Barrier Removal and Range Expansion of Sea Lamprey: Quantifying Habitat Conditioning in Atlantic Salmon Nursery Streams

Research from the Pacific Northwest US demonstrates that mass spawning of semelparous anadromous Oncorhynchus  spp. impacts the structure and function of stream ecosystems strongly. Bioturbation from spawning affects streambed geomorphology, periphyton, and invertebrates, and marine-derived nutrients from decaying carcasses can subsidize aquatic and terrestrial food webs, especially fueling growth of juvenile salmonines. When dams impact spawning runs and escapement declines, aquatic and terrestrial productivity decline in concert and a positive feedback loop ensures. In coastal streams of the Atlantic Northeast US, dams have been in place for centuries, and populations of anadromous fishes have declined precipitously.  Little historic information exists on the importance of anadromous fishes in conditioning freshwater habitats and subsidizing food webs in the Atlantic Northeast, but we suspect that an assemblage consisting of Atlantic salmon, river herrings, and sea lamprey performed a role analogous to that of Oncorhynchus in the Pacific Northwest. As dam removals proceed in the Atlantic Northeast, our long-term study on Sedgeunkedunk Stream (Maine) shows that sea lamprey respond quickly to expand their current range and re-colonize historic habitat. We performed a series of natural and manipulative experiments to quantify effects of bioturbation and nutrient enrichment from spawning sea lamprey on stream food web metrics of interest. Sea lamprey spawning increased habitat heterogeneity through creation of mounds and pits during nest building.  Compared to reference reaches, mounds were characterized by shallow, fast water, and were dominated by loose cobble with substantial interstitial space available for juvenile Atlantic salmon sheltering; pits were deeper, slower, were more dominated by fine particles, but also contained more sheltering habitats. Heterogeneity varied by stream reach, in that the impact of nest construction differed between the reach below a former dam site and the reach above the former dam site (which in 2009 received its first spawning run in more than a century). Effects on habitat were detectable immediately post-spawn (July) and also three months post-spawn (October), although the magnitude of these effects varied seasonally. Results quantifying effects on invertebrate abundance are pending. A small-scale (within stream) carcass addition experiment indicated weak localized effects on stream nutrient status and periphyton standing stock, and results of a large-scale (among-stream) carcass addition experiment on nutrient status and standing stocks of periphyton, invertebrates, and resident fish are pending. Our preliminary results indicate that bioturbation and nutrient addition from spawning sea lamprey can condition in-stream habitat, potentially to the benefit of juvenile Atlantic salmon.