The stable carbon isotope ratios of fish and their invertebrate prey in four New Zealand rivers

1. Sources of carbon utilized by invertebrates and fish in four New Zealand streams were investigated using stable carbon isotope analysis. Complementary analyses were made to determine σ13C alteration (fractionation) by fish reared on a known food and to examine the distribution of 13C/12C ratios in different tissues. 2.σ13C values of hatchery-reared rainbow trout (white muscle tissue) were 13C-enriched compared with their food (trout pellets) by an average of 1.7%0. σ13C values of different organs and tissues never differed by more than 1.8%o in any fish species; white muscle always had an intermediate value. 3. σ13C values of invertebrates ranged from-28.4 to –20.4%o– At the more open sites invertebrates were 13C-depleted, reflecting some use of algal carbon. However, invertebrates from the most heavily shaded stream site were unusually enriched in 13C. An undetected carbon source of terrestrial origin, perhaps 13C-enriched dissolved organic carbon in groundwater, may have been contributing indirectly to invertebrate biomass carbon. 4. σ13C values for fish were not closely linked to aquatic invertebrate ratios; instead, at all sites their carbon was isotopically similar to terrestrial organic carbon. The reasons for this apparent anomaly are unclear