Scientists work to explain — and prevent — eerie fish kills along the California coast following last weekend’s massive anchovy die-off in Marina del Rey
By Joe Piasecki
Marine biologists and public officials have concluded that lack of oxygen is most likely to blame for the sudden deaths of tens of thousands of fish last weekend in the waters of Marina del Rey, but whether pollution and other environmental factors played a role remains uncertain.
Increased water temperature, algae-feeding urban runoff and too little water circulation may have each played a role in depleting dissolved oxygen amounts below levels capable of sustaining life, according to scientists.
The silver blanket of dead anchovies that coated the surface of the northwest harbor basin between Tahiti Way and Bora Bora Way on Sunday morning was the result of a die-off that began overnight, according to witnesses.
Marina del Rey residents Bruce and Angela Philpott said they heard a commotion in the water shortly after midnight and were surprised by what they saw.
“The first thing we noticed was the noise. When we looked out across the water, fish were schooling on the surface in such great numbers that it looked like the sea was boiling,” said Bruce Philpott, 35, an intellectual property attorney. “It stretched for as far out into the water that we could see under the lights.”
After daybreak, birds and sea lions arrived to feast as Los Angeles County Dept. of Beaches and Harbors crews set about the task of removing dead fish from the water. Workers collected about 7,000 pounds of northern anchovy carcasses over several hours, filling 35 large bags for transportation to a composting facility, Beaches and Harbors spokeswoman Carol Baker said.
County workers returned Monday after thousands more fish floated to the surface.
Crews employed an inflatable boom to contain the dead fish at the back of the basin while removing them with a debris-collection boat — a small, open-deck vessel that utilizes an internal conveyer belt mechanism to skim the surface of the water and pull objects into the boat.
On Tuesday, biologists with the Santa Monica-based nonprofit group Los Angeles Waterkeeper conducted tests in the harbor that determined the waters where the fish had died, known as Basin A, were severely oxygen-depleted.
“The main channel in the marina had six to seven times the amount of oxygen than was present in Basin A,” Los Angeles Waterkeeper Executive Director Liz Crosson said.
California Dept. of Fish and Wildlife spokeswoman Janice Mackey said Tuesday that while conclusive results of state water and fish necropsy tests are pending, investigators believe the anchovy school likely died of oxygen deprivation after losing their way in the harbor and using up all of its dissolved oxygen.
“Why they went into the harbor is hard to tell, but most likely they were trying to seek cover from a predator species. They kind of got trapped in there, panicked and used up all the oxygen in the water,” Mackey said.
Similar fish die-offs have occurred for similar reasons elsewhere on the California coast. In 2011, a die-off of about six tons of sardines occurred in Ventura Harbor, and millions of various small fish died suddenly that spring at King Harbor in Redondo Beach.
USC marine biology professor Dave Carron led a team of scientists that concluded the sudden arrival of a large and densely packed school of sardines used up all the oxygen in the waters of King Harbor.
“In most of these events, you’re like an ambulance chaser. You get to the scene after the accident has occurred. In this case we had information from sensors that were already in the water before the event,” Carron said.
The data ruled out concerns about water temperature changes, algae bloom and pollution.
“This was a huge number of fish, and they were most likely to blame for taking the oxygen down to critical levels,” Carron said. “The calculation was those fish could use up all the oxygen in the water in six hours.”
Due to cutbacks in federal research funding, the network of water sensors that solved the mystery of the Redondo Beach fish kill — sensors once also in place in parts of Marina del Rey — are no longer active, Carron said.
Tom Ford, director of marine programs for the nonprofit Bay Foundation, said the man-made harbors such as Marina del Rey harbor are not well designed to support large populations of sea life.
“They minimize water movement—that’s the point of a harbor. The suppression of water movement leads to less water circulation, which can set up this kind of scenario,” Ford said. “Anchovies swim around constantly and require a lot of oxygen. It’s like sending you out for a run with a garbage bag to breathe out of.”
Dana Roeber Murray, a marine scientist with Heal the Bay, said a multitude of factors could have reduced the dissolved oxygen in Marina del Rey harbor’s Basin A before the ill-fated anchovies arrived.
Unusually hot weather may have increased the water’s surface temperatures, further slowing down water circulation and suppressing aeration. Heat combined with nutrient rich runoff common to the marina may have also stimulated algae growth that further depleted oxygen levels.
“The marina is a place where there are intense stressors on marine wildlife,” Murray said.
Ford said the Bay Foundation’s recently completed restoration of Malibu lagoon focused on restoring water circulation to restore dissolved oxygen to levels that could support an array of marine life.
How to improve water circulation in Marina del Rey is a complicated question, and over the years some have suggested installing pumps or aeration systems to stimulate water movement deep inside the harbor.
Carron envisions a network of sensors up and down the California coast that could help scientists monitor water quality and warn of potential ecological disasters, either from low dissolved oxygen levels or chemical pollution.
“We have the technology. What we don’t have is the funding to use it on a regular basis,” Carron said.