Mystery as Thousands of Anchovies Invade California Harbor

Thousands of tiny fish have been found gathered in Santa Cruz Harbor, California, turning the water dark.

These shoals of baitfish—thought to primarily be anchovies—began clustering in the harbor and along the nearby coast last week for reasons unknown, attracting hoards of hungry seabirds.

The swarms of fish sparked concerns that the water would be sapped of oxygen, causing a mass die-off of the fish and other marine life nearby.

"When large schools of fish inundate the harbor, it can reduce oxygen levels to a point where fish populations become stressed or die off en masse. This has occurred several times in the harbor's history, with the last large-scale die-off occurring in 2014," Santa Cruz Harbor explained in a Facebook post.

Mass fish die-offs occur on occasion around the world. In March this year, millions of fish clogged up the Darling River near the Australian town of Menindee, triggered by floodwaters receding and washing tons of organic matter into the river, reducing the oxygen concentration in the water. Mass deaths of fish can also be triggered by drought, overpopulation, increasing water temperatures, infectious diseases, chemical pollution, and algal blooms.

Santa Cruz Harbor saw its last mass die-off in 2014, causing a strong fishy smell as a result of the rotting fish and the feces of the seabirds feeding on the mass of bodies.

To prevent a similar thing happening this time, the harbor turned on aerators to re-oxygenate the waters.

"We have aerators throughout the harbor," Santa Cruz Port District Harbormaster Blake Anderson told the Santa Cruz Sentinel. "They put oxygen back into the water. The oxygen is being depleted as the fish swim and as they breathe. It's a lot like your home fish tank. We're trying to aerate that water and bring more oxygen to the water."

"Not only does it provide more oxygen to the water, which is ultimately what we need, we believe that it creates a nuisance sound that maybe they don't like," said Anderson. "We're hopeful that that will help to drive them out of here, too."

For now, the oxygen levels appear to have stayed high enough for the fish. However, there were fears that if more fish crowd into the waters, they will simply overwhelm the efforts of the aerators.

"It's a large biomass," said Anderson. "If the fish stay in here for long enough, there are so many of them that no amount of aeration or mitigation is going to help. The biomass will just overwhelm the aeration system and that's when we have the die-offs."

The reasons behind this swarming of fish is unclear, but Anderson suggested that it may be due to predators like whales or sea lions pushing the fish towards the coast, or a lack of oxygen further off the shore.

"Up and down the coast in Santa Cruz, we're hearing about the same thing occurring," said Anderson. "Over at Cowell's and Sunny Cove and Black's Beach all the baitfish are up against the shoreline—almost in the waves. That's also happening in South County. Our partner agencies are telling us there's tons of bait fish near the shore. Something is pushing all of the fish in this area toward the shore."

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