Algal toxin identified as culprit in Arctic tern die-off, thanks to abundant incriminating evidence

In the years of successive seabird die-offs in Alaska’s warming waters, scientists and coastal residents have pondered a question: Are algae-caused toxins, which are becoming more prevalent as water temperatures rise, causing or contributing to those deaths?

Now there are some definitive answers, at least about a localized die-off of Arctic terns in the Juneau area during the record-hot summer of 2019, according to a new study published in the journal Harmful Algae. Those birds were poisoned by saxitoxin, the compound that causes paralytic shellfish poisoning in people who eat tainted shellfish.

“It’s well-confirmed now. We feel confident in the sense that saxitoxin was a direct contributor,” said lead author Caroline Van Hemert, a U.S. Geological Survey wildlife biologist who has been studying bird health and the increasing risks to Alaska sea life of harmful algal blooms.

The 2019 die-off affected a few dozen birds, about half those in two nesting colonies, and mostly chicks.

The scientists investigating the die-off were able to analyze not just tern carcasses but the sand lance and herring that the birds were eating — and which adult birds were feeding their chicks. “That was the missing link in this,” Van Hemert said. They also had data on saxitoxin in mussels in the area, as well as documentation of a big bloom of the algae that produces saxitoxin, which has the scientific name Alexandrium catenella.

In most cases, all those different pieces of evidence cannot be gathered, Van Hemert said. When bird carcasses are retrieved – something that is not always possible – they are often too decomposed to provide meaningful information about algal toxins. Getting information about algal toxins in birds’ food sources is also difficult, she said. “If you have a lot of information like mussels sampling, you probably have some idea of what’s happening in the environment,” she said. “But in most places in Alaska, we don’t have that.”

Analysis of sand lance was important to the study. The slender silver fish are notorious for accumulating saxitoxin – and for causing exposure to animals that forage on them. In the case of the Juneau-area Arctic terns, the scientists think the amounts of saxitoxin found in sand lance “was sufficient to kill an adult bird,” Van Hemert said.

The lesson from the study is that algal toxins should be considered in cases of bird die-offs, even though it can be very difficult to tease out differences in causes of death, Van Hemert said.

Additionally, concentrations of saxitoxin in forage fish need not be very high to affect seabirds, she and her colleagues have found. For human consumption, the danger level in saxitoxin-containing shellfish is 80 micrograms per 100 grams, or less than one part in 12,000, but seabirds’ high metabolic rates and constant eating probably mean their danger threshold is a much lower concentration. “Birds could be exposed to a potentially lethal dose because they’re eating so much forage fish,” she said.

There have been no large-scale seabird die-offs reported this year in Alaska waters, but the picture has been complicated by the ongoing wave of highly pathogenic avian influenza, Van Hemert said. “Biosecurity requirements for testing for avian influenza preclude just going out and picking up dead birds,” she said.

Questions about saxitoxin risks to marine life extend beyond seabirds.

Farther north in waters off Alaska in the summer of 2019, saxitoxin was found at all levels of the food web, from tiny phytoplankton and zooplankton to big Pacific walruses and even bowhead whales, as described in a different study published in May in the journal Harmful Algae. The collections were made by a research cruise that sailed waters from the northern Bering Sea to the western edge of the Beaufort Sea in the Arctic Ocean north of Alaska. The marine mammal detections were made in fecal samples left by the animals; the walrus samples showed much higher saxitoxin levels than the trace amounts found in the bowhead samples.

Research into algal toxins and impacts to wildlife is continuing. From mid-July to mid-August, for example, a research cruise led by the Woods Hole Oceanographic Institute is set to document the distribution of harmful algal species in the Arctic waters of the Chukchi and Beaufort seas.

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