Marine Life / Scientists in Action!

Why Might Warmer Water = Sea Lion Starvation?

Throughout the warm summer morning, I carried crates and boxes up the gangplank of the R/V Melville to the tune of barking sea lions. Stopping to watch them flop across the rocks near the port reminded me that I was no longer in North Carolina, but was about to board an oceanographic research ship in San Diego.

As a marine scientist, I was boarding the Melville to embark on a month-long research cruise off the California coast. But despite my love of all things marine, I saw the sea lions mostly as a loud distraction from my quest to study something much smaller—microscopic, plant-like organisms called phytoplankton. That’s what the cruise, for me, was all about.


Phytoplankton in action!

Our cruise depended in part on finding the kind of water that the phytoplankton I study (called diatoms) love: cold, and nutrient-rich. Normally, dipping your toes in the ocean off a California beach is shockingly cold–a product of the fact that the water there has recently risen to the surface from much deeper in the ocean. But that summer, a unusually large mass of warm water almost derailed our cruise plans (that’s #marinescienceproblems for you) and although I knew that this would affect phytoplankton growth (and my research), I never gave a thought to how the sea lions would cope. Yet the sea lions and I have much more in common than I realized: we may have been searching for the same cold water all along.

Home, sweet home: the R/V Melville.

Home, sweet home: the R/V Melville.

So far this year, almost a thousand sick and starving sea lion pups have been found along California’s coastline—4 times the normal number. Marine mammal rescue centers having been working to rehabilitate the pups, but are starting to be overwhelmed due to the sheer number of animals needing care. And this is all happening months before pups usually begin to strike out on their own. Something, it seems, is driving these animals away from their offshore nurseries long before they have learned how to hunt by themselves.

Sea lion pups being cared for at a rehabiliation facility. Credit: NatGeo.

Sea lion pups being cared for at a rehabiliation facility. Credit: NatGeo.

One explanation for this die-off is the same warm water that I encountered on my research cruise. This past summer, the Melville and its scientists spent a lot of time steaming up and down the coast, looking (ironically) for a certain kind of water–cold, “upwelled” water that had risen to the surface from the deep ocean. We figured it would be easy to find, but eventually we had to sail all the way to Oregon in order to find an upwelling area.

Normally, upwelling is a common occurrence off the California coast, since winds blowing down from the north (in conjunction with the rotation of the Earth) push warmer surface water offshore. This allows the aforementioned deep water to rise to the surface, replacing the displaced surface water. But during our cruise, these winds had stopped, in part (meteorologists think) because of a huge area of high pressure in the northeast Pacific. This high pressure system has stuck around for far longer than anyone guessed it would (from before my research cruise into the present day, and still going strong), and is potentially causing sea lions to starve by depriving the pups and their mothers of easy access to food.

How upwelling occurs.

How upwelling occurs. Deep ocean water contains more nutrients (i.e, phytoplankton food) and when it rises to the ocean’s surface phytoplankton can “bloom” with the help of sunlight. These microscopic creatures provide food for many other marine organisms, including (indirectly) sea lions.

The high pressure area is denoted in red.

The high pressure area is denoted in red.

The phytoplankton that “bloom” in the high-nutrient, upwelled water are a food source for many marine creatures. Fish and other organisms thrive when phytoplankton are abundant, which in turn gives sea lion moms lots of food to feed their pups. The moms leave their pups behind when they hunt, and the presence of warm, phytoplankton-less water means that they have to swim further away for their dinner. Expending more energy on themselves means they have less food to give their pups. Without enough food, the pups are getting so hungry that they venture into the ocean to hunt long before they know how to do so for themselves.

An unhealthily thin sea lion pup. Credit: NOAA.

An unhealthily thin sea lion pup. Credit: NOAA.

This is one explanation. However, things in ecology are never black and white: another possible (or concurrent) explanation for this sea lion pup die-off is that the sea lion population has simply expanded beyond what resources can support–there just might not be enough fish to go around. Sea lions along the western U.S have recovered in recent years, and now number around 300,000. This event, and a similar one in 2013, could be nature’s way of keeping populations at levels that the surrounding ecosystem can support.

Regardless, it’s amazing how interconnected these processes are—how weather differences cause changes in the microscopic food web, which then affect larger organisms and, eventually, humans. Some people think that part of the reason for California’s current drought is this same Pacific high pressure system that’s affecting phytoplankton and sea lions. In this case, there’s little evidence so far that this event is directly connected to climate change. The world’s oceans go through natural cycles that are years to decades long, and regional warming and cooling are part of that.

It’s hard to know if any one event is caused by human-caused climate change. But we can know, and accept, that nature is and will always be “red in tooth and claw.” Things like this sea lion die-off will always happen as ecosystems rebalance. We humans have a place as part of that rebalancing: we’re red in tooth and claw just as much as any other animal. Our consciousness of that fact–of our capacity for brutality and destruction–means that we have the ability to transcend it. We can create a better world for the other creatures living in it if we choose to. The sea lion pups in rehab centers, for one, would thank us for that if they could.


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