In a darkened storage room filled with specimens from the Smithsonian Museum of Natural History, ecologist Megan Feddern picked up the skull of a harbor seal that had met its death 92 years ago in northern Puget Sound.

Holding the skull in her left hand and a high-speed rotary drill in her right, Feddern directed the screaming tool into the natural hole in the base of the skull. As a plume of bone dust rose from the workbench, she removed a tiny core of bone, just 1/8 inch across, from the inner surface of the skull.

Gordon Holtgrieve, who heads the UW’s Holtgrieve Ecosystem Ecology Lab where Feddern is working, said an intriguing question is what the seals were eating years ago when there were more salmon and fewer seals.

“Were they eating more salmon at that time, or were they eating other species like hake?” he asked. “Then you move forward and ask what was happening at a time when hatcheries started pumping out a lot of fish.”

It’s a complex system, Holtgrieve noted, and a major concern of the day is how much the competition for salmon among seals and other species could be pushing the southern resident orcas to extinction.

Isotope study examines the food web

The old saying “you are what you eat” generally holds true, experts say, provided that you account for chemical changes that take place after food enters the body. Accounting for changes in particular amino acids — the building blocks of proteins — helps to understand who is eating whom in the food web. That’s the basis of Feddern’s studies with seal skulls.

A diagram of the food web shows plants, algae and other primary producers on the bottom and high-level predators at the top. Since plants and algae produce their own food, they are not dependent on other living things. In the hierarchical framework of the food web, they occupy the bottom level, known as Trophic Level 1, or TL1 for short.

Herbivores, which eat only plants or algae, are found at the next level up, or TL2. Animals that eat only herbivores come in at TL3, and so on.

Typical diets of higher animals are not as simple, often consisting of a mixture of prey species from different trophic levels, so their average diet can put them between two trophic levels. For example, one analysis determined that modern harbor seals fall between trophic levels 4 and 5, specifically TL4.8 — among the highest of all marine predators.

Thanks to curators at five museums and officials at NOAA’s National Marine Mammal Laboratory, Feddern has been able to collect samples of bone from seals that died in various locations since 1928. In the laboratory, she is able to extract collagen from the bone samples to provide amino acids for her analysis. These amino acids were absorbed into the seals’ bodies from the food they ate during the previous year or so, leaving chemical signatures that can be traced back to their source.

Following the path of amino acids

To determine the trophic level of long-dead seals, Feddern analyzes the stable isotopes in the amino acids, particularly nitrogen-14 and its slightly heavier brother nitrogen-15. The key is how the ratio of N-14 to N-15 changes as amino acids in food are passed up the food web from prey to predator.

Phenylalanine, for example, is an essential amino acid, originating from a plant source. Because animals cannot produce phenylalanine, this “source” amino acid is passed up the food web essentially unchanged, and the N-14/N-15 ratio remains the same as in the original plant.

Meanwhile, nonessential amino acids, such as glutamic acid, are produced in animals through chemical transformations that tend to give up N-14 in exchange for N-15. Since the process gets repeated with each predation, researchers can use the isotope ratio in glutamic acid to determine the trophic level. Because of this, glutamic acid is considered a “trophic” amino acid.

These methods don’t provide direct evidence of the specific prey species eaten by seals, but they reveal an average trophic level of the various prey species. Feddern has learned that harbor seals living along the Pacific Coast are generally eating higher on the food web than seals in Puget Sound. The finding seems to hold true throughout the study period.

“Those on the coast could be eating different species or just more adults in general,” Feddern said, noting that adults of a species often shift to consuming higher-level species as they grow larger. Since adult fish are higher on the food web than juvenile fish, seals eating more adults would move up the food web.

A greater number of young salmon (smolts) living in Puget Sound before migrating to the coast could explain the lower trophic position for Puget Sound seals versus those on the coast, she said.

The new trophic findings for seals seem to align with historical records going back to the 1970s that show yearly estimates for the abundance of prey— primarily herring, hake and young salmon, she said.

“They seem to eat what is available,” Feddern noted, which fits the well-known description of seals as “opportunistic feeders” and raising this important question: Would seals today eat less salmon if more herring or hake were available?

During the 1980s, when hatcheries were producing excess baby salmon, Feddern’s data suggest that seals were feeding at a lower trophic level, as one might expect.

If the abundance of species is driven by environmental conditions, including long-term cycles like the Pacific decadal oscillation, then future cycles influenced by climate change could play a major role in the entire food web, she said.

“When the PDO is positive (less productive), the harbor seals are feeding lower on the food web,” she said, explaining that the relative amount of food declines more at higher trophic levels when overall productivity goes down.

No significant differences in trophic level were seen between male and female harbor seals at any time during the study, although other studies suggest that females may be feeding closer to shore and consuming different prey. Nor did the size of the animals show a notable difference when it comes to what they were eating.

Feddern is preparing two research articles for publication, one about the effects of ocean conditions on the food web, the other about historical changes in the diets of harbor seals. Meanwhile, stable-isotope studies continue in the Holtgrieve lab to complete a picture of the food web by filling in the blanks for other marine species.