Source: NOVA: "Sea Behind the Dunes"
NARRATOR: There are certain fish that live at sea but must return to breed each year in the freshwater ponds where they were hatched. They have a sense of taste so acute that in salt water, they can detect the diluted flavor of the minerals from the exact ponds where their lives began. That flavor becomes a homing beacon in April, when the breeding season arrives.
This fish is the alewife. They enter the bay, and each is drawn to the stream that leads to the pond where it was hatched. The journey is not without hazards (gulls crying).
This is one of the streams that lead the alewives inland from the bay. Fish can see things well above the surface, but they move on despite the threat. The mass is undeterred, and they press on. They run for weeks and by the thousands...to the pond where they have laid their eggs for centuries.
Each female lays 200,000 eggs a season, but in her life, only one may survive to breed. After three days, there is a nervous system and a minute but steady heart. When they have eyes, they are ready to struggle free.
They remain in freshwater until midsummer. By then, the unique scent of this pond will be so firmly imprinted on their memories that in four years, each one that survives can return with an unerring instinct for home.
Autumn is on its way. In the freshwater ponds above the bay, the alewife fry have been developing all summer. It's time for them to begin their descent to the sea. It's known how they will return to the pond, but no one knows for sure how or why they find their way from the pond to the ocean. Perhaps it's the noise of running water, perhaps the pull of the current.
It is thought that in the beginning, the alewife was a freshwater fish, because its eggs cannot survive in salt water, and neither can the very young. But their body chemistry changes in the fall, and then they must move to join their parents and face the rigors and the hazards of life at sea.
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