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Background. Fish have photoperiod-dependent rhythms (circadian cycle of about 24 h), including the enzymatic secretion cycles, and this information has not been available for Nile tilapia, Oreochromis niloticus (Linnaeus, 1758). The knowledge on the digestive enzymes in Nile tilapia may have practical implications in fi sh nutrition. The aim of the presently reported study was to determine the circadian cycle of digestive enzyme production of Nile tilapia, in fasting and feeding regimes, in hope to receive information that might improve feeding schedules of this fi sh in aquaculture. Materials and methods. In juvenile Nile tilapia the circadian cycle of concentrations of total soluble protein, protease, pepsin-like, trypsin, chymotrypsin, amylase, and lipase were determined. The baseline (fasting) and feeding conditions (ad libitum) were sampled and monitoring every hour for 24 h. Results. The basal peak of enzyme activity in the intestine occurred at 18:44 h for amylase, at 19:57 h for proteases, and 20:29 for trypsin. The minimal activity for most enzymes, appeared between 4:51 h (amylase) and 10:13 h (lipases). In the feeding treatment (ad libitum), stomach activity (pepsin-like) had maximal activity at 20:06 h and minimal activity 05:46 h. Intestinal amylase activity covered an extended period of low enzymatic activity beginning at the 05:46 h and ending at 12:59 h. The peak of digestive enzyme activity occurred within 18:44–20:29 h. In general, secretion of digestive enzymes was positively stimulated by food, for all enzymes assayed. Conclusion. Nile tilapia has a higher digestive enzyme activity at night than during the day. Knowledge of the circadian cycle of digestive enzymes, and modifi cations initiated by food, is useful to establish feeding times. If feeding schedules are adjusted to coincide with maximum natural peaks, feed effi ciency will increase, which will be refl ected in weight gain of the fi sh and provide more profi table yields for aquaculture.

acta ichthyologica et piscatoria

Circadian cycle of digestive enzyme production at fasting and feeding conditions in Nile tilapia, Oreochromis niloticus (Actinopterygii: Perciformes: Cichlidae)