A new study has warned that Arctic and Subarctic marine mammals are facing rising mortality rates and declining reproductive success due to the increasing frequency and intensity of marine heat waves (MHWs). These extreme ocean temperature events, which can last for days or even years, pose a serious threat to species such as cetaceans, sea lions, seals, and walruses.
The Arctic Ocean is projected to become a future hotspot for MHWs, with sea surface temperatures surpassing historical norms for extended periods. Scientists define an MHW as a temperature anomaly that exceeds the local 90th percentile threshold for at least five consecutive days. These heat waves have already been linked to widespread disruptions in Arctic and Subarctic ecosystems.
The study, published in Frontiers in Environmental Science, analyzed past MHW events, including the 2017–2019 heat waves in the Bering and Chukchi Seas, as well as the infamous “Blob” event of 2014–2016 in the northeast Pacific. Researchers found that MHWs disrupt entire food webs, affecting not only marine mammals but also foundational species like phytoplankton, zooplankton, fish, and seabirds.
Delayed but Severe Effects on Marine Mammals
Arctic marine mammals, including narwhals and bowhead whales, are particularly vulnerable to rising ocean temperatures. While their thick blubber reserves provide insulation and energy during food shortages, they also limit their ability to dissipate excess heat. Narwhals, for instance, struggle to adjust their swimming and diving behavior in extreme temperatures, making them highly susceptible to thermal stress.
Past MHWs have already caused mass mortality events among marine mammals. Humpback and fin whales suffered high death tolls following the 2014–2016 Gulf of Alaska MHW. Additionally, warm conditions in the northern Bering and Chukchi Seas led to high mortality rates and poor pup health among ice-dependent species like spotted seals.
MHWs also contribute to the spread of disease. The study highlighted the first recorded death of a polar bear due to avian influenza in 2024, likely caused by scavenging on infected seabirds a grim indication of the broad conservation and health risks posed by these climate-driven changes.
Disruptions Extend Beyond the Arctic
The consequences of MHWs extend beyond the Arctic, as changing ocean conditions force marine species to shift their ranges. Following the Gulf of Alaska MHW, altered whale migration patterns led to an increase in whale entanglements in fishing gear further south, due to greater overlap between whales and fisheries.
While some Subarctic species can adapt by moving to colder waters, Arctic-dwelling animals do not have this option. As a result, many face habitat loss, food scarcity, and heightened mortality risks.
The study underscores the urgency of addressing climate change and its impacts on marine ecosystems. With the Arctic warming at nearly four times the global average, the frequency of MHWs is expected to rise, posing an escalating threat to marine biodiversity. Scientists stress the need for immediate action to mitigate greenhouse gas emissions and develop conservation strategies to protect vulnerable species from further decline.