Field Assessment of Fish Passage to Support Connectivity Restoration Decisions in Great Lakes Tributaries

Great Lakes migratory fish are unable to access 86% of the total tributary length in the Great Lakes Basin (GLB) due to the presence of 7,091 dams and 268,818 road crossings. A landscape predictive model has been used to estimate the passability of every road crossing in the GLB to fishes based on road characteristics, upstream area, stream gradient, and slope at culvert sites. These estimates form the core of an online decision support tool (greatlakesconnectivity.org) that evaluates alternative barrier removals to optimize habitat gain per removal cost at any scale within the GLB. A major limitation of the landscape-based passability model is that it had not been validated with field surveys of fish passage. We conducted systematic field surveys in Lake Michigan tributaries during the spring spawning migrations of suckers and other native species to compare alternative perspectives on accessibility of spawning habitat upstream of a series of potential barriers.  Our field surveys provide a conservative estimate of migratory fish passage, and using them to calibrate interpretation of modeled passability can enhance the management value of the decision support tool.