1958) from Rawasan stream of Garhwal,Central Himalaya,India

The population of the freshwater animal was declined day by day due to the degradation of habitat and use of illegal fishing methods, which cause a great threat to these aquatic animals. The main purpose of this study was to scrutinize the wild population and fecundity of freshwater prawn Macrobrachium assamense peninsulare in Rawasan stream to assess the health status of prawn. Fecundity was estimated by a random sampling of prawn from five selected sites of Rawasan stream. A wild collection of a total of 35 ovigerous females was collected for an interval of 2 years (August 2013–July 2015) from May to September month, that is breeding session. The maximum number of eggs was 102 eggs in an individual having 52 mm total length and 2.66 g body weight. It was concluded that fecundity of Macrobrachium assamense peninsulare was low, and egg dimension was larger than other closely related species of freshwater prawn. Fecundity of Macrobrachium assamense peninsulare showed a positive relationship with a total weight (R² = .71) and moderate with the total length (R² = .38). A strong correlation exists between egg mass and total weight, and between egg mass and total length of female (R² = .849 and R² = .77 at p < .05) respectively. The numbers of eggs, that is fecundity, in a female were increased with the body growth of an individual, which results in a linear graph.     

Nursery rearing of kalbasu, Labeo calbasu (Hamilton), at different stocking densities in outdoor concrete tanks

The growth and survival of kalbasu, Labeo calbasu, was evaluated at stocking densities of 5, 10 and 15 million spawn ha⁻¹ in nursery rearing in concrete tanks of 50 m², each for a period of 25 days. Survival of fry was density dependent and significantly higher (54.5%) at 5 million ha⁻¹ than those at 10 million ha⁻¹ (50.1%) and 15 million ha⁻¹ (46.9%). Similarly, growth and specific growth rate were inversely related to the stocking density and varied significantly among the three densities.     

Cryopreservation of silver barb Puntius gonionotus (Bleeker) spermatozoa: effect of extender composition,cryoprotective agents and freezing rate on their postthawing fertilization ability

The effects of extender composition, cryoprotectant concentration and freezing and thawing on the fertilization efficiency of cryopreserved spermatozoa of Puntius gonionotus were evaluated. Computer‐aided motility analysis of semen was conducted to check the suitability of spermatozoa for cryopreservation after mixing with different extenders and cryoprotective agents (CPAs). Extender‐4 with an osmolality 260 mOsmol kg⁻¹and pH 7.6 was used for the cryopreservation study. Among the CPAs, dimethyl sulphoxide (DMSO) was least toxic and more than 60% fertilization was achieved when used at 1.4 M at 0 °C for 10 and 30 min, whereas the toxicity of all CPAs to spermatozoa was evident when tested at 30 °C. Semen frozen at −16 °C min⁻¹ with 1.4 M DMSO showed 70% fertilization, which was significantly higher (P<0.05) than other freezing rates. Samples thawed at 35 °C water showed a fertilization rate comparable with that of fresh semen. Computer‐assisted semen analysis of fresh and frozen semen after thawing showed variations in different types of motility in spermatozoa and in their class. There was no significant difference in motility before or after cryopreservation; however, significant differences could be observed in the average path velocity (VAP), straight line velocity (VSL) and curve linear velocity (VCL). Semen of silver barb could be cryopreserved with extender‐4 by addition of 1.4 M DMSO to a final cryopreservation medium (MED 2) cooled at a rate of −16 °C min⁻¹, stored in liquid nitrogen (−196 °C) and utilized after thawing at 35±2 °C.