Born to Run

Witness the annual migration of the alewives

By North Cairn

They are some of the most tranquil places to watch the fight for life.

The Cape’s herring runs – which extend all along the peninsula – mark the sites from which it is still possible to witness the annual swarming upstream of the alewives from salt water to fresh water. If it can be said that there is a “start” to a cycle that goes round and round from one generation to the next (or in and out, like breath or the tides), it might be set as early as March. During the late days of winter, isolated “scouts” – from a single fish to ten or twenty individuals – can be seen making their way from Cape Cod Bay toward fresh water.

But they are mere harbingers of the throngs of river herring – made up of alewives and blueback herring – that will fill the waters of streams and creeks in Brewster, Bournedale, Mashpee, Harwich and several other Cape towns before the spawning ritual is completed. From April into May (and as late as June in Buzzards Bay), the great struggle of the alewives is enacted and re-enacted, sometimes by thousands of fish. It draws crowds of humans to watch – particularly in Brewster, where the run has become one of the signature events of spring and a central metaphor of natural history in the literature the town’s writers have spawned.

The monthlong run of the alewives, like other types of migration across species of birds and animals, is understood only in part. The timing has to do with lengthening light and the relative temperatures of Cape Cod Bay and the interior ponds toward which the fish strive in an effort to spawn, biologists believe. When the temperature of the inland fresh water flowing out into the sea is warmer than the saltwater, the alewives become restless and begin to move toward the rivers and creeks that lead back to their birthplace. There they will spawn in water that has reached just over 50 degrees. On Cape Cod, that occurs during what might be called “high spring,” when the season is a sure thing but not yet fully accomplished.

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