Challenges for Diadromous Fishes in a Dynamic Global Environment

<em>Abstract</em>.-While there has been much recent research conducted on the effects of semelparous anadromous (sea-run) fishes on nutrient cycling and productivity in freshwater lakes, we know very little about how iteroparous anadromous fishes may affect these processes and ecosystems. Although iteroparous sea-run fishes do not contribute mass postspawning carcass additions to freshwater systems, nutrients may still be transported from marine to freshwater environments through deposition of eggs, metabolic losses, and postspawning/overwintering mortality. As well, some iteroparous sea-run species spend significant time growing to juvenile and smolt stages in freshwater where they may affect top-down or bottom-up changes in food web structure. The goals of this study were to (1) conduct a preliminary investigation into the effect of sea-run Arctic char <em>Salvelinus alpinus </em>on food web structure in coastal lakes in the central Canadian Arctic, and (2) semiquantitatively assess the direction of net nutrient transport achieved by anadromous Arctic char during annual migrations to the sea. In 2006, fishes and macroinvertebrates were collected from four lakes near Hope Bay, Nunavut, and analyzed for stable C, N, and S isotopes to examine food web structure and anadromy. Preliminary results indicate that the presence of sea-run Arctic char may impact the ?¹⁵N-determined trophic position and condition of sympatric populations of lake trout <em>S. namaycush</em>. When stable S isotope results were applied to a two-source mixing model, marine food items accounted for 84-100% of the diet of mature, anadromous Arctic char. A semiquantitative assessment of char-mediated nutrient transport between marine and freshwater environments at Nauyuk Lake, Nunavut, indicated that masses of nutrients imported by char to freshwater systems are likely negligible in relation to ambient water chemistry. There was significant among-year variability in the magnitude and direction of nutrient transport, and the complex life history of anadromous Arctic char requires more research before detailed nutrient budgets can be calculated.