- Harmful algal blooms (HABs) in the ocean produce toxins that can sicken humans and marine animals.
- A new study has found an increase in large-whale deaths and injuries near HABs on the U.S. East and West Coasts, mainly due to ship strikes and entanglement in fishing gear.
- The authors propose the reason may be that HAB toxins disorient whales, decreasing their ability to avoid ships and fishing gear. However, they say more research is needed to confirm the link.
- They recommend using advanced HAB prediction models to reduce whale deaths by limiting activities like fishing when HABs and whales overlap.
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.
Algal toxins threatening whales
The study also looked at the kinds of algae present in the blooms putting whales at risk. The most common genus on both coasts was the saxitoxin-producing dinoflagellate Alexandrium. Experts note that this toxin can affect marine mammals’ nerves and muscles, dulling the senses and reducing the ability to move properly.
The runner-up was Pseudo-nitzschia, the study found, although it was more common on the West Coast than the East Coast. This diatom produces domoic acid toxins that can cause marine mammals to become confused, move strangely, or even have seizures, experts say.
Algal toxins can persist in marine ecosystems, entering whales’ bodies through the krill and fish they eat, the paper notes. They could affect the whales over the long term, potentially diminishing whales’ capacity to detect or avoid fishing gear and approaching vessels, the researchers wrote.
NOAA research ecologist Elliott Hazen, who wasn’t involved in the study, expressed skepticism over this explanation for the increase in whale deaths and injuries due to human causes during HABs, and proposed other potential causes. For instance, Hazen said, warm water can cause HABs but can also drive anchovies and their whale predators closer to shore, where pot fishing is more common, thus potentially increasing the risk of entanglement. “The correlation is the dominant factor, with warm water often leading to increased HABs and inshore compression of whale prey,” he said.
On the other hand, Hazen said, fishery closures resulting from HABs can actually reduce the entanglement risk, further complicating the picture.
Similarly, when it comes to vessel strikes, Hazen said that while it’s possible HABs cause domoic acid toxicity in krill-eating whales that impairs their ability to avoid ships, there was no evidence to support or refute it. “Again, the enabling conditions of warm water may have effects on both parts of the ecosystem,” he said.
Hazen also noted that the difficulty in detecting and documenting entanglements and ship strikes — especially when they occur far from shore — makes it harder to definitively determine whether HABs are behind whale injuries.
Silber, noting that this is the first study to explore the link, acknowledged that more research is needed to confirm it, including routine testing for low-level algal toxins in whale carcasses to assess their role in entanglements or ship strikes. He also emphasized the importance of standardizing whale mortality data collection and consistently tracking HAB events and severity.
Growing whale populations, ongoing risks
The populations of many large whale species, particularly humpback whales (Megaptera novaeangliae), are steadily increasing in U.S. waters. Nevertheless, in the U.S., nearly all large whale species are listed as endangered, and three populations, including Atlantic humpbacks, are undergoing unexpected die-offs significant enough to earn the NOAA designation of “unusual mortality events.”
Ship strikes and entanglement in fishing gear are major threats to large whales globally. The Silber paper noted that on both U.S. coasts, 1,067 whale injuries and deaths were caused by fishing gear interactions during the study period, 325 were caused by vessel strikes, and 14 had other anthropogenic causes, such as marine debris. Nine species were affected, with humpback whales accounting for about half the incidents on both coasts.
These threats seem likely to persist. On Jan. 15, NOAA withdrew a proposed vessel speed regulation aimed at better protecting North Atlantic right whales (Eubalaena glacialis), whose numbers hover around 370 and whose leading cause of death is vessel strikes. Government and shipping industry officials hailed the move, but conservationists decried it. The New England Aquarium, a hub of long-term North Atlantic right whale research, called it a “serious setback for the recovery of this critically endangered species that is facing extinction.”
Meanwhile, Silber expressed concern that increasing HABs could lead to more vessel strikes. “It appears whales are generally capable of withstanding (or adapting to) toxin loads from algal blooms. However, whale deaths in fishing gear and from ship strikes are a problem globally. As whale populations grow — some are doing quite well — some can sustain low death levels from these sources. However, many cannot; and for them it remains a serious concern,” he said.
Predictive models to protect whales
If HABs do contribute to whales’ fatal collisions and entanglements as the paper proposes, there may be a way to reduce deaths, the authors write.
“National and regional governments should be able, with relatively little computing outlays, to couple algal bloom models with information on whale and commercial fishing/shipping distribution,” Silber said. “By recognizing where overlaps are greatest, management authorities can shift or limit those commercial activities to minimize whale injuries and deaths.”
Hazen said NOAA is “developing a lot of new abilities” to predict events like HABs or compression of whale habitat toward the shore. “These predictions can allow better preparation and more proactive approaches to reduce whale risk to ship strikes and entanglements,” he said.
Banner image: A humpback whale (Megaptera novaeangliae) breaches. Image by ArtTower via Pixabay (Public domain).
Citations:
Silber, G. K., & Silber, K. M. (2024). Co-occurrence of harmful algal blooms and whale deaths. Frontiers in Marine Science, 11. doi:10.3389/fmars.2024.1454656
Silber, G., Weller, D., Reeves, R., Adams, J., & Moore, T. (2021). Co-occurrence of gray whales and vessel traffic in the North Pacific Ocean. Endangered Species Research, 44, 177-201. doi:10.3354/esr01093
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