Intertidal migration of the four-eyed fish Anableps anableps in North Brazilian mangrove creeks

The movement patterns of mangrove fish during tidal cycles are virtually unknown, yet needed to understand fish habitat use. The intertidal migration of the four-eyed fish Anableps anableps L. (Anablepidae, Cyprinodontiformes), a surface-swimming species, was observed along 2 large mangrove-lined creeks in North Brazil. The number, direction and size of the fish crossing fixed line transects were recorded at 5 min intervals using visual surveys during daylight at spring, mid-and neap tide cycles. Migration took place in a surge and was predictably structured and directly related to water level, current speed and direction, and recurred independent of creek and tide. The fish rode the early flood tide upstream towards the creek heads. After the ebb current peak, they returned in a surge along the same pathways. The intertidal distances travelled ranged from 0.7 to > 2 km per tide. Fish density maxima occurred at early flood and late ebb tide, with more fish swimming alone during flood tide and largest group sizes during ebb tides. At neap high tides, fish aggregated in fewer accessible creek heads. When spring tides inundated wider creek areas and made additional creek heads available, maximum fish dispersal occurred. Small fish underwent migrations at shallower water depths than larger fish, thereby optimizing foraging and refuge time. Creek heads were prime feeding grounds, and intertidal creeks were the pathways connecting trophic flows of A. anableps, highlighting that in the management of mangrove ecosystems, complete drainage systems deserve protection. Striking similarities in tidal migrations of resident salt marsh species suggest that equal evolutionary pressures resulted in universal migratory strategies of estuarine resident taxa.