Ospreys, bald eagles, cormorants and great blue herons prey heavily on migrating alewives each spring, at a time when some of these birds are nesting and rearing their chicks. In polluted rivers, organic chemicals and heavy metals (such as PCBs, dioxin and mercury) bioaccumulate in the tissues of resident fish, and these toxins are then passed to birds that consume them. Alewives are a healthier prey item for fish-eating birds than resident freshwater fish because they have not accrued these toxins in their bodies. Thus, a large alewife run will help to curtail the harmful effects of biomagnification on fish-eating birds.

Some anglers worry that anadromous alewives might have a detrimental effect on sport fisheries, but nothing could be further from the truth. Adult anadromous alewives do not compete with freshwater fish because they essentially stop feeding during the migration and spawning period, and do not resume feeding until they reach brackish water on their way back to sea. Since alewives are planktivores, the vast majority of their food is zooplankton, small crustaceans and insect larvae. Even at sea, fish comprise a miniscule proportion of an alewife’s diet. One 1994 study published in the Fishery Bulletin, examined 1,215 alewife stomachs and found that larval fish represented only 1.4 percent by volume of total prey items.

Migratory alewives and their offspring are key forage for fish predators. Striped bass, northern pike, pickerel and lake trout are among the fish that consume adult alewives in freshwater or estuaries, in addition to the large number of predators in the marine environment. Striped bass will follow migrating alewives for many miles up estuaries and rivers, providing a recreational fishery in May and June. Some scientists have speculated that damming of coastal rivers contributed to the collapse of the cod fishery in the Gulf of Maine by reducing the abundance of alewives, one of the cod’s principal prey items. Restoration of pelagic and groundfish stocks in the Gulf of Maine would likely benefit from restoration of alewife populations.

Young-of-the-year alewives present a spring, summer and fall picnic for our important game fish. They live in freshwater for three to seven months and grow two to five inches before descending the watershed and entering the ocean. They are eaten by many fish such as perch, bass, salmon and trout. In studying both blueback herring and alewife in Massachusetts coastal lakes, scientists determined that Alosa was the most important fish prey consumed by largemouth bass (based on number in individuals consumed), and that Alosa are an energy-rich prey that provide a high growth potential for largemouth bass. The scientists concluded that “Our simulations show that the presence of ‘trophy’ largemouth bass found in southeastern, coastal Massachusetts was not solely related to water temperature across the state but rather was related to predator diet and, specifically, to the presence of anadromous herring in largemouth bass diets.”