The trophic interactions of young Arctic grayling (Thymallus arcticus) in an Arctic tundra stream

Abstract

1. Young (0+) Arctic grayling (Thymallus arcticus) have the potential to control the trophic structure of Arctic tundra streams through consumption, nutrient excretion and the modification of prey behaviour. The effect of young grayling on three trophic levels (algae, invertebrates and fish) was investigated by manipulating fish density and by fertilizing the river with phosphorus (P).2. Nutrients, epilithic chlorophyll a, benthic invertebrates and fish biomass were measured within each fish density treatment (0, 4, and 40 fish m–2) within the P‐limited reference zone and the P‐enriched fertilized zone of the Kuparuk River, Alaska.3. Epilithic chlorophyll a increased with increased fish density in both reference and fertilized zones, while mayfly density decreased 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 limited.5. River nutrient status and river discharge may modify the strength of top‐down control by young grayling.