Another deadly challenge for the sea otter
Pity the poor sea otter.
It’s been a struggle for the furry, button-nosed critter to make a comeback since being hunted nearly to extinction along California’s coast.
They get chomped by great white sharks. They must scrounge in overexploited waters to find enough shellfish to eat. Their immune systems are weakened by polluted runoff and under attack by parasites that wash into coastal waters from the feces of domestic cats and opossums.
Now it turns out that some of these playful marine mammals are also being poisoned by an ancient microbe — a type of cyanobacteria — that appears to be on an upsurge in warmer, polluted waters around the world.
The discovery was made by Melissa Miller, a state wildlife veterinarian and scientific sleuth investigating the multitude of things killing otters faster than they can reproduce. The Southern Sea Otter population has dropped for two years in a row, the U.S. Geological Survey announced last month. An estimated 2,711 otters remain in Central and Southern California waters.
The first clues came when nearly a dozen otters mysteriously died in Monterey Bay in 2007. Their carcasses were taken to the California Department of Fish and Game laboratory in Santa Cruz, where Miller and others do postmortem analyses.
“I started getting otters that were clearly jaundiced, with bright yellow gums and yellow in the whites of their eyes,” Miller said.
Performing necropsies, she found swollen livers that fell apart in her hands. She initially suspected a bacterial infection, leptospirosis, known for outbreaks in sea lions and found occasionally in sea otters as well. Yet all the tests turned up negative.
“I sat down and said, ‘I’m seeing a new problem. I’ve got to back to the basics.’” She began to rule out potential causes. Poisonous mushrooms? No. Poisonous plants? No. Iron toxicity? No. Drug overdose? None of these made sense.
Then she dredged something out of her memory from veterinary school. The damaged livers were like those of a dog or a cow that died after drinking out of a scum-choked farm pond. The culprit in those cases was a toxin, microcystin, produced by a type of cyanobacteria called Microcystis.
She found a lab to run tests and, sure enough, the liver sample tested positive for the Microcystis toxin. Yet it raised a question: How could a toxin produced in fresh water poison a sea otter?
Miller called the State Water Quality Control Board and learned that Microcystis blooms seemed to be occurring more often in lakes and estuaries. One was Pinto Lake, about five miles inland from Monterey Bay, where some of the yellow deceased otters had been found.
“I sent a lab tech to look at this lake,” Miller said. “She called me on her cell and said, ‘This is gnarly. I’m going to take pictures.’” She also took some samples.
“The best way to describe it? The lake turns the color of automobile antifreeze with chunks of broccoli floating in it,” said Robert Ketley, water quality manager for the nearby city of Watsonville. “It’s that grotesque. When the scum dries, it has a turquoise color to it.”
The city owns much of the 100-acre lake and used to draw on it for water — but no longer. Ketley now posts signs warning people to avoid contact and to keep their pets away.
The city is investigating what’s prompting the toxic bloom. A nearby pig farm is gone. Yet other farm fields drain into the lake, which also has shoreline homes that rely on septic tanks for sewage disposal.
Wayne Carmichael, professor emeritus of aquatic biology and toxicology at Wayne State University, calls Microcystis a premier organism. “We find it everywhere you have nutrient enrichment: nitrogen and phosphorous in warm, stagnant water. It’s been documented in every country in the world.”
It’s a type of cyanobacteria, an ancestor of modern-day bacteria and algae, which dominated the planet more than 2.5 billion years ago. Scientists have found that different strains are reemerging with the buildup of pollution and nutrients from expanding agriculture and the modern industrial society.
The samples collected by Miller’s lab tech found a super-bloom underway in Lake Pinto, with highly toxic readings. Miller teamed with Fish and Game chemists and UC Santa Cruz biologist Raphael Kudela to follow the toxic trail from Lake Pinto and other local lakes down rivers that reach Monterey Bay. Some of the toxins were also detected in ocean waters at Santa Cruz wharf.
But the results didn’t explain how otters might ingest a lethal dose.
So Miller and her colleagues designed a lab experiment to test a hypothesis. With the lab reeling from state budget cuts, Miller shelled out money to buy six heaping shopping bags of live oysters, mussels and crabs to place in seawater tanks. Then they added some contaminated water from Lake Pinto.
Most shellfish filter the seawater to feed themselves, gathering microscopic food and anything else in the water. Tissue samples revealed the shellfish in the tanks had accumulated the toxin in their digestive tracts at concentrations that were 107 times higher than in the surrounding water.
The study, published by the Public Library of Science’s peer-reviewed journal, documented the first case of a freshwater toxin poisoning of a marine mammal. The toxin was responsible for the death of at least 21 sea otters, a species listed as threatened with extinction.
The study also suggests that humans may be at risk if they consume shellfish harvested from river mouths, especially after the first fall rains flush toxins built up in the lakes. Public health officials do not test shellfish for freshwater toxins, only for marine toxins such as the one that causes paralytic shellfish poisoning.
Carmichael, the expert on Microcystis, isn’t too concerned about acute human poisonings because the dose would be too small. Sea otters consume about 25% of their body weight a day in shellfish, creating perfect conditions for toxic poisoning.