Food relations between coexisting native Hawaiian stream fishes

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In this study, an ecomorphological perspective is used to examine the role of feeding morphology in shaping patterns of food resource use and coexistence for Awaous guamensis and Sicyopterus stimpsoni, two native gobies (Gobiidae) which inhabit mountainous streams of the high Hawaiian Islands. Using data from underwater census, gut content analysis, and benthic sampling, I determined that A. guamensis, the generalist, had nearly unchallenged use of invertebrate foods. Overlap in fitness for algal use, however, resulted in a partitioning of benthic algae, with A. guamensis having domain over most green algae (43.0% of diet), whereas S. stimpsoni the algal specialist, fed predominantly on blue-green algae (22.6% of diet) and diatoms (54.2% of diet). Cladophora sp. (Chlorophyta) and pennate diatoms (Chrysophyta) were determined to be the ‘primary algal foods’ of A. guamensis and S. stimpsoni, respectively, and were utilized in a mutually exclusive manner with other ‘secondary algal foods’ depending upon availability. Heterogeneity, found in the abundance and composition of algal and invertebrate foods in the benthic landscape both spatially and seasonally, may be regulated by stream flow and periodic disturbance. This changing mosaic of foods is suggested as having provided opportunities for minimizing competitive conflicts and enhancing the potential for stream species to coexist. Competition for preferred foods, created by inter-specific overlap in ecomorphology and spurred by constraints placed on food diversity by the extreme geographic isolation of the Hawaiian Islands, is hypothesized as having played an evolutionary role in shaping resource use patterns which facilitate coexistence.