Fish do not feed at any time of the day and on whatever food itemthey encounter in the wild, but they show
daily rhythms of feeding activity and dietary selection. The aim of this research was to investigate the daily
rhythms of behavior in Nile tilapia self-fed with plant-based diets supplemented with different levels of
exogenous phytase (an enzyme that hydrolyzes non-digestible phytate and improves the nutritional value
of the diet). To this end, ten males were individually kept in 50-L tanks, each equipped with two selffeeders
and one infrared photo-cell connected to a computer. The selection factors investigated were the
level of phytase supplementation (0 IU kg−1 vs 1500 IU kg−1; 1500 IU kg−1 vs 4000 IU kg−1) or sodium
phytate (1% phytate vs 1% phytate+1500 IU kg−1 phytase). The results revealed that 66.7% of total daily
activity occurred during the day, while feeding was strictly nocturnal, with 93.0% of the daily food demands
occurring at night. Tilapia preferred the diet with 1500 IU kg−1 phytase rather than the control or
4000 IU kg−1 diets. When exogenous sodium phytate was added to the diet, tilapia preferred the phytase
diet. In conclusion tilapia self-feed at night (although locomotor activity was mostly diurnal) and chose
plant-diets containing phytase, which should be taken into account when designing feeding strategies and
practical diets for tilapia aquaculture.