Many studies have been made about the physio‐logical effects of isolated chronic or acute stress. However, few studies have been made to assess the combination of both responses. The fish submitted to chronic stress may be subjected to an additional acute stressor. The aim of the present work was to evaluate the acute stress response in Nile tilapia Oreochromis niloticus (L.) previously subjected to chronic stress. For this, two experiments were performed. In the first experiment, the fish were subjected to chronic stress followed by an additional acute stress. In the second experiment, the fish were submitted only to an acute stress. The data showed that Nile tilapia fingerlings can adapt to chronic stress situations, and this decreases, but does not eliminate, their capacity to respond to an additional acute stressor. In both experiments, plasma cortisol levels reached a peak 1 h after administration of the acute stressor. In fish previously submitted to chronic stress, the highest concentration of plasma cortisol measured was 196 ng mL–1. This value was significantly different from the cortisol concentration obtained in the second experiment (267 ng mL–1) with non‐chronically stressed fish. The data also suggest that the chronic stress response can provoke a reduction in performance and growth rates compared with non‐stressed fish.

Several researchers have demonstrated that the stocking density and social interactions between conspeci®c ®sh have signi®cant in¯uence on stress response. Nile tilapia Oreochromis niloticus (L.) is a teleost ®sh with hierarchical organization. Nile tilapia ®ngerlings (20 6 5 g) were distributed in 30 tanks, 12 tanks with single ®sh and six with paired, groups of ®ve and 10 ®sh, respectively, and held in these 100-L plastic tanks for 60 days. Six tanks of single ®sh and three of other groups received an acute stressor on the 61st day. The control ®sh reared singly and in pairs showed a plasma cortisol concentration similar to that normally found as a resting cortisol concentration in other ®sh. One surprising result was the absence of the formation of a hierarchical dominant/submissive relation between the ®sh held in pairs. One possible explanation for this result is the volume of the tank, so that each ®sh exerted a territorial position in a corner of the tank, and the ®ghting and antagonistic encounters did not occur. In the ®veand 10-®sh groups, the resting plasma cortisol concentrations were higher, which may indicate a chronic stress response during the 60 days of the experiment, attributable to social stress. The application of an acute stressor signi®cantly increased the plasma cortisol levels in all stocking densities, with values reaching 135±298 ng mL ±1 . When comparing the stressed ®sh, the 10-®sh groups showed a more intense stress response in relation to the other groups.

No abstract

The jundia Ł (Rhamdia quelen) is a siluriform with great potential for aquaculture in South America. Fish oil is a raw material in diets for ¢sh. However, the ¢sheries that provide ¢sh oil have reached their limit of sustainability. Thus, the use of alternative sources for this ingredient is primordial. The aim of this study was to evaluate the performance and body composition of the jundia Ł fed with di¡erent sources of the vegetable oils. Jundia Ł (1.0 AE 0.2 g) were fed for 31 days with ¢ve isonitrogenous (37%) and isoenergetic (19 kJ g À1 ) diets, in which the following oils were added: 50 g kg À1 corn oil (CO), 50 g kg À1 ¢sh oil (FO), 50 g kg À1 linseed oil (LO), 33.4 g kg À1 ¢sh oil and16.7 g kg À1 linseed oil (1/3LO),16.7 g kg À1 ¢sh oil and 33.4 g kg À1 linseed oil (2/3LO). The performance did not show di¡erences between treatments. The ¢nal fatty acid pro¢le and n-3/n-6 ratio of the ¢sh were highly in£uenced by the diet. Fishfed diets with linseed and/or ¢sh oil showed superior n-3/n-6 ratios to the minimal recommended by the World Health Organization; whereas ¢sh fed diets with corn oil showed an inferior value. Albeit in the present study the commercial size of ¢sh was not attained, these results show a clear tendency. The desaturation/elongation capacity was evidenced, in this species, for the ¢rst time. Linseed oil can be utilized as a substitute for ¢sh oil in diets of jundia Ł without a¡ecting their performance and for producing goodquality ¢sh. However, more studies are necessary to con¢rm these results for commercial size. Performance of Jundia Ł fed on di¡erent dietary lipids R J Vargas et al. Aquaculture Research, 2008, 39, 657^665 r 2008 The Authors à AVIPAL, Porto Alegre, RS, Brazil. wIRGOVEL, Pelotas, RS, Brazil. zETNA, Canoas, RS, Brazil. ‰Giovelli Industria De Oleos Vegetais, Guaran|¤ das Misso ì es, RS, Brazil.zDigestible energy (kJ g -1 ) 5 (23.6 kJ g -1  %protein  0.9)1(39.8 kJ g -1  %lipid  0.85)1(17.2 kJ g -1  %carbohydrate  0.5) (Jobling 1983). kDigestible protein (DP) 5 0.9  %protein (Jobling 1983). HUFA, highly unsaturated fatty acid. Aquaculture Research, 2008, 39, 657^665 Performance of Jundia Ł fed on di¡erent dietary lipids R J Vargas et al. r 2008 The Authors Journal Compilation r 2008 Blackwell Publishing Ltd, Aquaculture Research, 39, 657^665 à Means (n 5 2) AE SD, with di¡erent letters in the same line are signi¢cantly di¡erent according to Tukey test (Po0.05). Only ¢sh of ¢nal treatments were including in the ANOVA. An SD 5 0.0 implies an SD of o0.05. wANOVA, one-way analysis of variance; CO, corn oil; FO, ¢sh oil; LO, linseed oil; SAT, saturated fatty acids; MUFAs, monounsaturated fatty acids; PUFAs, polyunsaturated fatty acids; HUFAs, highly unsaturated fatty acids. Aquaculture Research, 2008, 39, 657^665 Performance of Jundia Ł fed on di¡erent dietary lipids R J Vargas et al.

Fish polyculture is based on the assumption that each species has its own feeding niche and may increase the maximum standing crop of a pond by exploring a wider range of available food and ecological niches. In order to identify the better species ratio and to introduce jundia (JN) (Rhamdia quelen Quoy & Gaimard) and Nile tilapia (NT) (Oreochromis niloticus Linnaeus) in to the carp polyculture practiced in South Brazil, a 162-day experiment was conducted, in 12 250-m 2 earthen ponds (1.2 m deep).