Marine officials were trying to determine Tuesday what caused thousands of sardines to turn up dead in Ventura Harbor, the second mass fish die-off in local marinas in as many months.
Roughly 6 tons of the small silvery fish were found floating in the harbor early Monday. Officials said their initial theory is that the sardines died after using up all the oxygen in a corner of the harbor.
The scene in Ventura Harbor — crews churning up the water with aerators and volunteers scooping nets full of fish up from the surface — was reminiscent of the cleanup effort in Redondo Beach six weeks ago when officials discovered a thick blanket of dead sardines coating King Harbor.
Scientists are looking into whether the two die-offs share a common cause.
A spokesman for the California Department of Fish and Game said a warden visited the harbor and concluded the die-off was the result of oxygen deprivation, not water pollution, toxins or algae blooms — the usual causes of fish kills.
Last month's massive die-off occurred after millions of sardines swam into King Harbor and suffocated. It took days for crews to scoop and vacuum up about 175 tons of fish carcasses from the harbor.
Those sardines tested positive for domoic acid, a neurotoxin generated by algae blooms, but scientists believe the fish — perhaps disoriented because of the toxic algae — swam into the enclosed harbor in such huge numbers that they died as a result of critically low oxygen levels, not poisoning. Still, what caused them to swim into the marina remains a mystery.
The die-off in Ventura appears to be much smaller.
Large schools of fish started to swim into Ventura Harbor about a week ago, Harbormaster Scott Miller said; it was unclear what drove them there. Dolphins, sea lions and seabirds followed, feasting on the heavy concentration of easy prey.
"We just think they moved in there, and it was just like crowding too many people into a room," he said.
An algae bloom along the coastline in recent weeks has poisoned dozens of sea lions, dolphins and seabirds and left them stranded on beaches across Southern California, but scientists have not linked either of the fish kills to the bloom.
USC biology professor David Caron said his lab was requesting fish specimens from Ventura Harbor to test for specific toxins related to algae blooms.
"It would be very nice to know if these fish that are undergoing mortality events are in anyway impacted or have toxins in their bodies," he said.
Fish kills are usually preceded by an algae bloom that entices large numbers of plankton-eating fish to the area, said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium, who studies low-oxygen "dead zones" of decaying algae near the mouth of the Mississippi River.
Because low-oxygen zones are fed by sewage, fertilizer and other nutrient-rich runoff, water pollution is usually considered an ultimate cause of such die-offs.
More severe fish kills have besieged Ventura Harbor before, but have typically followed algae blooms known as red tides. A 10-day red tide killed more than 80 tons of fish in 2003.
Miller said the die-off seemed to be subsiding after the cleanup and aeration.
"We think we got about 90% of the fish," he said. "And the birds this morning probably got the rest of them."