All prey are not the same: marine predators face uneven nutritional payoffs

(Press-News.org) The hunt is on and a predator finally zeroes in on its prey. The animal consumes the nutritious meal and moves on to forage for its next target. But how much prey does a predator need to consume?

Following a period of massive starvation among animals living along the California coast, University of California San Diego scientists began asking questions about predator sustenance and the value of prey consumption. Their investigations found that all prey are not the same, and this can have significant impacts on predator foraging. Even prey of the same size and weight within the same species can drastically vary in terms of the sustenance they provide, the study showed.

The new findings carry implications for understanding ocean food resources and the impacts that changes in these resources can have on animals that depend on them.

Former UC San Diego graduate student Stephanie Nehasil, who earned her PhD in the School of Biological Sciences, and Professor Carolyn Kurle, along with coauthors affiliated with the Southwest Fisheries Science Center (NOAA) and UC Santa Cruz, were initially interested in a mid-2010s marine heat wave that caused marine mammal and seabird starvation across the California Current Ecosystem, a rich ocean area along the West Coast that stretches for thousands of miles and supports a wide variety of sea life. These questions eventually led to a multi-year investigation of ocean food sources and the daily nourishment required for marine predators. To explore these questions, the team focused on one of the species most affected by the heat wave: the California sea lion.

The study’s results, published in the Journal of Animal Ecology, revealed that members of the same species of anchovy, sardine and squid can widely vary in their energy density, which is the standard by which prey are measured in terms of assessing how much nutritional value they provide for predator subsistence. Using multiple measures of prey quality, the researchers discovered that relying on lower-quality prey can force predators to double their intake, despite being the same size as more nutritious prey.

“We were surprised by how much variation there was within a single species,” said Nehasil, now a postdoctoral researcher at Stony Brook University. “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. 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.”

The new findings offer more precision for data featured in “bioenergetics” models, which are used to understand ecosystems and make key assessments of the state of marine animal populations. Previously, prey animals of the same species and similar size and weight factored the same in terms of their value to daily consumption by predators.

“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 Kurle, a faculty member in the Department of Ecology, Behavior and Evolution. “This information will help us understand the complexity of these ecosystems that so many care about, especially the fishers and others who use the ecosystem.”

In 2014 an immense heat wave in the North Pacific Ocean caused a significant disruption of the normal structure of the marine food web. The event disrupted the typical upwelling flow of nutrients from the deep ocean to the surface, thus curtailing the source of food for zooplankton, which in turn feed the fish that serve as prey and are commercially valuable for humans. A massive wave of starvation resulted that rippled through communities of marine mammals, seabirds and other species.

“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.”

In particular, the researchers witnessed emaciated sea lions and their pups, indicating that mother sea lions were not able to adequately feed their offspring and themselves, which initiated the researchers’ search for the underlying mechanisms.

Nehasil spent several painstaking years collecting data and measuring the energy density of specimens from a multitude of sources, including NOAA fish surveys, bait barges and samples from the California Department of Fish and Wildlife. In collaboration with Ocean Discovery Institute, a nonprofit that engages youth from the underserved community of City Heights in San Diego, she also involved local students in the research, providing hands-on science experience for the next generation.

To assess the true nutritional value of northern anchovy, Pacific sardine and market squid, rather than simply measuring length and weight, she used a time-tested instrument known as a bomb calorimeter that measures the energy content of a sample. The greater the temperature change as a specimen burns, the more energy and nutritional value it holds.

The measurement data were fed into a model that estimated the energy density and content of prey collected across different oceanographic regions, seasons, sizes and maturity stages. These estimates were then used to calculate how much prey would need to be consumed to meet sea lion energetic demands, revealing vast bioenergetic differences.

"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."

Fish within the same species can vary widely in energy value, depending on regional ocean productivity, seasonal nutrient supply, life stage and size. While regional and seasonal differences reflect environmental conditions, variation by size and maturity is driven by how energy is allocated — either toward growth or reproduction.

“We all want to have healthy ecosystems and there are many stakeholders who want these systems to succeed. We want everyone to have the best data possible to make important analyses and estimations,” said Kurle. “Anytime we can provide a little more understanding of ecosystem complexity it’s valuable and important.”

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