When scientists compared the diets of California sea lions in the Channel Islands, U.S., and the Gulf of California, Mexico, they anticipated uncovering a straightforward reason for the booming populations in California versus the shrinking numbers in Mexico. However, their findings revealed a more intricate picture.
The study discovered that the overall energy value of sea lion diets in both regions was nearly identical, suggesting that diet alone does not account for the divergent population trends. “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.
Environmental Heterogeneity Over Diet
Pozas-Franco emphasized that the variability in local conditions plays a significant role. “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,” she noted. The findings suggest that management plans should focus on the unique conditions of each colony and the factors influencing population dynamics regionally.
Dr. Andrew Trites, a professor at the Institute for the Fisheries and Oceans and Director of UBC’s Marine Mammal Research Unit, co-authored the study and noted that the results 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.”
Historical Trends and Data Analysis
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, researchers analyzed four decades of data on sea lion populations and diets.
Researchers measured two main aspects of “diet quality”: energy density and diversity. 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. Yet the Gulf’s colonies were declining.
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.
Impact of Environmental Changes
The composition of each diet told a different story. The wider menu in the Gulf of California didn’t make them better off. 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,” he explained.
Conservation and Future Outlook
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.
