THE TROPHIC INTERACTIONS OF YOUNG ARCTIC GRAYLING (THYMALLUS-ARCTICUS) IN AN ARCTIC TUNDRA STREAM

1. Young (0+) Arctic grayling (Thymallus arcticus) have the potential to control the trophic structure of Arctic tundra streams through cons umption, nutrient excretion and the modification of prey behaviour The effect of young grayling on three trophic levels (algae, invertebrate s and fish) was investigated by manipulating fish density and by ferti lizing the river with phosphorus (P). 2. Nutrients, epilithic chloroph yll a, benthic invertebrates and fish biomass were measured within eac h fish density treatment (0, 4, and 40 fish m(-2)) within the P-limite d reference zone and the P-enriched fertilized zone of the Kuparuk Riv er, Alaska. 3. Epilithic chlorophyll a increased with increased fish d ensity in both reference and fertilized zones, while mayfly density de creased with increased fish density in the fertilized zone only. Final mean mass of young grayling in the 40 fish m(-2) cages was lower than mean mass in the 4 fish m(-2) cages. 4. Young grayling may produce a top-down cascading trophic effect in areas where nutrients are not lim ited. 5. River nutrient status and river discharge may modify the stre ngth of top-down control by young grayling.