Behaviour and Circadian Rhythm of the Fish Bathygobius Soporator Valenciennes (gobiidae) under the Influence of Environmental Salinity and Temperature

The behavioural patterns and their circadian rhythms may be adaptive to the peculiar environmental conditions of subtropical brackish waters where Bathygobius soporator Valenciennes, 1837 live. Adult fish were caught at the southern Brazilian coast from mangrove rivers and rocky shores in a bay, where temperature and water salinity vary during the day and through the year. Observation on the behaviour of the animals was undertaken in salinity 8.5ppt, 17.0ppt, 25.5ppt and 34.0ppt, each one in temperatures of 18°C and 28°C. Temperature and salinity affect the frequency and intensity ofsome ofthe behavioural events, more than its pattern or rhythm. Swimming is rare, decreasing along the day and with temperature increase, being even lower at low salinity; aggressiveness is the highest in the morning being not affected by temperature, but by salinity, being higher the higher it is; territory defence decreases along the day and is lower at high temperature and extreme salinities; fish hide more at high temperature and with the decrease ofsalinity, but this is not rhythmical; a higher proportion offish rest in vertical position when salinity and temperature are high, increasing slightly at the beginning ofthe afternoon; respiratory frequency increases with temperature, salinity and in the afternoon; the colour of the fish is mainly light with spots in all hours of the day and in all temperatures and different levels ofsalinity, but with a tendency ofthe presence ofsome dark fish during the morning and some light ones in the afternoon, showing a higher valiability of colours at low temperature and extreme salinities. Besides temperature, salinity and light, feeding seems to be one of the determinant factors for the performance of the typical behaviour of B. soporator.