Effect of background color on the social stress of Nile tilapia (Oreochromis niloticus)

It was studied the behavior and physiologic answers of juvenile of Nile tilapia, Oreochromis niloticus, submitted the different backgrounds colors and social situations. The animals were maintained isolated in fishbowls covered with colored paper-card, composing five treatments (black, green, brown, blue and white). Through weekly filmings and in different and alternate stages (isolation and presence of a mirror), it enrolled the following parameters: distribution in the column of water, coloration, motility, agonistic behaviors, position of the dorsal fin and posture. For quantification of the glucose, triglycerides, total proteins and cortisol levels, were collected samples of blood after each filming. The obtained averages were analyzed statistically through the no-parametric Kruskal-Wallis method. The fish maintained in the black and green background presented low frequencies of agonistic behaviors, while those maintained in the white background, high frequencies, even so with low numbers of the alert pattern and not altering its motility. Animals submitted to the brown and blue colors presented the highest frequencies of agonistic behaviors, and larger motility. The fish stayed in all the treatments with the clear coloration, occupying, with larger frequency, the bottom of the column of water. Significant differences were not observed for the glucose, triglycerides and total proteins concentrations among the treatments, even so it was obtained a high level of cortisol for the animals maintained in the blue and brown backgrounds when submitted to the reflection of the own image in mirror. These results showed that there is influence of the background color on the social stress, in particular in the agonistic interactions among individuals of the same specie and in the concentration of the hormone cortisol. It was ended that the green and black colors are recommended to the maintenance of the species, for they liven up the agonistic interactions and the stress, while the brown and blue should be avoided for they stimulate these answers.