Determining the Efficacy of a Standard Denil Fish Ladder for Alewife Passage Using PIT Tags in a New Hampshire Coastal River

River herring were once more abundant in east coas rivers, but like many anadromous fish species, have exhibited a precipitous decline. Causes are widespread, but exclusion from freshwater spawning areas by dams is a likely contributor. Population estimates of New Hampshire river herring are derived by passage counts through fish ladders during spawning migrations. Inaccuracies in these population estimates may arise if significant numbers of fish fail to ascend the ladders but successfully spawn downstream from the dam. To evaluate passage efficiency and behavior of river herring movements within a standard denil fish ladder, a tagging study was conducted using PIT tags. During the 2013 spring spawning run, 622 adult alewives were internally tagged with 23mm HDX PIT tags and their ascent monitored with an eight antenna array within the ladder. Preliminary efficacy results indicate peak attraction of approximately 62% to the fish ladder with 61% successful passage. Behavioral observations suggest that during high-use periods the ladders may become saturated and limit additional fish ascension. Diel and seasonal motivation was determined and repeated attempts to ascend the ladder were enumerated. This information increases our understanding of fish ladder use by alewives and can improve the accuracy of spawning population estimates.