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The effects of stress on body cortisol and reproductive performance of guppy, Poeciliar eticulata, was evaluated on  90 immature female fish (0.29 ± 0.03g body weight). The experiment, carried for 30 days in three replications, included: the control (without any manipulation), handling stress (daily holding in a net for 2 minutes) and feeding with the cortisol-treated diet (500 mg per kg food). After this period, reproduction was induced by adding male fish to the remaining females in each tank. No significant difference was observed in growth rate among the treatments (p=0.001). The mortality rate and body cortisol content in both handling and cortisol-fed treatments were significantly higher than that in the control group (p=0.000). The minimum and maximum number of embryos was observed in the control and cortisol-fed  groups, respectively. Our results indicated that such manipulations have negative effects on survival and reproductive efficiency and change whole-body cortisol content. Therefore, the measurement of whole-body cortisol content could be a suitable criterion for determining physiological condition in small laboratory fishes.

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The effects of dietary nucleotide (NT) were evaluated on some serum biochemical parameters and acute stress responses of the catfish (Pangasianodon hypophthalmus). Five experimental diets including 0, 0.25, 0.5, 0.75, and 1% NT were supplied to catfish fry for 10 weeks. At the end of the experiment, fish fed the control and 1% NT diets were subjected to handling and crowding stress. The results showed that the fish supplied with 0.25-0.75% NT had a lower level of lactate dehydrogenase (LDH) (P<0.05) while other serum enzymes including alkaline phosphatase (ALP), asparate transaminase (ASP), and alanine transaminase (ALT) were not significantly reduced by NT inclusion in the diet (P>0.05). The fish which received the highest doses (0.75 and 1%) of NT exhibited higher levels of triglyceride than the other groups (P<0.05) while they showed no significant differences in other biochemical parameters including total protein, albumin, globulin, cholesterol, and glucose (P>0.05). To investigate stress responses, cortisol (primary response), serum glucose, and serum ion concentrations including sodium, potassium and calcium (secondary responses) were measured. The results showed significant fluctuations in all the tested components during the sampling intervals for up to 48 h post-stress; the exceptions, however, were glucose in the group on the 1% NT diet as well as serum cortisol and calcium levels in those supplied with the control diet. Based on the results obtained, it may be concluded that dietary NT can improve liver function in iridescent catfish, but it has no obvious positive effects on other serum biochemical parameters and stress responses.