Native Fish Migrations Deliver Subsidies to Great Lakes Tributaries

Catostomid fishes constitute a large portion of fish biomass in lakes and streams across North America, and millions of white and longnose suckers migrate from the Great Lakes into tributaries each spring to spawn. Worldwide, most migratory fishes are iteroparous, but studies showing that nutrients delivered by fish migrations can increase stream productivity focus primarily on Pacific salmon, which are semelparous. We used a temporary barrier to test whether carbon and nutrients delivered through excretion, carcasses, and gametes during sucker migrations influenced nutrient dynamics and growth of stream organisms. To track sucker materials through the food web, we took advantage of their isotopic enrichment in nitrogen and carbon relative to resident stream biota.  The sucker migration significantly increased nutrient availability relative to reference reaches above our barrier. Macroinvertebrates and algae became isotopically enriched and grew faster in the presence of the sucker migration. We conclude that spawning catostomids can influence stream productivity through the delivery of resources to nutrient-poor tributaries. Obstruction of suckers and other native fish migrations by anthropogenic barriers has likely altered ecosystem function in Great Lakes tributaries.