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“These sea-run fish migrate up into fresh water to spawn because there’s a biological advantage,” explains Lewis Flagg, Deputy Commissioner for the Maine Department of Marine Resources. “The ocean environment is loaded with serious fish predators. In fresh water, which is relatively safe, the young fish can get a start in life. By the time they get ready to migrate to the sea, they've developed the size and swimming ability to escape many predators.” Other anadromous fish include species of lampreys, smelts, sturgeons, and icefishes.
The life cycle of catadromous fish, on the other hand, is the opposite: they spawn in the sea but live primarily in fresh water. This is typical of freshwater eels. After the eels spawn in the sea, currents carry the larvae from the ocean towards estuaries. The juveniles migrate upriver and develop into adults. Eel larvae may drift for up to three years before reaching a river in which to mature. At the end of ten years or so, they migrate back to the ocean, spawn, and die. Eels can travel immense distances. For example, the European freshwater eel swims more than 5000 kilometers to spawn in the North Atlantic’s Sargasso Sea. American eels, which live in Maine’s Kennebec River, also spawn in the Sargasso Sea. Various species of mullets and some gobies are also catadromous.
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