Abstract 

Impacts of adfluvial spawners on ecology of Great Lakes tributaries 

by 

Lori Nicole Ivan

Co-Chairs: Edward S. Rutherford and J. David Allan 

Anadromous fishes are known to increase productivity and biomass of lower trophic levels and resident fish growth in oligotrophic streams of the Pacific Northwest.  In more eutrophic streams, variable impacts on lower trophic levels have been observed.   The potential for adfluvial fishes in Great Lakes tributaries to affect resident fish growth is great, especially as several highly fecund native adfluvial fishes spawn in these tributaries. 

I conducted field surveys, manipulations and simulation models 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 manipulated 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.  Walleye egg survival was low in uncovered traps compared to covered incubators, and survival was higher in warmer years, suggesting predation and cold water temperatures are an important source of egg mortality and results in the low natural recruitment observed in the Muskegon River.  Field experiments showed little impact of spawning salmonids on stream chemistry and macroinvertebrates, with variable responses between streams and years.  In the natural stream, density of adult trout increased during spawning in all seasons.  Resident trout that consumed eggs increased their energy intake but not growth.  Model output revealed YOY steelhead increased growth through egg consumption and not carcass decomposition, and impacts were greater in lower nutrient regimes.  Results indicate adfluvial spawners may impact growth of Great Lakes resident fishes by providing energy rich eggs as food and not through effects on stream nutrients and lower trophic levels.