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A 90‐day experiment was conducted by rearing 1020 prawn juveniles (0.54 ± 0.03 g) in water supplemented with three different concentrations of probiotic bacteria viz. T1 (107 cfu L−1), T2 (108 cfu L−1), T3 (109 cfu L−1) and the control (C) (unsupplemented water), to evaluate probiotic effect of Lactobacillus plantarum. In the present study, the growth parameters (WG%, SGR) and feed utilization parameters (FCR, PER) significantly improved (P < 0.05) in T3. The growth and feed utilization parameters though improved marginally in T1 and T2, the difference was not significant (P > 0.05) compared to the control. The gastro‐intestinal Lactobacillus sp. count increased significantly (P < 0.05) in all the treatment groups, whereas the decrease in harmful bacteria was significant (P < 0.05) in T3 compared to the control. Similarly, the Lactobacillus sp. count in culture water increased significantly (P < 0.05) in all the experimental groups, whereas the decrease in harmful bacteria was significant (P < 0.05) in T2 and T3. The immune parameters (THC, PO and RB activity) and clearance efficiency significantly improved (P < 0.05) in T3 with concurrent decrease (P < 0.05) in cumulative mortality against Aeromonas hydrophila challenge. However, water quality did not improved (P > 0.05) in any of the treatment groups. The results indicate that Lactobacillus plantarum at a minimum concentration of 109 cfu L−1 could be used as water additive to confer its probiotic effect in prawn, Macrobrachium rosenbergii. Moreover, future studies with higher probiotic concentrations should be conducted for its efficient commercial scale field application.

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a b s t r a c tDecapterus russelli (Indian Scad) is an important pelagic carangid distributed on both east and west coast of India. Despite its wide distribution, the stock structure of the species is not well known. The present study was conducted to investigate stock structure of D. russelli, based on body shape morphometrics using truss network system. A total number of 360 samples of the species were collected from two centres, Digha and Visakhapatnam in Bay of Bengal from east coast and on the west coast from Mumbai and Cochin in Arabian Sea. A truss network was constructed by interconnecting 11 landmarks to form a total of 23 distance variables extracted from digital images of samples using tps Dig2 and PAST software platforms. The transformed truss measurements were subjected to factor analysis and classification by cross-validation of discriminant analysis. Factor analysis showed meaningful loading of the middle portion, the portion below the second dorsal fin, above anal fin, and the caudal portion on first and second factor, respectively. The factor analysis revealed the existence of two morphologically different stocks of D. russelli between east and west coast of India. The discriminant analysis was conducted by the combination of the truss distances that loaded on Factor-1 and Factor-2. The measurements that belonged to the middle portion and caudal portion of the body produced minimum misclassification rate of 5% between the coasts; whereas, the misclassification was 28% for all the four stocks. The misclassification was higher between the stocks within the coast. The high rate of misclassification observed within the coast is probably the result of no demarcation in fishing area. The occurrence of a separate stock on each coast may be the result of different physical and ecological condition of Bay of Bengal and Arabian Sea.