Fifty years of Cook Inlet beluga whale ecology recorded as isotopes in bone and teeth

Abstract

Beluga whales (Delphinapterus leucas) are found across the Arctic and Subarctic in seasonally ice covered waters. Five stocks of beluga whales are associated with the waters near Alaska for at least part of the year and four of those five stocks are abundant and commonly hunted by Alaskan Natives. The belugas resident in Cook Inlet are also an important cultural and subsistence resource to Alaskan Natives in the area, but a ~50% decline in abundance in the 1990's led to the stock being designated as depleted under the Marine Mammal Protection Act in 2000 and listed as endangered under the Endangered Species Act in 2008. Numerous studies of beluga whales in relation to stranding events, predation (killer whales), parasitism, disease, contaminants, and other potential population threats have not identified the reason for their inability to recover. Changes in diet have been considered, but are difficult to study because observations of feeding in muddy water and beluga stomachs are difficult to obtain. To investigate the past feeding ecology of beluga whales from Cook Inlet I sampled bone and teeth for isotopic analyses. I sampled bone from 20 individuals that died between 1964 and 2007 for stable carbon and nitrogen isotope analysis (values expressed as δ¹³C and δ¹⁵N values). I also micro-sampled annual growth layer groups in the teeth of 26 individuals representing the years from 1962 to 2007. Bone and tooth data showed a general decrease in δ¹³C and δ¹⁵N values over time. The δ¹³C values from analyses of growth layer groups declined from -13.4‰ to -16.2‰ and δ¹⁵N values declined from 17.2‰ to 15.4‰. Although these values are consistent with a change in feeding ecology over time, the magnitude of the decrease in δ¹⁵N values (~2‰) is insufficient for a full trophic level shift (~3‰).The relatively large decrease in the δ¹³C values over the same time period (~3‰), however, is much greater than a full trophic level shift (~1‰) and suggests an increase in prey associated with freshwater, which typically have lower δ¹³C values than prey associated with marine water. To test this hypothesis I analyzed the strontium isotope composition (⁸⁷Sr/⁸⁶Sr ratios) of growth layer groups in teeth from a sub-set of individuals. The resulting ⁸⁷Sr/⁸⁶Sr ratios trended away from the global marine signature (0.70918) over time and toward the more freshwater signatures measured in rivers flowing into the upper reaches of Cook Inlet. These results indicate that the diet of Cook Inlet beluga whales has changed over time. This could be from feeding on different, more freshwater derived prey species, or from feeding on the same species, but on individuals from locations with a more freshwater influence. Both of these interpretations are consistent with population survey data indicating a retraction in beluga range into the upper reaches of Cook Inlet. This study presents the first evidence of a long term (~50 years) change in Cook Inlet beluga whale feeding ecology. The consequences of this change toward more freshwater-influenced prey, and how this change relates to Cook Inlet beluga whales' decline or recovery remains unknown. However, to better examine this change in feeding ecology a follow-up study will; 1) develop a strontium isoscape for the Cook Inlet watershed; 2) analyze more teeth to better analyze changes in feeding ecology by demographic group (sex, age); and 3) analyze growth layer groups from Bristol Bay beluga teeth for a comparison with Cook Inlet belugas to determine if the changes represent an ecosystem change within Cook Inlet or a broader scale change affecting another region. This study builds towards a better understanding of the changes in Cook Inlet beluga feeding ecology and will help to determine if changes in diet could be a factor in their recovery.