If you’ve ever been to San Francisco, chances are you’ve been to Fisherman’s Wharf and stopped off at Pier 39 to check out one of the city’s most famous tourist attractions, the sea lions. Hundreds of sea lions call Pier 39 home, but since last fall the population has decreased dramatically. Sea lions are migratory animals so they do move around a lot.  However, scientists believe El Niño is to blame for the disappearance due to changes in water surface temperatures and lack of upwelling.

Sea surface temperature across the world. El Niño is characterized by unusually warm temperatures. Image: NOAA

Ocean upwelling is essential in order to mix the deep cold water at the bottom of the ocean with the warm water at the surface. This allows the cold water and nutrients that fertilize aquatic plants to rise to the surface. Without upwelling, the nutrient-filled waters aren’t brought to the surface and fish begin to migrate to other parts of the ocean to look for food. 

Ocean upwelling. Image: Encyclopedia Britannica 

El Niño is the periodic warming of the eastern Pacific that can have a significant effect on fish stocks and on other animals that feed on fish. As El Niño warms the Pacific, fish are fleeing to colder areas, leaving the sea lions with little to eat. Marine experts believe that the sea lions might have traveled 500 miles north, looking for fish and colder waters along the Oregon coast.  An increase in sea lions off the coast of Oregon did begin to occur in October, just as they were leaving San Francisco.

Sea lions at Pier 39 in San Francisco. Image: flickr/wallyg