River Herring return to Tidmarsh
River herring is the general name for two anadromous fish species – alewife and blueback herring. These so-called “forage” species spend the majority of their life at sea but migrate to freshwater rivers, streams and lakes along the Atlantic seaboard to spawn in the early spring. Alewives spawn in lakes and ponds; bluebacks spawn in the main stem river or stream channel.
Spawning adults enter rivers in Massachusetts from early April to early June to spawn. The ideal water temperature range for spawning is 12.8-15.5 °C. Each female releases 60,000-100,000 eggs, depending upon the size of the individual fish. Fertilization is external. Once the eggs and milt have been released, the majority of the surviving spent adults make their way back to the ocean. Eggs hatch in 3-6 days. The juveniles remain in fresh water for 3 months or more before they begin their seaward migration.
Earthen dams and water control structures used in cranberry farming, typically block the passage of migrating river herring. At Tidmarsh, vertical boards set into culverts allowed the farmer to regulate the amount of moisture on the surface of the bogs; even if they get to the dam, most herring were not ascend the steep vertical rise of the boards. When these water control structures were removed in 2015-6, herring were able to travel through Tidmarsh, and alewife were successful in reaching Fresh Pond, the main fresh water spawning pond of this watershed.