Food resource use by juveniles of the endangered sleeper Eleotris oxycephala in the Sagami River system, Japan

To ascertain the early food resource use of the endangered sleeper, Eleotris oxycephala, the food habits of juveniles were studied in a major stream (which received treated sewage discharge and had a low level of vegetation cover) and a tributary (which received little artificial inflow and had extensive vegetation cover) in the Sagami River system, eastern Japan in November 2015 (winter), May 2016 (spring), and August/September 2016 (summer). The stomach content compositions of 174 individuals and the stable isotope ratios of 30 individuals and their prey animals were investigated along with the species composition of prey animals in the environment. Various aquatic animals, including fish eggs, mollusks, crabs, palaemonid shrimps, atyid shrimps, isopods (asellids), chironomid larvae, and dragonfly larvae, were present in juvenile E. oxycephala stomach contents. In the tributary, they mainly preyed on atyid shrimps, which were abundant in the environment, with lower feeding intensity during winter season. In the major stream, throughout the year (including winter), they fed mainly on chironomid larvae, which were smaller and slower than atyid shrimps, and so easily caught, even by small recently recruited juveniles. There were abundant chironomid larvae in the major stream environment, where the water temperature seemed warmer and more eutrophic than in the tributary because of the treated sewage discharge, particularly in winter. These results indicated that feeding intensity differed between sites, particularly in winter, and this difference might be associated with differences in prey availability (i.e., chironomid larvae) or warm-water conditions. Moreover, juveniles could shift their use of food resources, in response to differences in prey availability. Thus, to conserve E. oxycephala in terms of early food resource use, it is necessary to conserve various aquatic prey animals such as atyid shrimps and chironomid larvae and their habitats.