We employed stable isotope analysis (δ13C, δ15N) to evaluate the sources of nutrients used by amphidromous gobiid fishes (Lentipes concolor, Sicyopterus stimpsoni, Awaous guamensis) caught migrating into and living in Hakalau Stream, Hawaii. Although considerable variation amongst the stable isotope values of stream items was noted across all 4 years of our study, the relationships between the fishes were relatively constant. Stable isotope values of recruiting gobies were consistently closer to those of both inshore plankton and freshwater adults than those of offshore plankton, suggesting that the larvae of these species derive much of their nutrition from inshore environments influenced by fresh water. Small differences between the stable values of these species further suggested that their larvae come from different inshore locations. After entering fresh water all species appear to swim rapidly upstream without feeding. Finally, once well upstream, adult L. concolor and A. guamensis appear to assume an omnivorous diet while adult S. stimpsoni rely upon autochthonous production within streams. We propose that freshwater food webs play an integral yet complex role in the lives of both larval and adult amphidromous Hawaiian fishes.
Bibliographical noteFunding Information:
This research was inspired by the ideas and enthusiasm of the late Jan Smith (University of Saskatchewan) who also collected many of the samples in 1997. Both Bob Nishimoto and Bill Devick (Division of Aquatic Resources of the Hawaii Department of Land and Natural Resources) encouraged and supported our study from the beginning as did Mike Fitzsimons (Louisiana State University). Anne Hansen (University of Colorado) kindly performed the scanning electron microscopy of the fixed samples. Fishes were collected with assistance of Jean Smith, Lance Vrieze, Cheryl Murphy, Lance Vrieze, Jared Fine, Heiko Schoenfuss, Darrell Kuamo’o, Bruce Kaya, Merlin Pratt, Lance Nishiura, Skippy Hau, and Wade Ishika-wa. Offshore marine plankton was kindly collected by students and staff aboard the SSV Robert C Seamans operated by the Sea Education association of Woods Hole, Massachusetts. Patricia Healy assisted with sample preparation, and Garth Parry provided isotopic analyses at the Department of Soil Science, University of Saskatchewan. Several annonymous reviewers greatly improved the manuscript. This research was funded by a grant from the Division of Aquatic Resources, Department of Land and Natural Resources, State of Hawaii and the Research Corporation of the University of Hawaii (Dingell-Johnson Wallop-Breaux Sport Fish Restoration Project F)14-R-18; P.W.S.) and the University of Minnesota Agricultural Experiment Station (P.W.S.).
- Awaous guamensis
- Lentipes concolor
- Sicyopterus stimpsoni