Marine Mammals and Marine Heat Waves
Learn About Marine Heat Waves and How You Can Help
The ocean can experience heat waves much like those that occur on land. One important difference is that heat waves in the ocean can last much longer—sometimes for months or even years. We use the term “marine heat wave” to describe unusually warm ocean temperatures that persist over an extended period.
Like weather on land, these events are driven by a combination of factors. Weakened wind patterns, El Niño conditions, and long-term increases in ocean temperatures (such as those caused by climate change) can all contribute to the development of marine heat waves.
El Niño is the warm phase of a natural climate cycle called the El Niño-Southern Oscillation (ENSO). During an El Niño, sea surface temperatures in the central and eastern tropical Pacific Ocean become warmer than average.
El Niño events typically occur every two to seven years and often last about nine to 12 months, although some can persist longer. These events can affect weather patterns around the world, bringing heavier rainfall to some regions and drought to others. An El Niño can also contribute to marine heat waves by raising ocean temperatures.
For marine mammals, these warming events can have devastating consequences. Warmer waters can disrupt the ocean food web, reduce nutrient-rich upwelling, increase harmful algal blooms like domoic acid events, shift prey availability, and force animals into new and often dangerous habitats.
What we’ve seen historically is that warming ocean conditions reduce populations of energy-rich prey such as sardines and anchovies that are staples for California sea lions. That means sea lion mothers who are nursing their pups must travel farther to find enough food. As a result, pups are left fasting for longer periods at the rookery, leading to lower body weights and poorer nutrition. In severe years, some hungry pups leave in search of food before their normal weaning age, even though they are not fully prepared to forage independently.
Warming ocean temperatures can also impact the growth of harmful algal blooms that produce a toxin called domoic acid. Sardines, anchovies and shellfish eat the toxic algae and then bigger animals like sea lions and dolphins unknowingly eat the contaminated fish.
Domoic acid is considered one of the foremost threats to marine animals by the National Oceanic and Atmospheric Administration (NOAA). Large-scale toxic algal blooms like the ones we’ve seen during major marine heat waves and El Niño events are capable of sickening hundreds of sea lions in a matter of weeks, causing them to wash ashore at an alarming rate – sometimes a dozen a day.
Marine heat waves can also trigger a domino effect that worsens the threat of entanglement for humpback whales. As waters warm and prey distribution changes, whales change their behavior too. In some cases, this results in the whales traveling through the same waters as fishing gear and crab traps, which makes them much more likely to get entangled in the lines.
From 2013 to 2016, the West Coast experienced the impacts of a large-scale marine heat wave event that led to thousands of malnourished sea lion pups washing up along the shores of California, predominantly in Central and Southern California counties, including Los Angeles.
The increased strandings led NOAA to declare an Unusual Mortality Event (UME) and launch an in-depth investigation into the root cause of the strandings.
Based on what researchers learned from examining rescued pups as well as those who died, the UME was attributed to malnutrition in young sea lions caused by shifts in prey availability. They also concluded that those prey shifts were most likely driven by unusual oceanographic conditions at the time due to a marine heat wave (known as the “Warm Water Blob”) and El Niño.
More than 8,000 young sea lions stranded during the UME from 2013 to 2016, and more than 90 percent of those animals stranded alive. The majority were California sea lion pups (6 to 9 months of age) who had been weaned too early from their mother’s milk and yearlings (1 to 2 years old) who had failed to thrive on their own. Many of these young sea lions were severely emaciated.
At Marine Mammal Care Center, we are closely monitoring rapidly changing ocean conditions and preparing for what may lie ahead. As environmental threats intensify, ensuring we have the medical supplies, patient food, rescue equipment, facility capacity, and emergency response resources needed to meet future stranding events is more important than ever. Your ongoing support helps make that preparedness possible.
You can help ensure a healthier ocean for marine mammals and humans alike by taking action in your everyday life.
Use Your Voice
Vote with the environment in mind. Discuss the importance of conservation and marine mammal protection with friends, neighbors and local leadership. Research candidates’ stances on the topic before heading to the polls.
Eat More Plants
Go meatless at least one day a week. Reducing meat intake lowers your carbon footprint and can help slow global warming.
Share the Shore
Give resting marine mammals space by maintaining a safe distance of at least 50 feet and keeping pets on a leash. Use the zoom on your camera rather than approaching for a selfie. Call 1-800-39-WHALE to report sick or injured animals.
Together, we can ensure MMCC remains ready to respond when marine mammals need us most, providing rescue, rehabilitation, research, and hope during an increasingly uncertain time for our ocean and the animals that depend on it.
