February 4, 2025 — When certain algae flourish in the sea, they produce neurotoxins that can sicken both humans and marine animals. Acute exposure to these toxins is known to kill whales and other marine mammals outright, but many carry the toxins chronically without displaying obvious symptoms. The authors of a new study suggest these chronic exposures may nonetheless prove lethal.
The study, published in Frontiers in Marine Science in November, found an association between so-called harmful algal blooms (HABs) and whale deaths due to human causes in U.S. waters. The authors propose the reason may be that HAB toxins disorient whales, increasing their vulnerability to ship strikes and entanglements in fishing gear, which were by far the main causes of death and injury in the data they analyzed.
“Non-fatal concentrations of algal neurotoxins may render whales more susceptible to injury because they are less able to respond to entangling fishing gear and oncoming ships,” study lead author Greg Silber, an independent researcher and former coordinator of whale recovery efforts at the National Oceanic and Atmospheric Administration (NOAA), told Mongabay by email. “Knowing this provides an opportunity to reduce whale mortality by using algal bloom prediction capabilities.”
More algal blooms, more whale deaths
To explore the link between HABs and whale deaths, Silber and his daughter and coauthor, Katy Silber, an ecologist at the Institute for Applied Ecology in Santa Fe, New Mexico, studied the timing and location of HAB events alongside large whale mortalities and injuries in U.S. coastal waters. They analyzed data from the East Coast (2000-2021) and West Coast (2007-2021), from UNESCO’s Harmful Algal Event Database, and NOAA’s National Marine Mammal Health and Stranding Response Program. And they looked only at cases of whale injury or death attributed to human activities, excluding those attributed to natural or unknown causes.
Their main finding? “The number of mortalities/injuries was frequently higher in years with large-scale or severe HABs,” the paper states.
Although yearly changes occurred, both human-caused whale deaths and injuries and HABs showed general increases over time. The paper suggests that while improved monitoring and public reporting may partly explain this rise, year-to-year variations in HABs are influenced by oceanographic processes and warming ocean temperatures linked to climate change. HABs are also intensified by human activities that provide excess nutrients for algae growth.
The study also found differences between the two coasts. On the Pacific coast, it found a clear correlation between HABs and human-caused whale deaths and injuries: In areas with active HABs, there were at least three more whale deaths or injuries compared to places without HABs. On the Atlantic coast, the connection was still there but weaker. This might be because the ocean conditions, algal species, or the way whale and HAB data are collected differ between the two coasts, the researchers wrote.