Last week, I mentioned how the Pacific herring (Clupea pallasii) had returned to Uyak Bay on Kodiak Island this spring in such large numbers, even bears recently out of hibernation noticed and were feeding on them in the shallow estuaries where they spawn. Pacific herring are an essential food source for many animals living in or near the North Pacific, including birds such as cormorants, murres, auklets, puffins, and bald eagles; fish, such as salmon, halibut, cod, and pollock, and marine mammals, including harbor seals, Steller Sea Lions, fin whales, humpback whales, and orcas. When a pursuing predator forces a school of herring to the surface, seagulls take advantage of the situation and can often be spotted noisily diving and feeding on the fish. Herring are loaded with nutritious oil and nutrients and are an important forage fish for many species.
A herring has a blue-green upper body, silvery sides, and large eyes. Its body is laterally compressed with large scales, protruding in a serrated fashion. It has no scales on its head or gills. A herring has a deeply forked tail, a single dorsal fin located mid-body, and no adipose fin. Pacific herring can grow to 18 inches (45.7 cm) in length, but they are usually smaller than 9 inches (22.9 cm).
Pacific herring live throughout the coastal waters of the Pacific Ocean. In the eastern North Pacific, they range from Baja California north to the Beaufort Sea, and in the western North Pacific, they can be found in the western Bering Sea to Kamchatka, in the Okhotsk Sea and around Hokkaido, Japan southeast to the Yellow Sea.
[Pacific herring reach sexual maturity when they are three to four years old, and they spawn each year after reaching sexual maturity. Spawning occurs in the spring in shallow nearshore areas in intertidal and subtidal zones. Females release eggs at the same time males release sperm into the water, and the eggs and sperm mix, fertilizing the eggs. A single female can lay 20,000 eggs.
Spawning is precise, and while the trigger is not well understood, researchers suggest the male initiates the process by releasing milt containing a pheromone which stimulates females to release eggs. The process seems to be synchronized, and an entire school spawns in a period of a few hours, producing an egg density of up to 6,000,000 eggs per square yard (square meter). The fertilized eggs then attach to vegetation such as eelgrass or kelp or to the bottom. Eggs hatch two weeks after they are fertilized, and the larvae drift in the ocean currents. As they grow, juvenile herring stay in sheltered bays until autumn and then move into deeper water where they spend the next two to three years. Juveniles remain separate from the adult population. Biologists estimate only one herring per ten thousand eggs reaches adulthood.
Juvenile herring feed on phytoplankton and zooplankton, and adults also eat bigger crustaceans and small fish. Pacific herring travel in large schools. They migrate inshore to the heads of shallow coves and bays to spawn and then offshore to feed. They also migrate vertically in response to their prey, remaining near the bottom during the day and rising toward the top of the water column at night.
Herring are susceptible to environmental changes. Since they depend on shallow, inshore habitats to reproduce, they are affected by storms, pollution, and warming water temperatures. The Pacific herring population in Prince William Sound collapsed in 1993, four years after the Exxon Valdez oil spill, and it has still not recovered.[
The biggest threat to Pacific herring is a loss of their spawning grounds. Spawning habitat can be degraded or destroyed by construction, dredging, log storage, oil spills or other pollution, and by global warming.[ If Pacific herring populations crash, their loss will affect the many species of fish, birds, and marine mammals which depend on them for food.
Robin Barefield is the author of three Alaska wilderness mystery novels, Big Game, Murder Over Kodiak, and The Fisherman’s Daughter. To download a free copy of one of her novels, watch her webinar about how she became an author and why she writes Alaska wilderness mysteries. Also, sign up below to subscribe to her free, monthly newsletter on true murder and mystery in Alaska.
