SABA, M.V.*; HORN, M.H.; California State University, Fullerton; California State University, Fullerton: Ontogeny of Diet and Trophic Position in Herbivorous and Carnivorous Prickleback Fishes (Stichaeidae): Dietary and Stable Isotope Analyses

We examined gut contents and carbon (&delta¹³C) and nitrogen (&delta¹⁵N) signatures of four related, wild-caught, and experimentally-fed (high-protein animal diet) prickleback fishes as well as isotopic signatures of invertebrates, algae, and a seagrass from the habitat to assess food origins and ontogenetic changes in diet and trophic position of the fishes. Two of the fishes, Cebidichthys violaceus and Xiphister mucosus, begin life as carnivores, then shift to herbivory at sizes &gt 45 mm SL, whereas X. atropurpureus and Anoplarchus purpurescens remain carnivores for life. We hypothesized that, consistent with the dietary analyses, stable isotope values will show that: (1) Wild-caught C. violaceus and X. mucosus shift to an algal diet and decrease in trophic position with increasing size; (2) Wild-caught X. atropurpureus and A. purpurescens shift their diet to somewhat larger animal prey and show a slight increase in trophic position with increasing size; and (3) On the experimental diet, the two carnivores exhibit greater &delta¹⁵N enrichment than the two herbivores. Results to date indicate that: (1) Three isotopically distinct groups of possible food sources exist in the habitat: algae and herbivorous and carnivorous macro-invertebrates; (2) Trophic positions (&delta¹⁵N) of wild-caught fishes reflect phylogeny more than diet in that C. violaceus, X. atropurpureus, and X. mucosus, all in the same herbivorous clade but with some differences in diet, were nearly one trophic level lower than A. purpurescens, in an adjacent carnivorous clade; and (3) Trophic positions of the four species on the experimental diet were similar, the fishes thus appearing to show a plastic response to diet.