Selective Trapping of Adult Migrating Sea Lampreys and Passage of Finfish

We tested if eel ladders can selectively trap migrating adult sea lampreys. We also report on pilot experiments to selectively pass over ramps that act as sea lamprey barriers. We compared entry of lampreys into eel ladder-style traps (ELST) with capture by funnel traps in two Michigan rivers. The traps caught several thousand lampreys, but only the ELST perfectly retained animals and had zero by-catch. Mark-recapture revealed trap happiness of ELST-caught lampreys. In one river, the ELST catch was strongly biased toward females and caught lampreys with lower average weight and a lower female GSI. The rate of completion of climbs on the ELST varied considerably between sites and years. Most of the failed attempts were aborted near the base of the ramp; those attempts appeared to be caused by a decision to turn around. Steeper ramp angle and more attraction flow lead to more passage. Eel ladder-style traps can selectively catch (or pass over obstacles) adult migrating lampreys. While we can’t explain the reason for the trap biases we observed, our observations underline that fish passage devices or traps that rely on complex behavior of the target animal will likely select for a subset of the population.