USING PREDATOR BONES TO DETERMINE TROPHIC DYNAMICS 

Reconstructing a Century of Coastal Productivity and Trophic Dynamics from Bone Specimens

Researchers used isotopic analysis to study changes in the place of Salish Sea seals within the food web.

Principal Investigator

Gordon Holtgrieve, University of Washington, School of Aquatic and Fishery Sciences

Co-Principal Investigators

Project

As part of the California Current Large Marine Ecosystem, Washington coastal waters host some of the world’s most productive fisheries. Fishermen and planners are interested in understanding what influences this regional productivity. The project provided an 80-year-plus perspective on food web dynamics involving harbor seals, comparing changing predator and prey abundances with shifting ocean productivity regimes. Using a novel technique called “compound-specific isotope analyses” to examine nitrogen isotope ratios in archived seal bone collagen, researchers assessed the role of increasing marine predator populations on coastal ecosystems.

Research Updates

Background

Following the implementation of the Marine Mammal Protection Act in 1972, the Puget Sound harbor seal population exponentially grew from about 2,100 to 18,000 in 2018. This change has been correlated to declines in forage fish and salmonids, upon which harbor seals prey, suggesting that harbor seals threaten species including endangered Chinook salmon. However, these trends also coincided with an environmental regime shift of the Pacific Decadal Oscillation from a cool phase to a warm one in 1977, which also impacted fish populations. This raises the question: to what degree are harbor seals responsible for the declines in each type of fish? Teasing the ecological drivers apart is challenging, yet important for informed ecosystem-based management.

Results Thus Far 

In 2018, the researchers obtained 150 harbor seal skull specimens from museums and measured the nitrogen isotope ratios of the relevant amino acids. These samples represented harbor seals from both the outer coast and Puget Sound from 1920 to the present, offering a basis for future comparison.