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Blue-green algae toxin found in tissues of bull sharks in Indian River Lagoon

Photo: Researchers sampling blood and tissue from a little bull shark.

Toxic blue-green algae blooms are threatening the health of bull sharks in the Indian River Lagoon, one of the most important blue shark nurseries on the Atlantic coast, according to Harbor Branch researchers.

Young bull sharks that spend the first few years of their lives in the Lagoon show microcystin toxin – a harmful chemical emitted by blue-green algae – in their tissues even when the algae that produce the toxin aren’t blooming in the estuary.

That’s the bottom line of a new, ongoing study by Harbor Branch Oceanographic Institute assistant research professor Dr. Matt Ajemian and his colleagues at the institute’s Florida Center for Coastal and Human Health.

The Center was established about a year ago to investigate harmful algal blooms in the lagoon and determine how they affect the health of people and marine creatures. 

Ajemian said bull sharks are a potential “sentinel” species in the study because they are relatively high up on the food chain and accumulate toxins from what they eat. Sentinel species are ones that give early indications of ecological problems that can affect humans as well as animals.

Microcystin, which is produced by some blue/green algae or cyanobacteria, is “a potent liver toxin and possible human carcinogen,” according to the EPA. 

It can sicken or kill fish, marine mammals, birds and pets if inhaled or ingested in enough quantity. In humans, it “may produce allergic reactions such as skin rashes, eye irritations, respiratory symptoms and in some cases gastroenteritis, liver and kidney failure or death.”

Ajemian and his co-workers took blood and tissue samples from 31 young bull sharks they netted throughout the lagoon from the St. Lucie estuary to Cape Canaveral during the fall of 2018 and the spring of 2019 when no harmful algal blooms were present in those waters. 

The samples, analyzed by researchers at University of Connecticut, revealed non-lethal levels of microcystin toxins in about 20 percent of the sharks – two out of seven in the northern lagoon; one out of seven near Sebastian Inlet; and one out of nine near St. Lucie Inlet.

“We’re seeing concentrations of harmful algal bloom toxins in animals at times that we are not having major blooms,” Ajemian said, going on to ask the question: “What levels of toxins are these animals able to withstand in the lagoon?”

Ajemian said now that the researchers are armed with some baseline information, they will be able to track how toxin concentrations in the animals change between times when there are no algal blooms and periods when the high concentrations of blue-green algae are present.

Meanwhile, the scientists have implanted acoustic tags in about 70 small bull sharks. Each tag emits a unique signal that is picked up by listening stations installed throughout the lagoon. Ajemian said tracking those animals’ movements will be important in determining how long they spend in certain areas and whether they flee or stay put during algae blooms.