Ocean predators may need twice the prey when food quality drops

In 2014, something strange happened to the fish off the West Coast of the United States. The ocean warmed fast and stayed that way for a long time. This wasn’t a small change – it was a massive heat wave, and it seriously messed with marine life.

Seabirds and sea lions started starving. People noticed sick and skinny animals washing ashore. The entire food web had been shaken up.

But the question scientists began asking wasn’t just how much food was out there. They started wondering about something more basic: Was the food still good enough to eat?

Nutritional quality of ocean prey

A team from UC San Diego, working with partners from the Southwest Fisheries Science Center and UC Santa Cruz, wanted to know what really drives starvation in predators like the California sea lion.

The species had taken a hard hit during the marine heat wave and showed clear signs of food stress. What was behind that?

The study revealed something surprising. The nutritional value of anchovies, sardines, and squid – the sea lions’ favorite snacks – varied a lot.

Some fish were rich in energy. Others, not so much. And they were all the same species, sometimes even the same size.

A fish is not just a fish

“You could have two fish side-by-side that are the same size but have a huge range of variation in the amount of energy they provide,” said Stephanie Nehasil, a postdoctoral researcher at Stony Brook University.

“In some cases, predators would need to consume tens of thousands of the smaller, lower-quality fish to survive, and that’s just not possible.”

When the ocean warms up and nutrient flows slow down, everything from plankton to fish changes. And that change trickles up.

In fact, energy density – the measurement of how much energy a prey animal holds – isn’t something you can guess just by looking at the animal’s size or weight.

Measuring energy content in fish

Nehasil and her team used an instrument called a bomb calorimeter to measure the actual energy content in hundreds of samples. It burns a small piece of tissue and tracks how much heat it gives off. That heat equals energy.

The researchers pulled fish from different seasons, sizes, and regions, and logged how much nutrition they packed.

The results were eye-opening. Even within one type of fish, energy values could swing widely depending on whether the animal was young, mature, or even where it lived.

What this means for sea lions

For California sea lions, this matters a lot. During the heat wave, female sea lions struggled to find food with enough energy to feed themselves and their pups.

As a result, many pups starved. That raised alarms among scientists. Was this just about food amount, or something more?

“We saw a lot of mortality, which led us to question whether prey abundance or availability had changed during the event,” said Nehasil.

“Looking more closely at shifts at the base of the food web, we began to wonder whether it wasn’t just the amount of food that had changed, but the quality as well.”

Turns out, they were right. Lower-quality prey was abundant, but it couldn’t sustain the sea lions. The animals needed more food than they could reasonably find or eat. And this wasn’t just a temporary problem.

Why old models fall short

The team’s findings improve the accuracy of something called bioenergetics models. These models help predict how much food animals need, how ecosystems work, and how changes – like warming seas – will affect wildlife.

Until now, those models often assumed that all same-sized fish were equal in value, but they’re not.

“This is another piece of information to help inform our models to get a better understanding of how these ecosystems work, so we can recognize subtle dynamics that might otherwise be overlooked and respond more thoughtfully to environmental shifts,” said Professor Carolyn Kurle, a co-author of the study.

These models are tools used by people who manage fisheries, protect marine mammals, and make policy decisions.

More precise data means better decisions. And when ecosystems are already under pressure from climate change, precision counts.

Ocean changes affect fish energy

The 2014 ocean heatwave did more than just raise temperatures. It blocked the natural upwelling of cold, nutrient-rich water from the deep sea.

That upwelling normally feeds tiny plankton, which feed small fish, which feed big fish – and on and on. Without it, the whole food chain weakens.

Fish energy density is closely linked to these changes. A fish born in nutrient-rich water will likely grow bigger and store more fat, which means it provides more energy to predators.

A fish from a less productive region might be lean and less useful. Multiply that across an entire population, and predators like sea lions face a serious shortage.

“You have to use the currency of energy value to inform bioenergetics models,” said Nehasil.

“To help us predict what will happen as our climate and oceans change we need this baseline ecosystem data, especially to understand prey dynamics and how predators are going to respond.”

Energy value of ocean prey

Nehasil’s research included local students from San Diego through a partnership with Ocean Discovery Institute.

The students got hands-on experience helping measure fish and understand how science connects to the health of the oceans.

Understanding the energy value of prey is more than a numbers game. It tells us what’s really happening beneath the surface when animals struggle. It’s not just about empty waters – it’s about empty calories.

“We all want to have healthy ecosystems and there are many stakeholders who want these systems to succeed,” said Professor Kurle.

“We want everyone to have the best data possible to make important analyses and estimations.”

The full study was published in the journal Journal of Animal Ecology.

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