Tens of millions of silvery sardine-sized fish pour into San Francisco Bay each winter to lay and fertilize their eggs. The water turns milky gray as the males release their sperm, and other animals gather to feast — pelicans, gulls, cormorants, seals, sea lions, sturgeon and more. Fishermen pursue the small fish, too; some use gear as trivial as baited hooks floated under bobbers, while others use boats and set gillnets that catch many tons at a time.
But the phenomenal annual spawning events of the Pacific herring, which often aggregate densely along the Richmond and downtown San Francisco shorelines, have been losing their grandeur. Estimated herring populations dropped steeply about five years ago. The fish have not recovered, and scientists studying the decline aren't exactly sure why.
William Sydeman, a senior scientist with the Farallones Institute, based in Petaluma, has studied long-term trends in Pacific herring abundance. He notes that San Francisco Bay's herring population has waxed and waned for decades, with estimates jumping up and down from year to year, as seen in bar graphs produced by the California Department of Fish and Wildlife.
But in late 2014, the population abruptly nosedived as the fish's total abundance, or "biomass" in ecologists' jargon, plunged from an estimated 60,600 tons in the winter of 2013-14 to less than 17,000 tons the next year. The population has remained below 19,000 tons since, with last year's number dipping to 8,500 tons — the second-lowest return on record after 2008-09's biomass of 4,800 tons.
Through the 1980s, 50,000 tons or more was typical for San Francisco Bay; through the 1990s, 30,000 tons or so.
Today's numbers seem to represent a fishery collapse.
"I think herring in San Francisco Bay are really in trouble," Sydeman said.
Herring are one of many small schooling species loosely termed as "forage fish." These species, including anchovies and sardines, provide food for dozens of larger animals. Many seabirds, salmon, tuna, sea lions and whales feed on forage fish. That's why any detectable dip in population is cause for concern.
Commercial gillnetters target herring and catch anywhere from a few hundred to 2,000 to 3,000 tons each year. The fish are primarily sold for use as fishing bait and feed for livestock and aquaculture operations. The females' roe — usually about 12 or 13 percent of the total catch by weight — is sold for consumption in Japan. The commercial fishery has been criticized as wasteful.
Recreational fishers also fish for herring. When the fish are spawning — as evidenced by the feeding frenzy of birds and pinnipeds in pursuit — people gather with buckets and hand-thrown nets at shoreline sites like Point Richmond or the Alameda Rockwall and harvest as much as they can carry away.
But Sydeman doesn't believe herring are victims of overfishing.
Rather, global warming may be the main culprit. Specifically, when the surface of the ocean warms, the upwelling cycles that deliver nutrient-rich bottom water to the surface — and which generally make California's coastal waters such a productive environment — can shut down. This affects the entire food web, from phytoplankton up to whales.
That's what happened in late 2013, when an area of warm water termed by scientists and media as "the Blob" formed off the West Coast and remained there for more than a year. That event threw the ecosystem into chaos as small fish like anchovies and sardines migrated to more productive regions. Sea lions starved by thousands.
And San Francisco's population of herring — which feed on plankton — tanked.
Tom Greiner, a biologist with the California Department of Fish and Wildlife who focuses on San Francisco Bay's herring, says "oceanic conditions" are probably the foremost factor affecting the herring.
"We had a strong El Niño, and also the Blob," he said. The El Niño event he's referring to is that of 2015-16, one of the three strongest on record. El Niños typically create warm surface waters as disrupted currents bring a halt to upwelling — which is bad news for small fish.
The decline in herring has not been limited to California. In 2017, Sydeman and several colleagues published a paper in which they found that, of 14 regional populations from California north into Canada, all but two showed long-term downward trends.
In Alaska, too, once-enormous herring populations crashed in the 1980s and 1990s and haven't recovered. In northern Europe and Japan, herring runs have diminished, as well.
In British Columbia, four of five major herring populations have nosedived. The fish are smaller in average size than they once were, and the geographical extent of where they spawn has contracted, says Vanessa Minke-Martin, who works on conservation campaigning for Pacific Wild, a British Columbia environmental group. Industrial-scale fishing of the late 19th and early 20th centuries may have depleted these populations. Historically, much of the catch was reduced into oil and fishmeal.
Minke-Martin says the Canadian Department of Fisheries and Oceans uses 1950 as a baseline for the populations.
"This makes things look pretty good, because by then their population was already way down," she says.
But historical and archaeological records suggest a much higher baseline population. Minke-Martin says traditional knowledge from coastal indigenous populations indicates a relatively huge geographical spawning range as recently as a century ago, compared to the shorelines along which they spawn today.
Minke-Martin's group has been calling for a closure of the fishing season in the Strait of Georgia between Vancouver Island and mainland British Columbia, the last region in the province where fishing for herring is still allowed. She says efforts also are underway to create spawning habitat by restoring damaged eel grass beds. By and large, though, herring are overlooked by fisheries researchers, especially as salmon and orcas face extinction in the same region, Minke-Martin says.
In San Francisco Bay, the most dramatic crash in herring abundance has come since 2015. However, in Sydeman's 2017 paper, he and his coauthors described a long-term decline in the San Francisco Bay herring population of about 2 percent per year from 1980 to 2013. If global warming is the main reason that San Francisco Bay's herring runs are thinning out, this trajectory seems likely to continue.
In fact, scientists have predicted that the world's oceans will become, overall, less productive as waters warm, stifling upwelling and reducing productivity.
"This has been very well documented in the climate models," Sydeman said.
Marine heatwaves, he said, are becoming more and more common.
"And the northeastern Pacific is increasingly becoming an inhospitable place for fish that like cold water, like salmon and herring," he said.