Sea Lions v. The Blob

The waters off the coast of Alaska are supposed to be cold, especially in winter. Yet, in the autumn of 2013, they cooled far less than they normally would. A great mass of warm water, 2,000 km (1,250 miles) across and around 100 metres (330 feet) deep, remained trapped in the North Pacific. Nicknamed 'The Blob', this was caused by the weather patterns over the region remaining stuck in a high-pressure mode, preventing the warm water from dissipating with the winds as it should do. With temperatures stuck at up to 4°C (7°F) warmer than normal, the high pressure did not dissipate for eight months.

When it did, tropical winds pushed the warm water up against the American coast, from southern Alaska to southern Mexico, where it basically sat until El Niño kicked off in 2015... and that kept things unusually warm for another year. Inland, this disrupted weather systems leading, among other things, to frequent thunderstorms that sparked what was (at the time) the worst wildfire season in California's history.

While wildfires are, to a certain extent, a natural part of the ecosystem and can even benefit some plants and animals in the short term, unusually large ones can tip over the balance. They aren't, however, necessarily much of a problem for sealife, which you'd think might actually enjoy the warmer weather. But that's not necessarily the case. The Blob disrupted the currents that normally bring nutrient-rich water south into the mid-Pacific, and also blocked nutrients from rising to the surface from the deeper, cooler waters beneath. 

This meant less nutrients for the phytoplankton, which meant less food for small invertebrates, which meant less food for small fish... and, yes, ultimately, we end up with top predators such as sea mammals.

The California sea lion (Zalophus californianus) lives in coastal waters from southern Alaska all the way down to Costa Rica. Across most of this range, however, they are visitors, rather than permanent residents, with the breeding rookeries being found only from central California to the southern tip of Baja California and across to the eastern side of the Gulf of California. A 2006 study in Baja California Sur indicated that their favoured foods there were argentines (a kind of fish related to smelt), hake, rockfish, and market squid. There is, however, good evidence that this changes, not just with location, but from year to year, presumably as certain types of fish become more or less available.

A dramatic change in sea water temperatures, such as caused by the Blob, would plausibly change that availability, forcing the sea lions to switch their diet. Worse, if waters were nutrient poor, there might have been fewer fish of any kind to ear, which could be bad news for sea lions, along with many other animals. 

It's in this light that we should note that an unusual number of California sea lions became stranded on US coasts between 2013 and 2016 and, in most cases, died as a result. Between 2006 and 2012, an average of around 800 sea lions a year washed up in this way, so it isn't something that doesn't normally happen at all... but over 8,000 washed up in total over the four-year period of the Blob, with well over 3,000 in 2015 alone. Most were young, and almost all were visibly malnourished.

There may not be much that we can do, at least in the short term, to stop this sort of thing from recurring. That does not mean, however, that it would not be useful to understand what is happening here, and if sea lions are of less immediate importance to the residents of California than the risk of wildfires consuming their homes, it's all part of the wider picture.

There are at least two ways of analysing the diet of wild animals. Perhaps the more obvious is the one used in the 2006 study mentioned above: collect the animals' droppings and examine them. What you're looking for here is identifiable bits of undigested material. In the case of fish, that could be bones or teeth, but is more commonly otoliths, the calcified nodules found inside fish ears that are part of the system that gives them a sense of balance. Combine that with squid beaks, and you should have a pretty good idea of what exactly the sea lion was eating recently.

An alternative approach uses stable isotope ratios. δ¹³C analysis compares the ratio of carbon-12 to carbon-13 in the animal tissue studied, which makes it possible to determine the ultimate source of that carbon, since different types of plant incorporate different proportions of the two isotopes, and this stays broadly the same as the plants are eaten by herbivores, and so on up the chain. In the case of sea lions, this could tell you where the animal has been feeding, because plankton out at sea has less carbon-13 than that close to the shore and, thus, so do the fish at those distances.

In contrast, δ¹⁵N compares nitrogen-14 to nitrogen-15 because this does change as the nitrogen moves up the food chain; the rare nitrogen-15 is concentrated slightly with each step, so that the more of it is present, the longer the food chain that the animal you're sampling sits at the top of. (That is, it's higher in carnivores than herbivores, and higher in apex predators than in carnivores that mostly eat herbivores).

This is not going to tell you exactly what the animal has been eating, but it can provide information about where it has been doing so and how long the local food chain is. By going back to samples taken at the time, a recently published study used isotope analysis to look at the hair of month-old sea lion pups, a technique that should show what their mothers had been eating in the last trimester of pregnancy, and in the month since birth.

The study was conducted at the sea lion breeding colonies on the San Benito Islands off the west coast of Baja California. In 2013, there were recorded to be 10,657 sea lions on the islands during the breeding season, but this was down by 22% in 2014, and halved again the following year. That was followed by a slow recovery, but the population was still only 5,244 in 2019 when it ended. So there's no question that the sea lions in the area suffered badly.

Scat analysis at the colony showed that the most important prey for the sea lions were hake and needlefish but the details changed significantly year on year. For example, pilchards were the third most important prey item in 2015, but completely vanished from the sea lions' diet after that, while needlefish vanished in 2019, with sand bass taking up some of the slack.

This confirms that sea lions are opportunistic feeders, eating whatever they can most easily acquire, rather than focussing on any one type of food in particular. That may help them survive temporary conditions such as those caused by the Blob but, as their population crash shows, it doesn't leave them unaffected. 

The isotope analysis showed drops in δ¹³C and δ¹⁵N. The latter indicates a shortening of the food chain, likely reflecting the change in diet, since the local pilchards are themselves placed reasonably high on the chain. The drop in the δ¹³C value is a different matter, however, and likely indicates that the sea lions were foraging in different waters than they normally would. The pattern could fit either with them being forced to forage further out at sea or further north into cooler waters - or both. 

What this means is that, even to the extent that the sea lions kept themselves alive, and were able to raise new pups, they were forced to work harder and travel further afield just to do that much. And still, half of them died. The population is now recovering, albeit slowly, but in the battle against the Blob, California sea lions came off worse.

[Photo by "Rhododendrites", from Wikimedia Commons.]