Seasonal Diet Shifts and Overlap Between Two Sympatric Catfishes in an Estuarine Nursery

Abstract

This study describes the seasonal feeding habits of different size classes of Cathorops spixii and Cathorops agassizii along an estuarine ecocline and the food overlap when different size classes occur together. These species were essentially zoobenthivorous, feeding mainly on Polychaeta Nereis sp., Copepoda Pseudodiaptomus acutus, Ostracoda Asterope sp., Gastropoda: Littorinidae, and Bivalvia Mytilus sp. and Anomalocardia brasiliana. However, during their life cycle and between different habitats and seasons, their trophic guild can change to zooplanktivore. The competition for resources was observed among C. spixii and C. agassizii, but was significantly reduced due to the seasonal diference in habitat use by different ontogenetic phases in the main channel of the estuary. The food niche separation was strongly influenced by environmental fluctuations, principally of salinity, resulting from rainfall and river inflow. High abundance of some preys, such as P. acutus (all estuary), Asterope sp. (upper and middle estuary), and Littorinidae (upper and lower estuary), could influence the significant diet overlap, principally during the rainy season, and call for more detailed studies of the benthic community structure. Moreover, dietary overlap was observed mainly between smaller sizes (intraspecific and/or interspecific) or between corresponding ontogenetic phases (interspecific), suggesting some differentiation in the diet in relation to the size class. Differences in prey type and size between the different ontogenetic phases of these ariids, principally among juveniles and adults, could be related to the size of the mouth, since adults are able to successfully capture larger preys or larger quantities of particular items.