Oreochromis niloticus has been introduced worldwide into freshwater reservoirs and represents an alternative food for local populations. Two annual marks on fish scales have been reported for fish in these environments, one of them is allocated to reproduction, while the other is thought to be formed following low temperatures, a reduction in oxygen concentration and/or high densities, among others. In order to explore the causes of this second ring, we designed an experimental study to analyze the influence of four factors: a reduction in food amount, an increase in density, a decrease in oxygen concentration and temperature. The experiment was carried out using five 215 L plastic tanks, each one divided into two sections, so as to have treatment replicates (each with 20 juveniles). The control conditions were optimal for the species: 5% of body weight in food, a density of 3 fish/L, a total oxygen saturation of 4.98 ± 0.5 mg/L, and a temperature of 25.0 ± 1.0oC; the treatments varied as follows: food of 2.5% of body weight, density of 6 fish/L, oxygen of 2.8 ± 1.2 mg/L and a temperature of 19.6 ± 1.0oC. Fish responses to stressors were estimated through variations in length, weight and inter-circuli spaces or circuli spacing in fish scales. Every third day it was measured the concentrations of ammonium, phosphates and nitrates with a photometer. Three stages for length/weight fish measurements and scales sampling were considered: 1) Initial stage: before experiment started, 2) Final stage: at the end of the experiment, and 3) Compensatory stage: when all the experimental treatments were adjusted to the Control conditions; the experiments lasted for three weeks. The results indicated that all the treatments caused a reduction in length, weight and inter-circuli spaces when compared to the controls (6.7 ± 0.5 cm; 5.2 ± 1.2 g; 5.4 ± 0.5 mm, respectively), but only the variables of temperature and food resulted significant (5.6 ± 0.4 and 5.8 ± 0.5 cm; 3.4 ± 0.7 and 3.3 ± 0.9 g; 3.4 ± 0.3 and 4.1 ± 0.6 mm for temperature and food treatments, respectively) (P < 0.05). When treatments were reversed to the control conditions for another three weeks compensatory growth was observed in the oxygen and density treatments, but not in the same way for the temperature and food treatments. We concluded that inter-circuli distances are a good indicator to stress periods in this species.

Keywords: Oreochromis niloticus, scales marks, growth, circuli spacing, juveniles, experimental study