The bloom of 2016
In the Southern Hemisphere's summer and autumn of 2016, the waters around Chiloé Island experienced a catastrophic harmful algal bloom (HAB). This event, locally called "Marea Roja" (Red Tide), was caused by a population explosion of the dinoflagellate Alexandrium catenella. Unusually warm ocean temperatures, partly driven by a strong El Niño, created ideal conditions for the algae. The bloom was so extensive that it led to the closure of up to 500 kilometers of coastline for shellfish harvesting.
The economic and ecological consequences were severe. The bloom killed an estimated 23 to 25 million farmed salmon, a loss of around 100,000 metric tons of product. The total financial impact on the salmon industry, Chile's second-largest export sector after copper, was estimated at $800 million. The crisis also devastated the local artisanal fishing communities, which depend on shellfish harvesting. Thousands of fishermen and seafood plant workers lost their incomes, leading to widespread social unrest and protests that isolated the island for 18 days. The situation worsened when controversial dumping of over 4,700 tons of dead salmon into the ocean by aquaculture companies, which some scientists believe may have supplied additional nutrients that intensified the toxic bloom.
Saxitoxin and paralysis
Alexandrium catenella produces a potent neurotoxin called saxitoxin. This compound is among the most toxic non-protein substances known; the lethal dose for an adult human is estimated to be just 0.57 milligrams if ingested. Saxitoxin is roughly 1,000 times more potent than cyanide. The toxin selectively blocks blocking voltage-gated sodium channels in nerve and muscle cells. This action prevents the transmission of nerve impulses, causing a condition known as Paralytic Shellfish Poisoning (PSP).
Filter-feeding bivalves like mussels, clams, and oysters consume the toxic algae, accumulating saxitoxin in their tissues to levels that are dangerous for their predators, including humans. The shellfish themselves are often unharmed. In humans, symptoms of PSP begin with tingling and numbness in the lips and extremities, progressing to muscular incoordination, and in severe cases, respiratory paralysis and death. During the 2016 Chiloé crisis, dozens of people were poisoned by the toxin. The high toxicity levels recorded in some shellfish were extreme; surf clams washed ashore were found with saxitoxin concentrations as high as 8,763 micrograms per 100 grams of tissue, more than 100 times the regulatory safety limit.