Foraging behavior of yellow-phase Japanese eels between connected fresh- and brackish water habitats

Abstract

Japanese eels (Anguilla japonica) utilize a broad range of habitats along the marine-freshwater ecotone during their growth phase in inland waters. This study aimed to analyze the foraging behavior of yellow-phase Japanese eels in connected fresh- and brackish water habitats and to connect foraging behavior and habitat use through the analysis of carbon (δ13C) and nitrogen (δ15N) stable isotopes. Stomach contents of eels collected in fresh- and brackish waters were analyzed to identify food sources. Values of δ13C and δ15N were analyzed in eels and their potential food sources, and used to predict the recent foraging patterns of eels in the Matsukawa-ura, a brackish water lagoon, and in three freshwater tributaries. Eels preyed on the benthic river community and the mudflat community of the lagoon. Gobiid fishes were found to be an important food source for eels in fresh- and brackish water habitats, while Japanese mitten crabs (Eriocheir japonica) and shore crabs (Hemigrapsus spp.) were major prey in fresh- and brackish waters respectively. δ13C values of potential eel prey differed significantly between fresh- and brackish waters and were used to classify three recent patterns of foraging by the 73 eels included in this study: 1) freshwater foraging, 2) brackish water foraging, and 3) multiple-habitat foraging. The data suggested that some eels recently or frequently moved between fresh- and brackish water habitats while others demonstrated a higher fidelity to either fresh- or brackish waters. Isotopic characteristics of prey in the respective foraging habitats revealed the plasticity of habitat use of yellow eels in the study area. This study demonstrates an integrated approach to study habitat use and foraging behavior of eels. Furthermore, the study underlines the need to consider freshwater and estuarine habitats and the connectivity between them for eel management and conservation.