Introduction: Seasonal and tidal variations in nutrient concentration and water quality were investigated in the western Sundarbans of Bangladesh during the post-monsoon, winter and monsoon seasons during 2010-2011. Methods: Water collected from the surface, middle and bottom layers at six locations of the Kholpetua-Arpangashia river system during high and low tides was analyzed for temperature, salinity, pH, dissolved oxygen (DO), total dissolved solids (TDS), electrical conductivity (EC) and dissolved nutrients (NO 3-N, PO 4-P, SO 4 and NH 3-N). Results: The study revealed that nutrient concentrations were influenced by seasonal changes. Mean nutrient levels during post-monsoon, winter and monsoon seasons, respectively, were in the following ranges

This study evaluates the effect of probiotics, Zymetin, on the immune efficiency of Macrobrachium rosenbergii juvenile against pathogenic Vibrio spp. and Aeromonas spp. The experiment was conducted in glass aquaria with same level of feeding under different treatments, that is, negative control (Cn), positive control with Vibrio spp. (Cv) and Aeromonas spp. (Ca), prawn juveniles fed with probiotics (Zymetin) at 5 g/kg of feed but without pathogen (T1), probiotic fed prawn challenged with Vibrio spp. (T2) and Aeromonas spp. (T3). The results demonstrated that T1 showed higher survival rate, total haemocyte count, non-granular haemocyte count, phagocytic activity, and clearance efficiency compared to other treatment groups. In contrast, decreased number of small and large granular haemocyte was observed in T1.Despite that, THC was found to be significantly different (p < 0.05) among all the controls and lowest was in Cv, followed by Ca. Besides, phagocytic activity and clearance efficiency against Vibrio spp. and Aeromonas spp. decreased significantly in Cn, Ca, and Cv fed M. rosenbergii juvenile, while the values were found to be higher in T1 followed by T2 and T3, in that order. In addition, the challenge test showed an increasing trend of total and beneficial bacterial density as well as declining trend of some harmful bacteria in the water and gut of prawn in all the groups tested (p < 0.05). K E Y W O R D Sclearance efficiency, Macrobrachium rosenbergii, phagocytic activity, probiotics wileyonlinelibrary.com/journal/are Aquaculture Research. 2019;50:644-657. 2 | MATERIALS AND METHODS 2.1 | Pathogen isolation, culture, and biochemical confirmation Before conducting the challenge test, the pathogenic bacteria (i.e., Vibrio spp. and Aeromonas spp.) were isolated from four diseased prawns which displayed typical symptoms of inactivity, poor growth leading to weight loss, anorexia, and frequent mortality. All the diseased GFP were collected from the farmer's pond of Dumuria, Khulna, Bangladesh. Infected prawns were brought to the Fish Pathology Laboratory of Fisheries and Marine Resource Technology AZAD ET AL.| 645

The blue mussel (Mytilus edulis) is known as a robust bivalve species, although its larviculture appears to be highly susceptible to diseases. In this study, we isolated 17 strains from induced mortality events in healthy wild-caught blue mussel adults and demonstrated that they caused between 17% and 98% mortality in blue mussel larvae in a newly developed, highly controlled immersion challenge test model. Eight of the isolates belong to the Splendidus clade of vibrios, while the other isolates belong to the genus Photobacterium. The genomes of the most virulent Vibrio isolate and the most virulent Photobacterium isolate were sequenced and contained several genes encoding factors that have previously been linked to virulence towards bivalves. In vitro tests confirmed that all 17 isolates were positive for these virulence factors. The sequenced genomes also contained a remarkably high number of multidrug resistance genes. We therefore assessed the sensitivity of all isolates to a broad range of antibiotics and found that there were indeed many strong positive correlations between the sensitivities of the isolates to different antibiotics. Our data provide an ecological insight into mass mortality in blue mussels as they indicate that wild mussels contain a reservoir of pathogenic bacteria.

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