Return of the River Herring

The Spring Migration Has Begun

Spring weather on Cape Cod is changeable and dreary much of the time; and the old adage "If you don't like the weather on Cape Cod, wait a minute" seems most appropriate this time of year. However, when that brief minute provides a perfectly clear sky and the temperature tops 60 degrees the only place to be is outdoors. One seemingly required ritual is a visit to the local herring run to see if the fish are in.

Each spring, usually beginning in mid to late March, river herring begin their annual migration up streams and rivers across Cape Cod to spawn in the ponds of their birth, with the eternal purpose of fostering a new generation. River herring are anadromous fish, meaning they spend their adult lives living offshore at sea and return to their freshwater origins to spawn. Other fish like the American Eel (Anguilla rostrata) are catadromous fish meaning the opposite is true with adults living in freshwater systems most of their life and then travel back to the sea to spawn. But the eel is a story for another day.

Two species of river herring use our local streams as corridors to their spawning sites: the Alewife (Alosa psuedoharengus) and the blueback herring (Alosa aestevalis). Alewives arrive first and spawn mid-March to May, when water temperatures reach 51 degrees or so. Bluebacks arrive a bit later, April thru June when water temperatures have reached 57 degrees or higher. The fish will spawn in the headwaters of the ponds that feed the streams, and then will return back to the sea in late summer or early fall. The new population of juvenile herring will follow shortly thereafter.

The two species are hard to tell apart for most of us, as their spawning times overlap and they are very similar looking while in the water. Alewives tend to have a grayish green back versus the bluish hue of the bluebacks, but the colors are often difficult to distinguish. Each species reaches a length that approaches twelve inches or so. Alewives do have a larger eye, and a somewhat dusky spot just behind the upper margins of the gill, but I can tell you that you aren't going to be able to see those features while they are swimming upstream.