We have been monitoring the dam since March for spawning alewives. The season is almost over with probably just one full moon to go. Yesterday afternoon we saw our first alewife swimming up the ramp of the dam. It would reach the top where it was then blocked by an impassable 1 foot vertical lip. Finally it would slip back down to the pool at the base of the ramp to try again. When I went back this morning near high tide I saw two, one of which I managed to net (photo). Unfortunately the photo is poor quality so you can’t see the abrasions on it’s belly and sides, most likely from its struggles up the rough wall of the dam.
Oddly enough I just started seeing a bunch of carp pooling downstream of the dam about a week ago. You don’t normally think of carp as being saltwater fish.. Even more bizarre the carp were also trying to negotiate the dam! I saw the same thing at this time last year at the dam; both carp and alewives attempting to get up the dam. The rest of the year the “saltwater” carp disappear.
Based on my extremely small sample size of N=2, it looks to me like 1) alewives can’t enter the lake unless the tide practically floods over the top, a rare event, 2) the current ladder is worthless (the fish have to positioned perfectly to enter the ladder and even if they do I doubt they could make it up the “rungs”), and 3) they can almost make it to the top by swimming up the ramp; it’s just that last vertical drop that keeps them from success. I would think that it shouldn’t take a great deal of engineering to allow successful entry. For example the ramp could be built up so it starts at the very top and slopes down from there.