Altered photoperiod affects behaviour in a key fish grazer, the thinlip grey mullet, Chelon ramada.

The aim of this study is to address experimentally the immediate impacts of an alteration of the photoperiod, simulating an extreme artificial light pollution, on the behaviour and stress of a key grazer fish common in temperate coastal ecosystems in Europe, the thinlip grey mullet, Chelon ramada. Fish behaviour, assessed by total distance travelled, swimming speed, distance between individuals in fish groups and angular velocity analysed over 6 periods of 5-min slots during 24 h, was shown to vary depending on the photoperiod conditions and time. During the 5-min sequences analysed during the day, fish in the control photoperiod condition (alternate photoperiod, AP) swam slower, travelled less distance and were closer to their conspecifics than fish in the altered conditions [direct or indirect continuous photoperiod (iCP), dCP and iCP, respectively]. For the 5-min sequences analysed at night, fish exposed to direct continuous lighting (dCP) swam significantly faster, hence travelled a longer distance and further apart than fish exposed to iCP or alternate normal photoperiod (AP). Angular velocity was similar for all conditions. Exploratory cortisol measures conducted in the water revealed lower quantities in the iCP condition than in the AP and dCP conditions. Although based upon an experimental design exaggerating the natural photoperiod alteration as compared with most real-life situations, results obtained highlight the importance of addressing behavioural responses of organisms at the individual level to tackle the global and specific impacts of diurnal cycle alteration by artificial light in urbanised aquatic ecosystems. This study is also the first to report on the behavioural response of a common wild species playing a key role in the functioning of aquatic ecosystems to a widespread stress in coastal environments.