Fish welfare and biological rhythms: time to regulate

In this article we introduce the concept of biological rhythms in fish welfare and critically discuss a neglected aspect: time-dependent issues. First we should consider that fish, like most living organism s , posses a clock that drives rhythmic biological functions. Therefore, fish stress responses are strongly affected by the time of stressor exposure (e.g. day/night) according to their daily pattern of behavior (diurnal/nocturnal). Furthermore, drugs affect fish differently depending of time of administration. For instance, commonly used substances such as anesthetics may have strikingly different effects depending on the time of day. MS-222 -the most widely used fish anesthetics- has fastest effects and higher toxicity when applied during daytime, but it is much less toxic at night. Surprisingly, no attention has been paid to this time-dependent factor when establishing dosages and protocols for fish anesthesia and euthanasia. In summary, there is a need to take into account properly the role of biological rhythms when discussing fish welfare issues and regulations by the Institutional Animal Care and Use Committee ( IACUC ) .