The Symbiotic Importance of Sharks and Pinnipeds

By Shannon Crosby

Contrary to the (ill-conceived) worldwide perception around great white sharks (Carcharodon carcharias), for many Californians that have a close relationship with the ocean, their first thought when pondering great white sharks is not attacks on humans, but their predation on sea lions and other pinnipeds (harbor or elephant seals, walruses, etc). The California sea lion (Zalophus californianus) is a common sighting for any ocean-loving beachgoer, surfer, sailor, scuba diver, boater, paddleboarder, swimmer, fisherman…you get the idea, Californians have a lot of ocean hobbies! In other words, sea lions - and by extension, great whites - are a staple of beach life along the California coast.

In fact, Los Angeles is one of the most popular destinations in the world for great whites, because there are two major hot spots nearby that contribute to the Californian sub-population: Guadalupe and the Farallon Islands. Great white sharks are nomadic, but often visit the same areas in seasonal patterns each year for the best combination of available prey and warmer water temperatures. As juveniles, white sharks are regionally endothermic. In other words, if you touch a young white shark (which is not recommended), they feel cold, but their body is actually warm inside. The deeper you go towards their core, the warmer they are. The difference between juveniles and adults are that younger white sharks do not retain as much heat, making them rely on surrounding environmental temperatures to maintain their body heat. This is why young white sharks hunt sting rays and fish closer to shore, where they can stay warm in the shallow waters and feel protected from predators in the open ocean (like bigger sharks). Once young white sharks have matured and grown into adults or near adults, they can regulate their body temperature internally when they are in colder waters, and they can save up heat energy to use in short bursts when they need to move quickly for hunting. This means that as white sharks age, they can begin migrating to hunt in cooler waters and go after mammals like elephant seals and California sea lions; they will sometimes even go after other sharks, porpoise and whale carcasses. What is fascinating about adult white sharks is the fact that they are an oceanic species, but travel frequently to coastal pinniped populations to feed, sometimes being mislabelled as a coastal shark.

Many cities along the California coast are known to act as white shark nurseries, including Ventura, Oxnard, Santa Monica Bay, Huntington Beach, and Dana Point. A study out of UC Davies in 2008 concluded that the sub-population of white sharks in California accounts for over half of the white sharks (adult or near adult) in the northeastern Pacific; since then, the number of white sharks that scientists have spotted and tagged off the coast of Southern California has increased significantly, which scientists at California State University Long Beach and Moss Marine Landing Monterey Bay attribute to increased marine mammal and shark protections since the 1970s-90s, as well as decreasing white shark migration as a result of rising water temperature levels. 

Generally, young white sharks will spend the summer months off of Southern California, and then will migrate to the warmer waters of Baja Mexico during the cooler winter months. Once matured, white sharks will also often migrate across the basin to Hawaii or spend months out in the open ocean – there is even a region that’s been dubbed “White Shark Café” by marine scientist Barbara Block; it lies between California and Hawaii, where white sharks congregate to hunt the abundance of squid, jellyfish, phytoplankton, and fish that live sluggishly in the cooler midwater column. Tagged white sharks that have traveled here have shown hunting dives up to 3,000 feet during the day to reach these prey and have been observed to stay for months at a time before moving on to Hawaii or back to California or Mexico. This is new information to scientists, who dubbed the behavior as “bounce dives” or “exploratory runs” because due to the lack of oxygen, the sharks do not spend much time at such depths.

With significantly more shark sightings occurring off the shores of California, many residents have expressed expectations of increased shark attacks or white shark encounters with humans. However, even with significantly more beach activity during the pandemic, the data has not shown this to be true – a few recent years have even shown shark attacks decrease. Human tendency to assume that white sharks are inherently vicious and prone to attack anything that resembles food is simply a narrative that has been passed down through the generations because of badly-worded press and over-emphasized hype on the few shark attacks that do occur. Society allows the fear to outweigh the data, which indicates that sharks only purposefully hunt their natural food sources, and when attacks do occur, they are cases of mistaken identity where a shark bites, realizes it’s mistake, and ceases to attack. 

Thankfully, the demonization of white sharks has slowly begun to transform in recent decades due to the determined transparency of a number of scientific organizations, documentary films, and educational programs. We are still far from bringing the respect of these predators into a more focused position of importance in human society, but great progress continues to be made. What might surprise some readers, though, is that in addition to the poor image that sharks have, sea lions are considered in an extremely negative light by the general public. 

Depending on perspective, Californians may view the constant sea lion presence in different ways. For example, a scuba diver will most likely be excited by the sighting of a “sea-pup” during a dive, because sea lions and seals tend to come play with diver bubbles and spin around to show off underwater, which is an extremely fun and exciting experience. A fisherman or boat owner will most likely view a sea lion sighting as a nuisance, assuming the sea lion is there to steal their fish or wreck their boat slip by rifling through the belongings there. A surfer will likely spot sea lions with a reluctant smile and consider with trepidation that a shark may be hunting nearby in the water. While sea lions can manifest these varied feelings from differing human angles, their image will never alter the fact that they play an important role in their ecosystems. In California, that sometimes means white shark prey, although not always. You may be surprised to the fact that sea lions are not white sharks’ favorite meal, as Shark Week tells us. Instead, they prefer the elephant seal due to their higher fat content and inferior agility compared to a sea lion.

