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Lateralization (eye preference, preference of one side over the other)was studied in Nile tilapia (Oreochromis niloticus), stocked in the laboratory in order to examine/determine how multiple tasks are performed simultaneously for their survival, tenacity, responses, predator detection, etc. In this study, mirror and dyadic tests were used to determine, and compare the lateralization and aggression levels of Nile tilapia with its mirror image, an opponent and a potential natural predator. Atotal of 120 apparently healthy mixed-sex Nile tilapia (mean plus or minus SE standard length 130.5 plus or minus 0.3mm and weight 89 plus or minus 0.1g)were used. Results showed that individuals showed more aggression towards (p = 0.001) mirror images than they did towards opponents in the dyadic test. Left and right eyes used in both tests showed individual differencesin the levels of lateralization. The individuals in the dyadic test used their left eye as much as (P= 0.83) they used their right eye in viewing their opponents. However, in the mirror test, their left eye was used more than their right eye when viewing opponents (P less than  0.001), so they are left eye biased. Furthermore, test fish were found to also use their left eye more (p less than 0.001) than they used their right when viewing a predator.

Keywords:Lateralization, aggression, mirror test, dyadic tests, eye preference, Nile tilapia