Researchers warn of population decline for sea lions in hostile environments

By Pol Allingham via SWNS

Male sea lions are growing larger and changing shape as food becomes scarcer and competition fiercer, according to a new study.

Usually, the body size of marine mammals decreases as the population grows, but sexual selection has been driving California sea lions to get bigger and strengthen their neck and jaw muscles that help them fight for mates.

Males have grown bigger but females have stayed the same, and the University of California Santa Cruz (UCSC) put this down to size mattering for male mating success but not female.

The males have upped their biting force and become more flexible too.

Both male and female sea lions diversified their diets to escape food shortages, even straying further from the shore to find new goods.

Consistently the females had a more diverse diet, approaching food more flexibly to maintain size without foraging too far away.

Meanwhile, their populations have kept growing. Since the Marine Mammal Protection Act was passed in 1972, the number of breeding females has almost tripled in the US, from around 50,000 to almost 170,000.

Only now are numbers starting to plateau.

Despite their population growth researchers fear sea lions will soon be living in “really hostile environments”, which could lead to a new decline.

Lead author Ana Valenzuela-Toro, a paleoecologist at UCSC and the Smithsonian Institution, said: “Body size reduction is not the universal response to population increase in marine predators.

“California sea lions were very resilient over the decades that we sampled and were able to overcome increasing competition thanks to prey availability.

“They’re like the raccoons of the sea: they can consume almost everything, and they can compensate if something is lacking.

“One male can breed with many females, and males in the breeding colony fight with each other to establish their territory.

“Bigger males are more competitive during physical fights, and they can go longer without eating, so they can stay and defend their territory for longer.

"Over time, some male sea lions were foraging further north.

“This is consistent with some anecdotal records that they’ve even been seen in Alaska, where they were not known to go in the past.”

Scientists analyzed museum specimens of California sea lions collected in central and northern California between 1962 and 2008.

Over 300 skulls were compared by size to estimate how big the mammals were and how that has changed over time.

Other skull features were also measured, like the size of the muscle attachment points which revealed how flexible the sea lions’ necks were and how hard they could bite.

Tiny bone samples were taken from some skulls to test the stable carbon and nitrogen isotope composition, which indicated where and what the beasts may have been foraging.

Doctoral student Valenzuela-Toro said: “Carbon provides information about habitat use—whether they’re foraging along the coast or offshore—and nitrogen provides insights about the trophic level of their prey, for example, if they’re consuming smaller or larger fish.

“The neck muscles are really important because they allow them to move their head and neck more agilely, bite harder, and, eventually, win when they are fighting other males in the breeding colony.

“They remain in a narrow zone around their breeding colony, but they still show a lot of flexibility in what they eat.

“We believe that skull morphology in the rostrum—which is related to the size and shape of the mouth—probably has something to do with this flexible foraging behavior.

“We found that the size and shape of the mouth of females is related to the size of the prey that they consume.

“All these dynamics occurred in an environment that was rich in prey: full of anchovies and sardines, two species that are super important for their diet.

“But over the last few years, the populations of these two fish have collapsed, so California sea lions are diversifying their diet to compensate, and apparently they are not doing so well.

“As climate change progresses, prey availability of sardines and anchovies will decrease even more, and eventually we will have more permanent El Niño-like warming conditions, reducing the size and causing a poleward shift of these and other pelagic fishes.

“It will be a really hostile environment for California sea lions, and eventually we expect that their population size will stop growing and actually decline.”