Fifty years of Cook Inlet beluga whale feeding ecology from isotopes in bone and teeth

Abstract

Beluga whales Delphinapterus leucas that reside in Cook Inlet (CIBW) are important to coastal Alaska Native culture and subsistence, tourism, and ecologically as a top-level predator. Due to a ~50% population decline in the 1990s, the distinct population segment in Cook Inlet was designated depleted under the Marine Mammal Protection Act in 2000 and listed as endangered under the Endangered Species Act in 2008. Diet changes are a concern in CIBW lack of recovery, but beluga feeding ecology is difficult to study. Skulls from 20 CIBW and tooth growth layer groups (GLGs) from 26 individual CIBW showed decreasing trends for both nitrogen and carbon stable isotope ratios (expressed as δ15N and δ13C values) from 1962 to 2007. The decline in δ15N values (~1 to 2‰) could indicate a trophic level shift, but the magnitude of decline in δ13C values (~3‰) is much greater (>5 times greater) than expected for a trophic level shift. A shifted baseline or increased use of freshwater prey could explain the decline in δ13C values. We compared the strontium isotope composition (87Sr/86Sr ratios) of GLGs with rivers that flow into Cook Inlet and used δ15N values from the essential amino acid phenylalanine to determine that declining δ13C values may be explained by 2 scenarios: (1) CIBW foraged in the same location while the environmental isotopic baseline changed, or (2) CIBW foraged in a different location with a different baseline. This study presents the first evidence for a long-term (~50 yr) change in CIBW feeding ecology.