Their basic salt-water diet is plankton. The copepods and amphipods--tiny animals similar to the well-known sand hoppers, beach fleas, water fleas, or small shrimp--provide the staple part. On their inland migration they will not eat much of anything. They are in the service of a mission, and they fast. Still there may be occasions when they snap at lures. I have heard of some, migrating inland, that were caught by fishermen casting with flies at the Herring River in Harwich, on the south side of the Cape. Those herds now in the brown waters of the channel seemed to be gradually moving inland as the day advanced, and I guessed that for most of them it was the journey up. As I watched them there was a slight, quick change of wind, a shift in the breeze that flicked the water, and in the crosshatches this made on the bright surface all the fish disappeared. Then the surface cleared and I could see them again, swimming through a rippling weave of light that was reflected on the channel floor. They meandered along, an occasional lively one dashing through the rest, or rushing up to the surface. I could hear light plops, faint flips along the water. I walked closer to the edge so that my shadow fell across them, and they turned back in one quick and graceful stampede, some of them dashing to deeper water under the opposite bank. That so many separate entities should have such an immediate response to one another that they all moved like the beat of a wing was hard for one individual to understand. Alewives are able to stand the quick change from salt to fresh water without any trouble. It would kill some other kinds of fish, but neither the alewife fry growing up in fresh water nor adults growing up in the sea seem to mind being taken out of one medium and plunged into another. They are adapted to both. On the other hand, repeated changes, such as occur sometimes under certain conditions in tidal estuaries, are said to be able to kill them. These changes, incidentally, are registered in an alewife’s scales. To read them is difficult and requires a competent biologist. In general they mark physiological changes, such as occur when young alewives go from fresh to brackish water, or when the adults spend some time in fresh water before returning to the sea. An alewife’s age can be determined because a record of each spawning migration is etched on the scales.