Using museum specimens to understand harbor seals’ role in the food web

Members of the Holtgrieve Ecosystem Ecology Lab, led by gradate student Megan Feddern, are trying to better understand the interactions between harbor seals, their prey and environmental changes to better inform management decisions from an ecosystem-based approach. We aim to do this by:

  • Creating a dataset of where harbor seals have been feeding in the coastal WA food web over the past 100 years from museum skull specimens.
  • Combining this dataset with other historic datasets of environmental drivers (Pacific Decadal Oscillation, El Niño Southern Oscillation, sea surface temperature) and important harbor seal prey species (herring biomass, salmon populations, Pacific Hake biomass) to identify what ecological components drive harbor seal food web position.
  • Integrating our results to develop ecological integrity indicators’ for the California Current Integrated Ecosystem Assessment (CCIEA). For this particular dataset the harbor seal food web position indicator will represent ecological interactions between harbor seals through competition, predation and environmental drivers.

Using a recent methodological advancement called compound specific stable isotope analysis of amino acids, we are able to measure the nitrogen isotope ratio (15N/14N) of eleven different amino acids preserved in bone collagen. Certain amino acids, called trophic amino acids, show an increase in 15N relative to 14N as an animal feeds higher in the food web. Other amino acids, called source amino acids, do not show this change and instead have the 15N/14N of primary producers. By using the 15N/14N of trophic amino acids and controlling for potential changes in distribution of 15N and 14N using source amino acids we can calculate food web position. We can decalcify a small piece of bone (50 mg) to access preserved collagen and measure the 15N/14N of the amino acids contained within that collagen and calculate the food web position of the harbor seal that collagen came from.