Effect of light and feeding regimes on the daily rhythm of thermal preference in Nile tilapia (Oreochromis niloticus)

Fish thermal preference varies throughout the day by allowing them to develop their biological processes under the most suitable thermal conditions. This study aimed to determine the effect of light and feeding regimes on the daily rhythm of thermal preference in Nile tilapia (Oreochromis niloticus). For this purpose, fish were allowed to freely choose their preferred temperature with different lighting conditions (experiment 1) and feeding schedules (experiment 2) using a multichamber tank with a thermal gradient from 26 degrees C to 34 degrees C or at a constant temperature of 30 degrees C (control). Fish were exposed to three different photoperiod combination [Light:Dark (LD) cycle, inverted LD cycle (DL) and constant darkness conditions (DD)] and four different feeding schedules [random (RD), feeding in the mid-light phase (mid-light, ML) or the mid-dark phase (mid-dark, MD), and fasting (FT)] . Regardless of the feeding/fasting protocol, a daily rhythm of temperature preference was observed, which suggests the role of LD cycles as the main synchronizer. In all the experimental phases, tilapia chose higher temperatures at dusk and cooler temperatures at dawn. Persistence of the rhythm for DD and FT indicated that an endogenous pacemaker may drive this rhythm. In both experiments, when fish were placed in the experimental tank on a thermal gradient for the first time, fish exhibited a stress-induced hyperthermia (SIH) response, characterized by the selection of higher temperatures. Conversely, the fish fed at MD selected cooler temperatures at night than the fish fed at ML or RD. Finally, FT resulted in selecting temperatures below the thermal optimum. These findings provide new insights into ecological and evolutionary aspects of fish thermoregulation, and might be useful for designing feeding protocols, which could impact fish welfare and, therefore, optimize fish production.