Diet alone doesn’t explain divergent health of California Sea Lions in US and Mexico

(Press-News.org) When scientists compared what California sea lions eat in the Channel Islands (U.S.) and the Gulf of California (Mexico), they expected to find a clear explanation for why populations were booming in California but shrinking in Mexico. Instead, they found something more complicated.

The study found that what sea lions eat may matter less than where they live. Despite large regional differences in population trends, the study found that the overall energy value of sea lion diets in the Gulf of California and the Channel Islands was nearly identical.

“What surprised us most was that, even though sea lion populations in the Gulf of California are declining while those in the Channel Islands are thriving, their overall diet quality was similar,” said Ana Lucía Pozas-Franco, a master’s graduate at UBC’s Institute for the Oceans and Fisheries. “This highlighted how this region is a patchwork of different environments - environmental heterogeneity, rather than diet quality alone, influences sea lion population trends in Mexico.”

Pozas-Franco says this variability underscores how much local conditions matter. “Because California sea lions in Mexico are under special protection due to overall population declines, management plans should focus on understanding the unique conditions of each colony and the factors influencing population dynamics regionally.”

Dr. Andrew Trites, professor in the Institute for the Fisheries and Oceans, Director of UBC’s Marine Mammal Research Unit, and one of the study’s coauthors, said the findings challenge common assumptions. “It’s tempting to assume that diet quality directly drives population size,” he said. “But these results show that the same prey energy can yield very different outcomes depending on local oceanography, prey availability, and human pressures. Each colony lives in its own ecological neighborhood.”

California sea lions range from British Columbia to Mexico’s Gulf of California, but their fates have diverged sharply over the past forty years. In the U.S., populations at the Channel Islands have increased by several percent each year since the 1980s, while most colonies in Mexico have declined at a similar rate. To understand why, the researchers analyzed four decades of data on sea lion populations and diets, comparing what animals in each region were eating and how their numbers were changing over time.

Researchers measured two main aspects of “diet quality.” One was energy density, which captures how much energy each gram of prey provides. The other was diversity, measured using the Shannon Diversity Index and the number of main prey species making up a sea lion’s diet. On both counts, the two regions were surprisingly similar. Sea lions in the Channel Islands had diets averaging about 5.4 kilojoules per gram of wet weight, while those in the Gulf of California averaged 5.3. In other words, both populations were eating equally rich food. Yet the Gulf’s colonies were declining.

The composition of each diet, however, told a different story. Sea lions in the Channel Islands fed mainly on a small number of schooling fish and squid species such as anchovy, mackerel, and market squid. In contrast, those in the Gulf of California ate an impressive 88 different main prey species, dominated by benthic species and schooling fish. The wider menu didn’t make them better off. The team found no simple link between how diverse or energy-rich a diet was and whether a colony was growing or shrinking.

During the 2014–2016 Pacific heatwave known as “The Blob,” sea surface temperatures rose across the region and disrupted prey availability. In the Gulf of California, sea lions shifted toward eating more benthic fish and fewer schooling fish and squid, which caused a measurable drop in the energy density of their diet. Yet even this shift had mixed effects. Some colonies experienced temporary declines, while others appeared unaffected.

The Gulf of California, Trites added, is a particularly complex neighborhood. “It is a mosaic of subregions shaped by different currents, temperatures, and prey communities. In the southern Gulf, around places like Los Islotes, sea lion numbers have remained stable or even increased. Farther north, colonies have struggled as warming and changing prey patterns made their foraging grounds less reliable.”

For conservationists, this means there is no single problem with the sea lion populations in the Gulf of California. Protecting the species requires understanding how each rookery interacts with its environment, how prey species change, how females alter their foraging trips, and how local fishing activity or pollution might tip the balance. The authors argue that better long-term monitoring is needed to track these shifts and to distinguish between natural fluctuations and human-driven change.

Instead of asking whether sea lions are eating the right food, this study asked how a changing ocean is shaping the food that’s available, and how adaptable sea lions can be in response. The answer, it seems, depends on where they happen to live.

“Environmental heterogeneity plays a bigger role than diet quality in driving divergent California sea lion population trends,” appeared in PLOS One.

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