Ecological consequences of damming on river herring throughout Maine watersheds determined by historical spatial and productivity analysis

The anadromous river herring, collectively alewives and bluebacks, were historically abundant in most northeastern U.S. coastal river systems. Dam construction, which disrupted diadromous fish migration pathways, is considered the earliest principal cause of reduced productivity and population declines. Using published surveys, GIS layers and historical documents, we compiled a database of 1359 dams constructed on Maine watersheds from 1600-2008 and created a timeline to examine their effect on river herring spawning site access and historical abundance. Spatial analysis, harvest and recruitment data were used to estimate the subsequent decline of spawning habitat and alewife productivity through 1900. Results demonstrate impassable dam construction at head of tide on all Maine rivers by 1850 and near total blockage of spawning habitat by the 1860s. Analysis of 19th century alewife harvests reveals a critical shift from primary rivers and multiple watersheds along the entire coast to three mid-coast secondary rivers. As a result, our estimates show a decrease in annual alewife productivity of at least 94% from 1680-1900 equaling 6.5 billion alewives lost statewide. Our techniques can be applied to other watersheds and diadromous species to help evaluate current management targets and approaches towards restoring these species and ecosystems.