A further assessment of trophic and spatial inter‐relations of galaxiids and salmonids in New Zealand

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Abstract
The trophic and spatial inter‐relations between 2 small native fish (Galaxias vulgaris Stokell and G. paucispondylus Stokell, Galaxiidae) and juveniles of 2 introduced fish (Oncorhynchus tshawytscha Walbaum and Salmo trutta L., Salmonidae) were investigated in a New Zealand stream. Interspecific overlap in timing of feeding among these four fish species was greatest between pre‐dusk and pre‐dawn (1730‐0530). Aquatic invertebrates were consumed exclusively by all fish species except O. tshawytscha, which also preyed on Trichoptera imagos, particularly at night. Dietary similarities were least between galaxiids, moderate between galaxiids and salmonids, and greatest between salmonid species. Both galaxiid species were found primarily in riffles, with G. vulgaris occupying slightly deeper (≤0.3 m) and slower (0.5–0.8 m. s−1) waters than did G. paucispondylus (≤0.2 m deep and 0.7‐1.0 m. s−1 water velocity); salmonids were mostly in pools and runs, with O. tshawytscha occupying slightly slower (0.1‐0.2 m. s−1) and deeper (≤0.5 m) waters than those occupied by S. trutta (0.2–0.5 m. s−1 water velocity and ≤0.4 m deep). The interspecific differences in feeding and microdistribution presumably reduce interaction between sympatric populations of galaxiids and salmonids.

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