Sea lions are often considered to be cute and playful to the average bystander, flipping through the water to show off to spectators on the beach, or lounging and napping in piles along the pier, so it can be off-putting to imagine them being killed by sharks for food. However, if their populations grew unchecked without natural predation, it could permanently damage the ecological health of western shores. Generally, scientists consider larger diversity of species within an ecosystem to correspond with better ecosystem health and functionality, because it encourages beneficial species competition and positive evolution of species over time. If white sharks were not preying on the pinniped populations, those pinniped populations could overrun their surrounding environment and push out competing species/diversity, and it would also create a top-down effect on the food chain (too many pinniped predators without a corresponding increase in their prey, ultimately leading to the desecration of pinniped prey populations). Occurrences like these would decrease the overall health of an ecosystem because it would bring it out of the natural balance it has achieved over evolutionary time. When coastal ecosystems suffer and become unbalanced, human societies feel the effects, even if they don’t realize these effects originate from the suffering of the marine ecosystem. 

For example, in the past 3 decades many communities in the Pacific Northwest began resenting sea lions because of their predation on wild salmon. Sea lions and seals are known to prey on salmon in estuary regions as the fish travel between salt and fresh water, which can prevent the salmon populations from maturing and reproducing properly for sustainable fishing practices (salmon migrate between salt and fresh water for different stages of maturation – they are often born in fresh water until they reach juvenile age and then travel to the ocean to fully mature and gain body mass; they return to fresh water as adults to spawn). While the significant decline in wild salmon over past decades was found to predominately be caused by habitat loss and degradation as a result of human society, sea lions and seals have also increasingly inserted themselves into salmon fishermen’s daily lives when hunting salmon from fishing nets or straight from the fishing boats after they’ve docked. This means that for many natives in the region, sea lions are a huge contributor to the difficulty of their livelihood as salmon fisherman, even though the sea lions are simply preying on their natural food source. Seafood shortages, increased shoreline erosion, and toxic algal blooms are all additional examples of potential effects of local marine ecosystem imbalance that have an impact on human coastal life. 

While it is important to ecosystem balance that sharks continue to prey on sea lions, it is also important to an ecosystem’s stability that predation by sharks does not occur unchecked. These animals, along with other predator/prey relationships in any ecosystem, essentially form a symbiosis – they are ecologically invaluable to each other, and both populations will suffer if the other is thrown out of balance. If sea lion populations plummeted because they were too easy a prey for white sharks (or if their food source is overfished like in the 1970s, which caused an upwards of 70% decline in Stellar sea lions until the 90s), that would also cause an imbalance and result in less available food for the shark population. Continuing along these lines, consider the human position within an ecosystem – we are essentially the top predator, the same way a white shark is a top predator. Accordingly, just as a white shark population suffers when their main food source is decimated, so does the human population suffer when we overexploit our food resources (which largely stem from the ocean worldwide). The reaction to this ecological suffering should not be to create a variety of alternative solutions, but to heal and rebalance the symbiosis between predator and prey resource. 

Therefore, it may make some readers hopeful to know that sea lions are not the type of prey to lay down and die when a shark decides to hunt. In fact, sea lions are very well evolved to give white sharks a difficult time turning them into a meal; researchers have captured a number of videos at the Farallon Islands and other white shark hunting grounds showing how sea lions are able to evade becoming dinner with their speed and agility in the water, wearing down a shark’s burst energy enough to escape. These researchers have observed that white sharks tend to pick out the sick or visibly weak sea lions or elephant seals, indicating that the sharks much prefer easy prey over expending the energy required for an intensive hunt. When a white shark has been spotted meandering around a rookery waiting for the right opportunity to chase, sea lions will also often form a pack to run the lingering white shark away from the rookery by essentially creating a hive around the shark and pestering it with body slams or charges until it decides the hunt isn’t worth the energy at that particular moment. 

Suffice to say, information like this goes to highlight the importance of the ecological roles predators and their prey play in an ecosystem and in their environment. Even further, it goes to show that the balance of these predator/prey relations is much more significant to human success than most people realize, and that significant changes in local ecology can signify some very high-level environmental changes or problems. This idea implicates the importance of protecting that balance. While sea lions and sharks can come across to people as nosy, competitive, smelly, scary, foreign, cute, playful, or a nuisance, their image should not result in a lack of their care and cultivation.  

While shark and marine mammal protections have increased in recent decades, there are still very serious threats that humans pose to the safety of earth’s shark and pinniped populations (and therefore marine ecosystem health). Plastic pollution, fishing net/line (and other debris) entanglement, and overfishing are major threats these animals encounter time and again. Climate change and ocean acidification also indirectly affect the health of these creatures, as they are adapted to live and perform most efficiently in specific water temperatures and pH levels; they also depend on the fish and other coastal species that are extremely sensitive to these environmental properties. Ultimately, these animals will need to evolve in the coming generations to combat some of these threats naturally to survive, but as humans invested in the maintenance of the status quo provided by these marine resources, society will also need to become more involved in protections against the threats that plague these marine communities.