Rodríguez-Rafael, E. D., Elorriaga-Verplancken, F. R., Enríquez-García, A. B., Martínez-Rodríguez, M. I., Cruz-Vallejo, R. A., Moreno-Sánchez, X. G., Moncayo-Estrada, R., Tripp-Valdez, A., Galván-Magaña, F. (2024). Effects of warm oceanographic anomalies on the foraging variability of northern elephant seals. Marine Biology, 171(7), 162. https://doi.org/10.1007/s00227-024-04476-5
You may have seen in the news that we exited an El Niño event this summer. El Niño and La Niña are climate anomalies that lead to higher or lower temperatures than normal. El Niño is characterized by sudden, abnormally warm sea surface temperatures that may last for just a few days, but can also stretch on for months. The progression of climate change and rising global temperatures has led to more intense and frequent climate anomalies. These climatic anomalies can have detrimental impacts on people. For example, El Niño events can lead to an increased risk of drought, storms, and extreme heat. But how are animals, particularly those residing in the Pacific Ocean, impacted by El Niño?
Recent research has found that these climate anomalies can impact the feeding, reproduction, and overall survival of some animals, including pinnipeds, which include seals, sea lions, and walruses.
The northern elephant seal and El Niño
The northern elephant seal is perhaps best known for the male’s long, trunk-like nose, but this species is also an important predator along the west coast of North America from northern Mexico to Alaska. Twice a year, female northern elephant seals migrate to feeding grounds in the Pacific. The first occurs after their breeding season, which takes place from February to April, and the second follows the molting period, from June to December. These feeding grounds are supported by oceanographic processes, such as eddies, as well as the subtropical and subarctic gyres, which are vortex-like currents rich in nutrients. When El Niño increases temperatures and changes wind patterns, this alters the gyres, currents, and eddies by changing the stratification of the water columns and decreasing nutrient transportation. This in turn can cause these regions to have less algae and plankton growth, leading to fewer fish for elephant seals to eat. Considering that northern elephant seals are highly dependent on these feeding grounds and that migration requires so much energy, a team of scientists decided to investigate how climatic anomalies may impact female northern elephant seal migration.
But how can elephant seal feeding sites be located? As mentioned before, female elephant seals migrate to feeding grounds after breeding. In pregnant females, the developing fetus grows a dark, soft coat called the languo coat. Since the languo coat is grown entirely from the food the mother eats during migration, it contains only the isotopic signals from the migratory feeding region, specifically stable carbon and nitrogen signals. Animals take up the carbon and nitrogen from the food they eat, which reflect the nutrients available in the feeding region, giving a clear indication of where the feeding takes place.
From 2013 to 2023, the researchers collected languo coat samples from recently weaned northern elephant seal pups. After the languo samples were broken down into a fine powder, the isotopes were analyzed, and the ratios of stable carbon and nitrogen were measured. Then these values were compared to the sea surface temperatures from each year to see if migration locations shifted in response to warm climatic anomalies.
What did we learn?
The ratios of carbon and nitrogen varied only slightly year by year, suggesting that overall, female northern elephant seal foraging is impacted very little by warm climatic anomalies. However, more extreme marine heat waves during the years of 2014, 2018, and 2021 caused a slight shift in the ratios of both carbon and nitrogen compared to other years, likely indicating changes in foraging strategies and locations in response to higher temperatures. Changes in the amount of isotopic nitrogen demonstrated a shift in foraging areas to different latitudes, while a difference in carbon suggested that female northern elephant seals are migrating towards more oceanic foraging areas rather than coastal ones.
So why are northern elephant seals so resilient when other pinnipeds are highly impacted by sea surface temperature anomalies? One reason could be that female northern elephant seals have a wide variety of foraging techniques; they may forage at the surface or dive to great depths.While climate anomalies tend to just impact the surface water to a depth of 100 meters, the average feeding dive for northern elephant seals is over 500 meters deep. This may mean that some of their food sources are not impacted by El Niño temperature changes. Therefore, the data from this study indicate that the shift of female northern elephant seal migration towards more oceanic regions may encourage more deep-diving behavior.
In the future, the researchers suggest that analyzing the stomach contents of female elephant seals could offer additional insight into how they are responding to climate anomalies. It is promising that northern elephant seals are displaying resilience in their migratory foraging patterns during warm climate anomalies, but it’s important to continue monitoring these key predators in the years ahead.
I am a recent graduate of Oregon State University with a bachelor’s degree in marine biology. I am most interested in marine ecology, sex-changing fishes, and intertidal nudibranchs. I also recently earned my AAUS scientific diving certification which has sparked my interest in the different applications of SCUBA diving in science. In the near future, I hope to pursue a master’s degree in tropical fish ecology. Outside of science, I also enjoy hiking, reading, and doing the wordle.
