Daily rhythms of digestive physiology, metabolism and behaviour in the European eel (Anguilla anguilla)

Abstract

Circadian rhythms are common in a wide variety of physiological processes. Our knowledge of biological rhythms in fish can be exploited to improve some key aspects of aquaculture such as feed optimization and fish welfare. The aim of this paper was to investigate daily rhythms in behavioural and physiological variables of the European eel. To this end, we first analysed the daily rhythms in locomotor activity measured by means of infrared photocells and feeding activity by means of self-feeders. The eels were then submitted to scheduled feeding (single meals once a day at a fixed time), and the levels of cortisol, glucose and digestive enzymes were analysed. Locomotor activity was predominantly nocturnal (63 % of daily activity occurring at night) with peaks located around light transitions, whereas feeding activity was less clearly defined. Cortisol and glucose showed daily rhythms, with acrophases located in the middle and at the beginning of the light phase, respectively, which should be carefully considered when using these parameters as welfare indicators. Significant variations were found during the day in acid protease levels, which increased in anticipation of mealtime. Taken together, these results describe for the first time a picture of the circadian physiology of the European eel, providing hints to improve feeding methods and welfare assessment.