The effects of stocking density and social interaction on acute stress response in Nile tilapia Oreochromis niloticus (L.) fingerlings

Abstract

Several researchers have demonstrated that the stocking density and social interactions between conspecific fish have significant influence on stress response. Nile tilapia Oreochromis niloticus (L.) is a teleost fish with hierarchical organization. Nile tilapia fingerlings (20 ± 5 g) were distributed in 30 tanks, 12 tanks with single fish and six with paired, groups of five and 10 fish, respectively, and held in these 100-L plastic tanks for 60 days. Six tanks of single fish and three of other groups received an acute stressor on the 61st day. The control fish reared singly and in pairs showed a plasma cortisol concentration similar to that normally found as a resting cortisol concentration in other fish. One surprising result was the absence of the formation of a hierarchical dominant/submissive relation between the fish held in pairs. One possible explanation for this result is the volume of the tank, so that each fish exerted a territorial position in a corner of the tank, and the fighting and antagonistic encounters did not occur. In the five- and 10-fish 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 significantly increased the plasma cortisol levels in all stocking densities, with values reaching 135–298 ng mL− 1. When comparing the stressed fish, the 10-fish groups showed a more intense stress response in relation to the other groups.