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Session C7: An Evaluation of Eel Ladders as Traps for Migrating Sea Lampreys

Abstract: We tested if eel ladders with the right spacing of pegs on slanted ramps can selectively trap migrating sea lampreys. We compared entry of adult sea lampreys into eel ladder-style traps (ELST) with capture by funnel traps during three seasons in two Michigan rivers. Both funnel traps and ELST caught several thousand lampreys, but only the ELST perfectly retained trapped animals and had zero by-catch of finfish. Markrecapture experiments revealed significant trap happiness of ELST-caught lampreys. In one river, the ELST catch was strongly biased toward females. At the same site, the ELST caught lampreys with lower average weight and a lower female GSI. We analyzed via PIT tags and video observations of hundreds of nocturnal approaches and attempts to swim up the exposed part of the ramp. The rate of completion of climbs 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 more often by a decision to turn around than a lack of stamina to swim all the way up. Manipulations of ramp angle and attraction/ ramp flow resulted in varying rates of attempt and completion: steeper ramp angle and more attraction flow lead to more successful passage over the ramp. We showed that eel ladder-style traps can selectively catch (or pass over obstacles) adult migrating sea 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 target population. After further fine-tuning of the ramp parameters and flow, modified eel ladders could become a valuable tool for management of adult lampreys.
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