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On August 2, as Damon Tighe was taking off from Oakland airport, he saw a dark channel in the water below forming near Alameda, an island on the eastern side of San Francisco Bay. At first, the community scientist thought it was an oil spill, but he soon realized it was algae. He still remained concerned. If the algae turned into a harmful bloom that spread across the bay, it could have disastrous consequences, Tighe knew. So, once he returned home, he kept an eye on it during his daily trips to monitor the bay’s shoreline.

By the end of August, Tighe’s worst fears had come true: As many as at Lake Merritt, a tidal estuary in the heart of Oakland connected to San Francisco Bay. Tighe walked less than a mile around the lake on August 29, counting more than 522 striped bass, 39 bat rays, and tens of thousands of northern anchovies, topsmelt silversides, and yellowfin gobies—all dead. He quickly organized an for community members to document “the harrowing event,” and soon sightings of white sturgeon, dungeness crabs, endangered green sturgeon, and other animals were also reported.

Located straight east across the bay from the Golden Gate Bridge, Lake Merritt’s 3.4-mile loop path is trafficked by thousands of walkers, runners, and bikers each day. “A good portion of the community for the first time was seeing the life of the lake,” Tighe says, “but it was only because it was dying.”

The exact cause of the harmful algal boom, also known as a red tide, remains unknown, but experts suspect the bay's rising water temperatures, more sunlight shining through the water's surface, and increasing nutrient levels from water treatment plants contributed to the bloom's size and severity. Stuart Siegel, interim director of the San Francisco Bay National Estuarine Research Reserve, calls the bloom “definitely exceptional” with its intense spread through San Francisco Bay, high number of fish deaths, and rapid development.  called the harmful algal bloom the bay’s “largest in recorded history,” with fish washing up dead across the bay, but in the highest numbers at Lake Merritt.

Earlier in the summer, before the red tide and fish die-off, a summer anchovy boom offshore provided plentiful food for birds. Bay Area residents even witnessed from feasting pelicans. “There were just so many fish,” says Paige Fernandez, biologist at just north of San Francisco. Then, the algal bloom hit. It didn’t immediately harm the birds’ fish supply; in fact, the bloom made the fish easier to catch. By the end of August, waterbirds enjoyed an of fish and mollusks weakened by the toxins created by the algae, which doesn't seem to affect the birds if ingested.

Now, Tighe and other experts are worried that migrating birds to and through the Bay Area could experience the opposite phenomenon: a severe lack of food. “The bay is a major stopover in the Pacific Flyway,” Siegel says. If August's red tide decimated fish populations, migratory birds in desperate need of sustenance might be in trouble. During peak migration, the Bay Area’s more than may face a food shortage if fish can’t reproduce quickly enough to replace those lost to the red tide. “Now that all of those organisms are dead, I just don't think [the populations] are going to bounce back in time,” Tighe says.

Shorebirds, including near-threatened Snowy Plovers and Ridgway’s Rails, have already started arriving. Overwintering ducks, such as Surf Scoters and Greater Scaup, will come in October and November. Hundreds of thousands of ducks will congregate in late winter to take advantage of the Pacific herring run—“the caviar of San Francisco Bay,” says Andrea Jones, director of bird conservation for . Fortunately, early indicate the red tide might not have hit herring as hard as other species. Anchovy, another fish commonly eaten by Bay Area birds, could be of greater concern. Tighe flagged anchovy as dying in great numbers when monitoring Lake Merritt and the bay's shorelines. “I would definitely be concerned with losing anchovy because so many birds do rely on that fish,” Jones says.

Soon researchers will have a better sense of whether the local fish populations are as depleted as they fear. If birds arrive and food isn't where they expect, starvation events could occur. After a long migratory flight, many birds arrive in declining condition and need to refuel quickly, says Krysta Rogers, a senior environmental scientist at the California Department of Fish and Wildlife. “If there's no food when they arrive, then they may or may not be able to make another trip to find food,” she says. Shorebirds and ducks also don’t have many options in the area—their populations have been squeezed into few remaining habitats in San Francisco Bay.

Lake Merritt, the and home to more than , is one of those habitats. Tighe says he hasn’t seen an increase in starving or dead bird sightings since creating the iNaturalist page, but he has heard reports of missing Black-crowned Night-Herons and other birds commonly seen at Lake Merritt. Whether their absence is due to the red tide die-off is hard to determine, but Tighe speculates it could be related.

“The red tide certainly affected fish populations,” Fernandez says. “It is possible that the birds [will] have a harder time finding fish to eat.” Even for non-threatened species, populations can suffer if a significant die-off event occurs. “In a very severe case, it is possible that we will see birds washing up on beaches that have died from starvation,” she says. Tighe also worries about the fish die-off from a recreation and human food-safety point of view: Many non-native striped bass died, which “a lot of families catch as a continual kind of food source around the bay.”

Researchers and officials are working to understand the causes of the bloom and fish die-off. Experts say fish deaths were likely driven by a drop in dissolved oxygen as algae decayed, a fish-killing toxin produced by the algae, or a combination of the two. The algae causing the bloom, Heterosigma akashiwo, is “a selective killer” that creates a toxin in fin fish but isn’t known to climb the food chain, says William Cochlan, who studies marine phytoplankton at San Francisco State’s Estuary and Ocean Science Center.

The San Francisco Bay Regional Water Quality Control Board awaits results for toxins in sampled fish at Lake Merritt. On August 29, they tested the water’s dissolved oxygen and found it had dropped to zero, likely due to its shallow water levels, high nutrients, and limited water exchange. Algae flourish in shallow water bodies that heat up quickly and have a small influx of fresh water flowing in, Tighe says. Eileen White, executive officer for the board, says “we don’t think the nutrients triggered the [algal bloom].” But they allowed algae to grow and spread rapidly.

Recently conditions have improved. The red tide has dissipated, and dissolved oxygen levels have returned to typical levels. Other than missing the fish wiped out by the bloom, White says "the bay right now is back to its more normal state." And so far, there hasn’t been a spike in dead bird reports, which community members can .

This lack of reports gives some hope that the worst-case scenario might be avoided during this migration. “We can make all these predictions, but sometimes the fish populations bounce back and sometimes the birds are a bit more resilient than we give them credit for,” Fernandez says. “It's hard to figure out what exactly is going to happen before it happens.”