Resource: Scent of an Alewife

In nearly every way, the alewife is a nondescript silver-colored little fish. Individuals spend their lives hidden in enormous schools of their own kind, flashing and darting away from would-be predators. One aspect of the alewife's life, however, is far from average. Unlike most fish species, which spend their entire lives in either fresh or salt water, the alewife, during critical stages of its life, moves intrepidly from the open ocean into a dramatically different freshwater environment -- and back again -- encountering innumerable dangers along the way.

The spawning behavior of the alewife mirrors that of another, larger and better-known group of fish: the salmon. Like salmon, alewives hatch in freshwater streams and ponds. After spending their first several months in fresh water, young alewife fry move out of their natal streams along the eastern coast of North America and into the Atlantic Ocean. They spend the next four or five years at sea, eating mostly plankton and smaller fish, and growing quickly in this productive ecosystem.

Although very little is known about the life of the alewife during its first several years in the ocean, studies have found that at least some groups of alewives migrate great distances -- as far as 1,200 miles along the Atlantic seaboard -- probably in search of food. By the age of four or five, however, the alewife has reached sexual maturity and is ready to spawn. Cued by changes in day length and water temperature, alewives begin swimming en masse toward their natal streams, where they face a gauntlet of predators and human-made obstacles.

The alewife's spawning voyages are inherently treacherous, but twentieth-century agriculture and industrialization, especially in New England and the Mid-Atlantic states, have made them even more so. For example, by the mid-1900s, dams had cropped up on stream after stream, impeding the efforts of alewives trying to get to their native spawning grounds. Pollution from fields and factories has also killed countless spawning alewives or the eggs they managed to lay.

Miraculously, the alewife has persisted despite the threats to its well-being. Through decades of environmental degradation, populations of alewives have continued to breed successfully, albeit in much smaller numbers. This is due in part to the fish's ability as a hearty generalist to withstand less-than-optimal conditions. Today, environmental groups and agencies are cleaning up coastal waterways, dismantling dams, and, where necessary, restocking alewife populations. Slowly these vigorous little fish are reclaiming the important place they once held in the coastal food chain.

The female alewife lays up to 200,000 eggs at a time, but only a fraction of that number will survive. What do you think happens to the others? Contrast this with another animal that has only one baby at a time. What are the advantages of each reproductive pattern?

Is the individual alewife important?

Would you describe the alewife as a freshwater or saltwater fish?

What is imprinting?