Attraction and Passage Rates of River Herring through Two Different Fish Ladder Designs Using Passive Integrated Transponder (PIT) Tags

Fish ladders are widely used to mitigate the effects of dams and weirs blocking diadromous fish migration. However, the passage performance of fish through these ladders greatly depends on ladder design and flow rate. Here we present the results of two studies using passive integrated transponder (PIT) tags to track  the passage of alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis) through two ladder types (slot-pool and steep-pass) on the Raritan and Metedeconk rivers in New Jersey. Over the course of two years, detection rates at both locations indicate that a very small proportion of fish are able to find the entrances to the ladders. On one river, data from a third antenna installed to detect fish arriving in the general area of the dam demonstrated that  77% of  river herring either abandon migration after tagging or otherwise fail to reach the dam. Even after accounting for abandonment, passage rates for river herring remain low (39% and 42%, respectively). Using these passage rates, concurrent video monitoring counts can be scaled to estimate population size. Our results support recent calls to move away from fish ladders and towards more natural passage designs, or dam removals, whenever possible.