Impacts of Adfluvial Spawners on Ecology of Great Lakes Tributaries.

Abstract

Anadromous fishes are known to increase productivity and biomass of biota in oligotrophic streams of the Pacific Northwest by depositing energy-rich eggs and nutrient-rich carcasses during spawning migrations. In more eutrophic Great Lakes tributaries, impacts of fish spawning migrations on stream ecosystems and fish production are poorly known but potentially significant, as several native adfluvial species are more abundant and fecund than introduced Pacific salmonids. I conducted field surveys, manipulations, and simulation modeling to study the impacts of adfluvial fish spawners on Great Lakes tributaries. I used egg mats and egg incubators to determine density and survival of walleye (Sander vitreus) eggs in the Muskegon River, Michigan. I conducted field experiments to determine the impacts of semelparous Chinook salmon (Oncorhynchus tshawytscha) and iteroparous steelhead (O. mykiss) spawners on the ecology and productivity of two Muskegon River tributaries. I sampled stream biota and water chemistry before and after introductions of salmonid carcasses and eggs in a treatment stream, and compared results with samples collected from a stream with natural salmonid runs. I also developed a cohort-based ecological model to determine the impacts of salmon carcass decomposition and salmon eggs on YOY steelhead growth and survival under varying nutrient regimes. I estimated walleye spawn 2-4 billion eggs annually in the Muskegon River. Walleye egg survival was lower in uncovered traps than in covered incubators, and survival was higher in warmer years, suggesting predation and cold-water temperatures are important sources of egg mortality that affect walleye recruitment in the Muskegon River. Field experiments showed little impact of spawning salmonids on stream chemistry and macroinvertebrate biomass. In the natural stream, density of adult trout increased during spawning in all seasons. Resident trout that consumed salmonid eggs increased their energy intake. Model output revealed growth and survival of YOY steelhead increased by consuming salmonid eggs but not by effects of salmon carcass decomposition on stream nutrients and steelhead prey. Impacts were greatest in lower nutrient regimes. Results indicate adfluvial spawners may impact growth and survival of Great Lakes resident fishes by providing energy rich eggs as food sources in low nutrient streams.