Insect Introductions and Diet Changes in an Endemic Hawaiian Amphidromous Goby, Awaous stamineus (Pisces: Gobiidae)l

Data are presented from gut content analysis of 94 Awaous stamineus (Edouyx & Souleyet) ('o'opu niikea) collected from the Wainiha River on the north shore of the Hawaiian island of Kaua'i during the period from June 1990 to May 1991. Also presented are results from gut analysis of 11 preserved museum specimens captured in Lii'iemalo'o stream, O'ahu, in February 1938 and June 1939. The results suggest that introductions of alien insects into the Hawaiian biota are changing the diet of this endemic fish. Comparison with the results of an earlier study indicate that A. stamineus is still dependent primarily on freshwater algae in the genera Cladophora, Rhizoclonium, Oedogonium, and Spirogyra; however, aquatic insect foods in the diet have increased from about 6% to nearly 13% in the current study. The findings suggest a reduced reliance on native aquatic chironomids in the genera Calospectra and Telmatogeton and an increased selection of immature stages of several recent aquatic insect immigrants, most notably two alien caddisfly species, Cheumatopsyche analis (Banks) (Tricoptera: Hydropsychidae) and Hydroptila arctia Ross (Tricoptera: Hydroptilidae), first reported in Hawai'i in 1967 and 1971, respectively. THREE ENDEMIC GOBIID species compose the major portion of the native fish fauna found in Hawaiian streams at higher elevations. These benthic gobies, or '0 'opu as they are known in Hawai'i, are remarkably adapted for life in torrential Hawaiian streams and are characterized.by fused pelvic fins that function as sucking disks. These fishes are amphidromous, spending their adult lives in running freshwater habitat but having marine larval stages. Awaous stamineus (Edouyx & Souleyet) or the 'o'opu niikea, which is the focus of the present study, is known to make seasonal migrations to river areas close to the ocean where spawning takes place (Ego 1956, Kido and Heacock 1991). Larvae hatch within 24 to 28 hr after spawning (Ego 1956, -l~do and Heacock [99-1) and are passlvely 1 Manuscript accepted I May 1992. 2 Kauai Research Facility, University of Hawaii, 7370A Kuamo'o Road, Kapa'a, Kaua'i, Hawai'i 96746. 3U.S. Fish and Wildlife Service, 300 Ala Moana Boulevard, Honolulu, Hawai'i 96850. 4Department of Zoology, University of Hawaii at Manoa, Edmondson Hall, 2538 The Mall, Honolulu, Hawai'i 96822. swept out into the ocean where they remain as part of the marine plankton for an estimated 150-169 days (Radtke et al. 1988) before returning to fresh water. All three endemic '0 'opu species have been cited as requiring special protection (Deacon et al. 1979); however, currently only Lentipes concolor (Gill), the o'opu alamo '0, is listed in the National Register for Endangered Species, where it is given Category I status (Dodd et al. 1985). Critical to effective management for preservation of these unique native Hawaiian fishes is a sound understanding of their basic ecology. Central to such understanding is research directed at elucidating their food and feeding habits. As several authors have pointed out (Hynes 1950, Pillay 1952, Berg 1979J,Oiet studies are of great importance in understanding the ecology of fishes, and analysis of gut contents is often an effective method of determining a fish's diet. The primary purpose of this study was to quantify the contemporary main foods of the endemic '0 'opu niikea, a subject that has not been seriously studied in many years. In 1936, entomologist F. X. Williams opened