The innate immune system of fish is considered to be the first line of defence against a broad spectrum of pathogens and is more important for fish as compared with mammals. Lysozyme level or activity is an important index of innate immunity of fish and is ubiquitous in its distribution among living organisms. It is well documented that fish lysozyme possess lytic activity against both Gram‐positive bacteria and Gram‐negative bacteria. It is also known to be opsonic in nature and activates the complement system and phagocytes. It is present in mucus, lymphoid tissue, plasma and other body fluids of freshwater and marine fish. It is also expressed in a wide variety of tissues. Lysozyme activity has been shown to vary depending on the sex, age and size, season, water temperature, pH, toxicants, infections and degree of stressors. Here, we review our current understanding of different types of lysozyme and their expression and its role in fish innate immune system.

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An experiment was conducted to study the stress mitigation and growth enhancing role of dietary L-tryptophan (TRP) under thermal stress in rohu, Labeo rohita fingerlings for 45 days. Seven hundred and twenty fishes were distributed in three major groups that are ambient temperature (26 °C), 34 and 38 °C in triplicate following a complete randomized design. Acclimation of fishes to 34 and 38 °C over average ambient temperatures were carried out at 1 °C/day. Each group was fed with a diet supplemented with 0, 0.36, 0.72 or 1.42 % L-TRP. Results showed that blood glucose and serum cortisol level were found to be significantly higher (p < 0.05) in the higher temperature groups than the ambient temperature group. Similarly, aminotransferase, lactate dehydrogenase, malate dehydrogenase, CAT, superoxide dismutase activities were found to be significantly higher (p < 0.05) in the control groups (0 % L-TRP) and decreasing activities of these enzymes were observed with the increasing level of dietary L-TRP. In different temperature groups, L-TRP-supplemented groups were found to have higher (p < 0.05) growth, RGR and PER. The results obtained in the present study indicate that dietary L-TRP mitigates thermal stress and enhances growth. From the present study, we can conclude that dietary supplementation of L-TRP at the 0.72 % level in the diet is found to be optimum to reduce thermal stress even up to 38 °C in rohu, L. rohita. The baseline data obtained here could be useful for the farmers to formulate feeds to culture the fish in different agro-climatic zones.

The study was undertaken to determine the modulation in innate immune response of rohu (Labeo rohita) during experimental freshwater lice Argulus siamensis infection. Results showed that serum a-2 macroglobulin (a-2M) activity, ceruloplasmin level and alternative complement activity were signi¢cantly (Po0.05) lower in ¢sh at di¡erent degrees of lice infection in comparison with uninfected control. No signi¢cant di¡erence (P40.05) in haemagglutination titre was observed in ¢sh with low-and high-degree lice infections as compared with uninfected control. The serum lysozyme level was signi¢cantly (Po0.05) lower in low degree of lice infection as compared with control ¢sh. The total serum antiprotease, myeloperoxidase activity and total protein level were not signi¢cantly di¡erent (P40.05) in di¡erent degrees of lice-infected ¢sh with respect to the control ¢sh. The study indicated that A. siamensis infection modulated the immune system of rohu by suppressing the a-2M, serum complement activities and ceruloplasmin level and through induction of stress response. The baseline data obtained in the present study have tremendous importance in understanding the susceptibility of rohu to di¡erent degrees of parasitosis and might be useful in controlling this dreaded ectoparasitic infection in ¢sh.

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This study was conducted in indoor, aerated plastic tubs to estimate the digestibility of dry matter and nutrients by the carps, Labeo fimbriatus (Bloch, 1795) and Cyprinus carpio Linnaeus, 1758, from three feed ingredients viz., azolla, soybean meal and silkworm pupa incorporated at levels ranging from 0-40% in a diet comprising equal amounts of rice bran and groundnut oil cake using crude fibre as the marker. Activity of the major digestive enzymes was also estimated in the gut of fish at the end of the experimental trial. Utilisation of azolla protein was comparable with that of control diet up to 20% incorporation in both L. fimbriatus and common carp. Major nutrients from soybean were better utilised up to 40% incorporation by the two species. Protein digestibility from silkworm pupae increased at 20% incorporation and decreased at 40% in L. fimbriatus, while fat and nitrogen-free extract digestibility was higher at 20-40% incorporation. In common carp, higher digestibility of protein and fat was recorded at 10-30% inclusion levels of silkworm pupa.

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