ENVIRONMENTAL-FACTORS INFLUENCING THE ZONATION AND ACTIVITY PATTERNS OF A POPULATION OF PERIOPHTHALMUS-SOBRINUS EGGERT IN A KENYAN MANGROVE

The activity rhythms and zonation of a population of Periophthalmus sobrinus were studied along the muddy mangrove banks of the Tana river delta (Kenya). A transect, perpendicular to the water channel, was subdivided into sections 1 m in width parallel to the river bank. The number of surface-active mudskippers and their zonation was recorded over several tidal cycles. Fishes found resting on their nests were also recorded. The observations were carried out over two periods of 48 consecutive tidal hours corresponding to a spring and a neap tide, Environmental variables such as tidal level, air temperature and relative humidity were also recorded. Periophthalmus sobrinus was active both diurnally and nocturnally and during spring tide all the observed mudskippers were active on the mud surface. At neap tide more than half of the total number of animals sighted were found resting on their nests. During this synodic phase, daytime sightings decreased drastically because the mudskippers took refuge inside the nests. Since P. sobrinus normally tends to avoid water, its mean hourly zonation at spring tide was more variable than at neap tide. Differences in mean hourly zonation were also found between night and day of the spring tide. Activity patterns and zonation of the mudskippers were directly influenced by the synodic and tidal cycles and depended more on environmental factors such as air temperature and relative humidity than on the diel light cycle.