Diet of the crayfish Paranephrops zealandicus in bush and pasture streams: insights from stable isotopes and stomach analysis

Crayfish (Paranephrops zealandicus White) stomachs from streams in both native bush (mainly tree leaves and dicotyledonous seeds) and exotic pasture settings (mainly grass stems and monocotyledonous seeds) were dominated by allochthonous material. More detritus occurred in stomachs in autumn-winter than in spring-summer, but quantities were similar in crayfish from native bush and pasture streams. The stomachs of larger crayfish contained a significantly greater proportion of detritus than smaller individuals. Aquatic invertebrates were the second most abundant dietary category by volume, with highest values in winter, but there were no significant differences between land uses or crayfish size classes. A wide range of invertebrates was eaten by crayfish, with mayfly nymphs, chironomid larvae, and snails predominating. The latter were numerically more prominent in crayfish from bush than pasture streams. Terrestrial invertebrates were recorded from 4% of stomachs, but there were no significant differences in relation to land use, season, or crayfish size class. Despite aquatic invertebrates making up <4% of stomach volumes on average, stable isotope analysis indicated a greater importance for invertebrate prey in terms of assimilation and incorporation into crayfish biomass. Allochthonous detritus and moss appeared to be unimportant. Whereas the results of stomach analysis provided some evidence of an ontogenetic shift, with detritus assuming greater importance in larger crayfish, this pattern was not supported by isotope analysis because invertebrate prey appeared more important to the diet than detritus. An unidentified carbon source, depleted in (13)C and perhaps of microfloral origin, seems to be an important energy source for crayfish in both stream types.