“They eat plankton, which reduces the phosphorus, and deposit it in the ocean, which can handle it better,” Woodbury said.
The alewife is chiefly a plankton feeder like the herring; copepods, amphipods, shrimps, and appendicularians were the chief diet of specimens examined by Vinal Edwards and by Linton [page 104] at Woods Hole. However, they also take small fish, such as herring, eels, launce, cunners, and their own species, as well as fish eggs. Unlike herring, alewives often contain diatoms even when adult. Alewives fast when they are running upstream to spawn, but when the spent fish reach brackish water on their return they feed ravenously on the shrimp that abound in the tidal estuaries and which they can be seen pursuing. We have often hooked alewives on an artificial fly at such times.”
Bigelow p.101
Alewives are important to the ecology of freshwater, estuarine, and marine environments. They provide an alternative prey item for osprey, eagles, great blue heron, loons and other fish eating birds at the same time juvenile Atlantic salmon are migrating downriver. Alewives provide cover for upstream migrating adult salmon that may be preyed on by eagles or osprey, and for young salmon in the estuaries and open ocean that might be captured by seals. It is important to understand that alewives tie our ocean, rivers and lakes together, providing vital nutrients and forage needed to make healthy watersheds. Between and within those various habitats,everything eats alewives: striped bass, bluefish, tuna, cod, haddock, halibut, American eel, brook trout, rainbow trout, brown trout, lake trout, landlocked salmon, smallmouth bass, largemouth bass, pickerel, pike, white and yellow perch, seabirds, bald eagle, osprey, great blue heron, gulls, terns, cormorants, seals, whales, otter, mink, fox, raccoon, skunk, weasel, fisher, and turtles.
http://www.maine.gov/dmr/searunfish/alewife/
Everything Eats Alewives 