Warmer ocean threatens California sea lion population

Photos of California sea lions

California sea lion harem at San Miguel Island rookery. Taken by Tony Orr (NOAA)

According to a recent Reuters article, biologists have reported worrying trends in the California sea lion population resulting from a warming ocean. This has seen both lower birth rates, and an alarming increase in young sea lions starving and being stranded on the beaches.

According to the National Oceanic and Atmospheric Administration (NOAA), since January this year, already over 2,000, mostly young sea lions have been found either dead or dying on the southern and central coasts of California.

This is apparently over twice the average number of stranded animals considered normal, yet comes nowhere near the record 4,600 beached sea lions found in 2015, predominantly in the first half of that year. (Of those animals beached in 2015, rescue teams were able to rehabilitate and release 1,300. The rest were already dead when found or died during rehabilitation.)

Rather than seeing this year’s reduction in stranded sea lions as a positive sign, however, NOAA biologists have proposed that this fall may in fact be a consequence of the declining birthrate. Both phenomena are thought to be due to rising ocean temperatures along the Pacific Coast, which has caused increasing scarcity in the sea lion’s food supply of sardines, anchovy and squid.

The California sea lion population has been estimated to be around 300,000; however this was prior to the dramatic rise in beached animals which began in 2013, the overall impact of which has not yet been calculated.

In contrast to other sea lion populations, California sea lions are not yet considered too be a species at risk. Nevertheless, if continued for a decade or more, this trend could pose “pretty dire consequences,” said NOAA’s Jeff Laake. 

According to Laake, “It’s all nutrition-based.” 

Scarcity of food around the island rookeries off Southern California has lead nursing mothers to venture further afield to feed their pups, which in turn has meant young sea lions being left on their own for greater periods of time. 

Normally, the pups fast for several days while their mothers are away, however, with longer to wait, many malnourished pups and juveniles stray from the islands in search of food, before being caught up in currents and washed up on mainland beaches.

The reproductive cycle of these marine mammals has also been severely affected. NOAA figures from the Santa Barbara coast in the Channel Islands show a 40 percent fall in sea lion births between 2014 and 2015. According to Reuters, this is also due to food-related stresses on adult female sea lions. The more energy required to find prey, the harder it is for them to successfully breed or carry their pups to term.

The rise in sea temperatures has been linked to a decline in winds which help bring cooler, nutrition-rich water from the depths of the Pacific up closer to the surface. It is unclear how long these conditions will persist and experts have postulated that this situation may have been exacerbated by the recent El Niño impact. 

In 1983, another El Niño year, California sea lion pup numbers on all rookery islands in the Channel Islands declined between 30 – 71 percent, according to the NOAA, and it took another 6 years before the recorded total born and that survived equalled those recorded in 1982. 

Sources include: Reuters and NOAA

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