The poles are often seen as the frontline for signs of climate change. But with global temperatures on the rise, the boundaries between temperate and tropical regions should also be recognised as key pressure points for animals and ecosystems adjusting to a wave of ‘tropicalisation’. Mexico’s sea lions are the southernmost of their kind, with California sea lion populations stretching from Alaska down into the Baja California peninsula. Whilst more northerly colonies are thriving, those in Mexico face an existential crisis. The sea lion population in the Sea of Cortez fell by 65% between 1991 and 2019, from 43,834 animals to just 15,291. Over the same period, a multi-decadal warming of the sea surface was measured, with mildly temperate waters shifting from an annual average of around 22°C to over 23°C. It may not sound like much, but this represents a drastic change to the ocean and is the most likely driver of sea lion decline.

Dr Fernando Elorriaga-Verplancken, professor and sea lion specialist from CICIMAR-IPN in La Paz, Mexico, has devoted 23 years to the study of pinnipeds. His one wish: “Stop the ‘tropicalising’ of our environment.” The long-term warming trend is accompanied by short- term spikes from El Niño events, whereby the ocean is unusually warm for a period of time. To an extent, these events are part of normal planetary cycles, but in recent decades they have increased in frequency and severity, with some researchers linking the change to higher levels of greenhouse gases.

2023 was another El Niño year and, when I visited the Sea of Cortez in autumn, its clear waters had turned green and warmed to an alarming 32°C. The usually resilient corals at Cabo Pulmo, the northernmost coral reef in the Eastern Pacific, were bleached and dying – one of the first signs of a marine ecosystem suffering from the heat. The warming of the ocean leads to a less productive sea, which affects food availability for sea lions. The upwelling of nutrient rich waters from the deep is inhibited, in turn reducing the production of plankton with repercussions throughout the food chain. Sardine fisheries in the Gulf of California have become highly unstable since the nineties, with dramatic peaks and troughs. Catches fell from over 500,000 tons in 2008 to 3,000 tons in just five years. Dr Elorriaga-Verplancken and colleagues believe that Mexico’s declining sea lion population is linked to the decreased availability of high- quality prey, as a result of climate change.

The sea lions are forced to eat ‘junk food’, less nutritious options such squid and rockfish, rather than a preferred diet of sardines, anchovy, and mackerel. Adult sea lions leave the rookery for hunting trips which last several days and span hundreds of nautical miles. As their food source becomes harder to find and less rich, the adult sea lions suffer from diminished health with cascading effects throughout the colony. Pups needing a longer lactation period will be slower to accumulate body mass and strength, delaying their independence and increasing the risk of juvenile mortality. Dr Elorriaga-Verplancken was left traumatised after witnessing pups dying due to these issues: “It was painful, it wasn’t a gentle death… they didn’t just go to sleep. It keeps you awake, wondering what had to happen to reach that scenario.”

Far offshore in the Eastern Pacific Ocean, I catch up to sea lion hunting packs as they compete for prey with other ocean predators such as marlin, mahi mahi, tuna, and whales. Working as a group, the sea lions are often able to intimidate and assert dominance over the food source, but in a race for fast-swimming prey, they lose their advantage as they struggle to keep up with faster competitors. Even in hunting mode, the playful, social character of the sea lions is still apparent.

Their attacks can seem lethargic compared to the more surgical billfish, and they still make time for joyful swoops through the water, with no obvious purpose beyond joie-de-vivre. The large bulls are less agile than the females and younger males, and less adept at pursuing the sardines. However, a hierarchy still reigns and if a younger male does catch a fish, the dominant bull will pursue the younger male, barking until it releases its catch. Once bellies are full, sea lions plank as a group at the surface, fins sticking out as their bodies create an odd-looking raft.