Understanding uncertainty in the effect of low-head dams on fishes of Great Lakes tributaries.

Small dams represent one of the most widespread human influences on riverscapes. Greater understanding of how these structures affect aquatic organisms is needed to ensure that decisions regarding their construction and removal strike an appropriate balance between components of human and ecosystem services. Within the basin of the Laurentian Great Lakes, the effects that in-stream barriers (dams) used to control the non-native, parasitic sea lamprey (Petromyzon marinus) on the diversity of non-target fishes is a significant concern for fishery managers. A previous study indicated that upstream changes in the species richness of non-target fishes observed in 24 streams with a sea lamprey barrier relative to paired reference streams (a measure of effect size) was variable across the basin. We examined the degree to which the variance in effect size could be attributed to imprecision in the field sampling protocol used to estimate effect sizes, differences in catchment-scale landscape attributes between barrier and reference streams within pairs, and differences in landscape attributes at different spatial scales among barrier streams. Simulation modeling and analyses of repeated field measurements made for a subset of streams demonstrated that a large variance in effect size is expected for the field sampling design and that estimates of effect size measured for individual barrier streams are imprecise. Regression models and multimodel inference methods based on Akaike's Information Criterion provided less support for hypotheses linking effect size to landscape attributes. Mean effect size, adjusted for the influences of landscape characteristics within and across stream pairs, provides the most reliable and least biased estimate of the effect of sea lamprey barriers on the richness of nontarget fish species. With the information currently available, landscape characteristics of catchments cannot be used to help decision makers anticipate effects sizes for candidate streams being considered for future barrier construction. Our findings will help fishery managers in the Laurentian Great Lakes make more informed decisions regarding the use and placement of sea lamprey barriers and achieve their objective of delivering an integrated pest management plan for sea lamprey control that is environmentally and economically sound and socially acceptable.