Nutritional Stress in Orcas

Published 2017-07-11

Photo: NOAA Fisheries

Researchers at the University of Washington have been tracking the Southern Resident killer whale population in the Salish Sea for several years in an effort to track their health and reproduction. The Southern Resident killer whales are endangered with continuing population decline. Debate exists on whether the decline in the whale population is due to food supply, pollutants, or boat traffic. In an effort to resolve this debate, Sam Wasser and colleagues at the University of Washington conducted a multi-year survey of the nutritional, physiological, and reproductive health of the Southern Resident killer whales.

In this study, the researchers obtained samples of fresh orca scat with the help of trained scat-sniffing dogs. The fresh fecal samples allowed researchers to determine the sex of the animal, the family pod, and identity of each individual. These fresh samples were later analyzed for hormone levels. Hormones found in these samples included reproductive and stress hormones, which were used to understand the reproductive status of female orcas and the level of nutritional stress in each animal.

The researchers found that in 69% of total pregnancies in this sample population failed, which is a lower success rate than other nearby orca populations. In females with failed pregnancies, low ratios of thyroid hormones relative to glucocorticoid hormones were detected, indicating that nutritional stress may play a large role in reproductive success. In addition, this study compared the levels of nutritional stress to records of Chinook salmon runs in the Columbia and Fraser rivers. Chinook salmon accounts for a vast majority of the Southern Resident diet and the stocks in these two rivers are important sources of food for the population. In years with poor salmon runs, signs of nutritional stress were more prevalent in the orca population. This study suggests that low reproductive output and lack of population growth in these whales is due to low or variable abundance in their prey.

For more information about the Southern Resident killer whales, visit the Center for Whale Research at https://www.whaleresearch.com/orca-population.

Journal Reference:
1. Samuel K. Wasser, Jessica I. Lundin, Katherine Ayres, Elizabeth Seely, Deborah Giles, Kenneth Balcomb, Jennifer Hempelmann, Kim Parsons, Rebecca Booth. Population growth is limited by nutritional impacts on pregnancy success in endangered Southern Resident killer whales (Orcinus orca). PLOS ONE, 2017; 12 (6): e0179824 DOI: 10.1371/journal.pone.0179824