Alien Rainbow Trout (Oncorhynchus mykiss) (Salmoniformes: Salmonidae) Diet in Hawaiian Streams

Abstract

Diet of rainbow trout, Oncorhynchus mykiss (Walbaum), introduced by the State of Hawai'i into tropical headwater streams of the Waimea River in the Koke'e area of the Hawaiian island of Kaua'i, was examined in this study through gut content analysis. In Wai'alae Stream, rainbow trout were found to be opportunistic general predators efficient at feeding on invertebrate drift. Foods eaten ranged from juvenile trout, to terrestrial and aquatic arthropods, to algae and aquatic mosses. Native aquatic species, particularly dragonfly (Anax strennus) and damselfly (Megalagrion heterogamias) naiads, lyrnnaeid snails (Erinna aulacospira), and atyid shrimp (Atyoida bisulcata), were determined to be major foods for alien trout. Terrestrial invertebrates (primarily arthropods), however, provided a substantial (albeit unpredictable) additional food supply. Based on results of the study, it is cautioned that large numbers of rainbow trout indiscriminantly released into lower- to middle-elevation reaches of Hawaiian streams could do substantial damage to populations of native aquatic species through predation, competition, and/or habitat alteration.