Because of the Horseshoe Pond dam.
At first glance this photo doesn't look like much.
When you look closer you can see a bit more clearly.
That the whitish haze on the rocks in the top photo are millions of rainbow smelt eggs.
Unfortunately all the rainbow smelt eggs in the above photo's are quite dead.
Because the rainbow smelt are forced to spawn below the dam their eggs wind up high, dry and dead when the tide goes out. Normally smelt would swim upstream above tidal influence to lay their eggs. The Horseshoe Pond dam prevents them from doing that here. Through thousands of years rainbow smelt have developed the instinct to lay their eggs above tidal influence, so that the above will not happen.
How long will it take us to figure out that these old mill dams are killers? If a fish with a brain the size of a pinhead can figure it out, why can't we?
Don't take comfort in the idea that rainbow smelt lay eggs in other parts of the stream below the dam. For reasons known only to the smelt, they lay their eggs where it becomes dry at high tide. About 80% of the eggs they lay below the Horseshoe Pond dam die.
More Weweantic info HERE