The Pacific herring (Clupea pallasii) is a schooling fish that at some times of the year contributes to the diet of many marine animals. Whelks devour herring eggs deposited on beaches, gray whales from Mexico slurp up eggs, humpback whales from Hawaii and Japan devour mouthfuls of fish in deep water, seabirds feed small herring to their chicks, seaducks eat herring eggs, and sea lions and eagles gorge on the schools of fish.

Herring arrive in a choreographed wave of silver that moves north along the coast beginning in February in northern California and continuing in to June in Alaska. Along many kilometres of shore, the jade green waters turn a milky white when the spawn beings. Into this cloud and out of sight of predators swim the females laden with unfertilized eggs. They slip into the shallow to release sticky tiny jewel like eggs that coat rocks and marine algae. Two weeks later, the eggs hatch into larval herring that gather into schools in shallow coastal waters.

Each spring, the arrival of Pacific herring signals the end of winter for hordes of animals that gather for the spring feast. The sheer numbers of animals that eat herring make it an important link in the North Pacific food web.

Gray whales arrive from their breeding lagoons in Mexico in time to track the herring wave all the way to the Bering Sea. Surf and white-winged scoters also follow the herring north. Harlequin ducks gorge on herring and then move directly to their nesting grounds in mountain streams. Turban snails also consume herring eggs that are eaten by crabs and birds completing another step in the food chain. And humpback whales make spectacular lunges out of the water to scoop mouthfuls of herring.   

This “wave” of spawning activity occurs during the same period that many birds and mammals are migrating up the coast in preparation for breeding.  This leads to a series of intriguing and important questions:

1) How closely is the timing of migration of birds and mammals linked to the northerly progression of herring spawning activity?  For species that are known to consume spawning adults and eggs, does the wave of migration correspond to the wave of herring spawning?  Do marked individuals move progressively from spawn site to spawn site, using them as staging or refuelling areas as they migrate?

2) Do predators build energy reserves while foraging on herring adults or eggs?  If so, how are these used – as migration fuel, or as energy and nutrition for reproduction?

3) How important are spawn sites for migrating birds and mammals?  Can we estimate the consequences of changes to numbers of spawning sites or abundance of herring?

Do surf scoters migrate north on the wave of herring, as proposed by the silver wave hypothesis? Thanks to Sea Duck Joint Venture funds, Dr. Dan Esler from PWLF and his Canadian and American colleagues have begun to answer this question.

Dr. Esler and his team used satellite technology to record the locations of surf scoters carrying miniature radios that migrated through British Columbia and Alaska. They discovered that thousands of scoters assemble on the south coast of British Columbia to eat herring spawn in February and March. From there, scoters follow either an inland or coastal route to their breeding grounds in western Canada. Twenty-two scoters with miniature radios in the southern Strait of Georgia, British Columbia and Puget Sound in Washington State fattened on herring eggs in early spring before flying across British Columbia to the breeding grounds in northwestern Canada. Twenty-seven other scoters fattened on herring eggs in the Strait of Georgia, before flying to the north coast of British Columbia and southeast Alaska to eat more herring eggs prior to turning inland to the breeding grounds. Herring eggs helped female and male scoters fatten in British Columbia. Scoters caught at the end of the herring spawn season in May weighed 300 grams more than those caught prior to the spawn in December. The team also attached conventional VHF radios to scoters in California, Puget Sound and Washington State and southern British Columbia. Nine scoters from San Francisco Bay, 4 from Puget Sound, and 5 from the Strait of Georgia were located mostly in the northern end of Lynn Canal, including Sullivan Island, Chilkoot Inlet, Chilkat Inlet, and Taiya Inlet. The results indicate that some but not all scoters time their migration to correspond with spawning herring as the wave moves north and that individual scoters fly north from site to site to refuel on the feast of herring eggs.