Effects of fish chemical cues on the interactions between tadpoles and crayfish

We studied the effects of predatory crayfish (Pacifastacus leniusculus), th e non-lethal effects of fish chemical cues (Oncorhynchus mykiss), and the c ombined effects of crayfish and fish chemical cues on the performance of ta dpoles of two co-existing anuran species, Rana temporaria and Bufo bufo, in experimental pools. We also examined grazing effects on periphyton, the ma in food source for the tadpoles. Crayfish significantly reduced tadpole sur vival, particularly by feeding on Bufo. Rana benefited from reduced numbers of competitors, resulting from crayfish predation, by increased growth rat e, whereas the growth rate of Bufo was unaffected by crayfish. The proporti on of Rana in refuges (in relation to the number of survivors at the end of the experiment) was unaffected by crayfish, whereas proportionally more Bu fo stayed in refuges in the presence of crayfish, relative to controls. Fis h cues had no effect on tadpole survival of either species. During the enti re larval period, Rana responded to fish cues by increasing the use of refu ges relative to controls, whereas Bufo, did not show any significant behavi oural response to fish cues. In accordance with these observations, the pro portion of Rana in refuges at the end of the experiment was high in the pre sence of fish cues, whereas the use of refuges by Bufo was not affected by fish cues. Predatory crayfish and fish chemical cues had effects on tadpole survival, growth and refuge use. Tadpoles in all treatments reduced periphyton biomass. Both crayfish and fi sh cues had positive indirect effects on periphyton biomass. The positive i ndirect effect of fish cues on periphyton was likely an effect of reduced g razing from Rana. Thus lethal, as well as non-lethal, predator effects on p rey populations can influence lower trophic levels.