Be Kind to Your Neighbor: the California Sea Lion

Authors: Rachel Apolinario, Nicole Crawford, Maia Cymerys Da Silva, Ze En Ling

If you’re enjoying a day at the beach anywhere in California, you’ve probably come across some Zalophus californianus, or as they are more commonly known, the California sea lion. It’s especially hard to miss them in Santa Cruz. Whether you are at the beach, getting lunch on the wharf, or going on rides at the boardwalk, you have likely seen or heard these large marine mammals barking at each other or basking in the sun. It is common to see the larger, dark colored male California sea lion surrounded by the smaller tawny brown females, lounging around remote areas or tourist attractions; really, wherever there is an available rock. It’s hard to imagine Santa Cruz without them, they’re simply everywhere.

But have they always been so abundant?

California sea lion populations have recently rebounded to new heights as of 2018, ever since their populations dropped after 2012. Due to an unusually strong climatic and oceanic El Niño event combined with the emergence of “The Blob”, the waters of the Pacific Ocean along the west coast rose in temperature. Because of this climatic anomaly, thousands of California sea lions pups had their immune systems weakened from malnutrition. According to the National Oceanic and Atmospheric Administration , the California sea lion population dropped to 250,000 in 2014. You can read more about how El Niño affects marine mammals in this journal(1).

But thanks to the Marine Mammal Protection Act (MMPA) and the passing of El Niño and “The Blob”, California sea lion populations are now healthy and abundant, thriving along the Californian coast at areas of coastal upwelling such as Santa Cruz.

But the more important question to ask here - will the California sea lions stay this abundant? Anthropogenic threats of environmental pollution and interference from human activity are unfortunately why we need to ask this question. 

We contribute to a stressful environment more than we realize. It’s tempting to approach a California sea lion once you spot one in the wild, but have you ever considered how your presence, even just standing there, affects them?

Consistently being harassed by any species can be very stressful, and it can lead to changes in physiology and even death. In general, human contact with California sea lions have more negative than positive impacts. Human disturbances in this context is defined as any human exposure within 50m of the animal. With an increase of human disturbances like ecotourism and boating, important factors for California sea lion population growth such as reproductive rate begin to decline. Although studies may show increases in other factors such as pup growth rate, this can possibly be attributed to more indirect consequences from human exposure. More information on human disturbance influencing California sea lion reproduction can be found here (2).

It’s not just direct human contact putting these animals at risk.

California sea lions, like all marine life, can be affected by changes to our oceans’ conditions. Eutrophication, an excessively high nutrient load in the water, can be caused by agricultural runoff. This can promote massive amounts of algae growth in water. In large quantities certain algal blooms can produce toxins which can negatively affect the health of the entire marine system. During ENSO (also known as El Niño) years, the water conditions favor these algae blooms, with high nutrient loads and warmer water temperatures. The toxins that the algae produce work their way up the food web ultimately accumulating in top predators, such as California sea lions. This toxin (known as domoic acid) can cause reproductive and developmental damage, as well as brain damage. This article goes further in depth on this toxin (3).

From 1998 to 2002, 209 pregnant female California sea lions were stranded due to algae blooms from eutrophication in the Monterey Bay. Of these stranded females, less of half of them survived, but not after suffering from abortion or reproductive failure to toxins from the algae. The Marine Mammal Science journal has an article on this topic that provides further information and details here (4).

Although the impacts of our actions indicate that we can be a threat to California sea lion population growth, there is still so much we don’t know. Future studies should focus on how changes in their environment, anthropogenic or not, affect the physiology of these marine mammals. With evidence of the negative consequences of direct and indirect human disturbances one thing is clear - we have to do something.

While all of this current research may seem quite daunting and out of our hands, there is actually a lot we can do to help our sea lion friends! Here are a few “quick” tips:

If you eat seafood, checkout the Monterey Bay Aquarium’s seafood watch program! They make it easy to be a responsible consumer! This guide provides the reader with tools to choose the best fish for your health and the ocean’s.

Reduce your ocean footprint! Here are some simple ideas courtesy of The Marine Mammal Center:

    • Reduce the toxins you put into the environment, as these will runoff into our waterways and oceans. This can be done by managing what you pour down the drain or changing the type of fertilizer you use for gardening.
    • The most common items found trashing our oceans are plastic based! Choose reusable items over single use ones and recycle the plastic if possible.
    • Buy items in bulk to reduce packaging waste.
    • Bring your own reusable water bottles and mugs with you instead of single use plastic bottles.
    • Cut apart plastic soda can rings before disposal.
    • In general, just recycle everything you can!

If you encounter a California sea lion (or any marine mammal for that matter) please remember:

    • Leave them alone! Do not approach any marine mammals as they are wild and also protected under the MMPA. (keep your pets away too!)
    • If you are concerned about the health of a (living) marine mammal please contact the Marine Mammal Center (415-289-SEAL). If you come across a deceased or stranded marine mammal be sure to contact your local stranding network (831-212-1272 for Santa Cruz Area)
      • Both of these organizations operate under special permits that allow them to assist our friends in need and to collect data on deceased animals for science and their conservation!

With all of this information in mind, we hope that we have provided you with the tools to be kind to the locals (California sea lion) and help your local populations persist naturally! California sea lions play an important role in their ecosystem, making their conservation ever important to maintaining our local ocean health. Do your part! And be a good neighbor.


(1) Banuet-Martinez, W. Espinosa de Aquino, F. R. Elorriaga-Verplancken, A. Flores-Moran, O. P. Garcia, M. Camacho, K. Acevedo-Whitehouse, Climatic anomaly affects the immune competence of California sea lions. PLoS One. 12, 1-14 (2017).

(2) S.S. French, M. González-Suárez, J. K. Young, S. Durham, L. R. Gerber, Human disturbance influences reproductive success and growth rate in California sea lions (Zalophus californianus). PLoS One. 6, 1-8 (2011).

(3) C. A. Scholin, F. Gulland, F. M. Gulland, Mortality of sea lions along the central California coast linked to a toxic diatom bloom. Nature. 403, 80-84 (2000).

(4) E. C. Brodie, F. M. D. Gulland, D. J. Greig, M. Hunter, J. Jaakola, J. S. Leger, T. A. Leighfield, F. M. Van Dolah, Domoic acid causes reproductive failure in California sea lions (Zalophus californianus). Marine Mammal Science. 22, 700-707 (2006).

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