A multi-year survey of the nutritional, physiological and reproductive health of endangered southern resident killer whales suggests that up to two-thirds of pregnancies failed in this population from 2007 to 2014. The study links this orca population’s low reproductive success to stress brought on by low or variable abundance of their most nutrient-rich prey, Chinook salmon.
The study, published June 29 in the journal PLOS ONE, was conducted by researchers from the Center for Conservation Biology at the University of Washington, along with partners at the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Centerand the Center for Whale Research. The team’s findings help resolve debate about which environmental stressors — food supply, pollutants or boat traffic — are most responsible for this struggling population’s ongoing decline.
“Based on our analysis of whale health and pregnancy over this seven-year period, we believe that a low abundance of salmon is the primary factor for low reproductive success among southern resident killer whales,” said lead author Sam Wasser, a UW professor of biology and director of the Center for Conservation Biology. “During years of low salmon abundance, we see hormonal signs that nutritional stress is setting in and more pregnancies fail, and this trend has become increasingly common in recent years.”
Southern resident killer whales typically feed from May to October in the Salish Sea, and spend winters in the open Pacific Ocean along the West Coast. Unlike transient orca populations that feed on marine mammals, more than 95 percent of the diet of southern resident orcas consists of salmon, with Chinook salmon alone making up about three-quarters of their total diet.
Scientists already knew that the southern residents, just 78 individuals in Dec. 2016, had a lower fecundity rate compared with orcas in northern British Columbia and southern Alaska. But the data gathered by Wasser’s team indicate that dwindling and variable salmon runs do more direct damage to the reproductive success of the southern resident population than increasing boat traffic in the Salish Sea. Impacts of nutritional stress on pregnancy failure are further compounded by the release of toxins, which accumulate in their fatty tissues.
To gather data about orca health and reproduction, Wasser and his team measured the breakdown products of key physiological and sex hormones in orca fecal samples, or scat. They also used orca DNA extracted from the scat to determine sex, family pod and identity of the individual responsible for the leavings.
Obtaining fresh orca scat is no ordinary task. Through the Center’s Conservation Caninesprogram, the team trained dogs to sniff out floating orca scat from the bow of research boats that trailed southern resident pods. The dogs could detect scat up to one nautical mile away. Using this approach, they collected 348 scat samples from 79 orcas between 2007 and 2014. On these fecal searches, the researchers also gathered extensive data on boat traffic in the area, which increased significantly during the study period.
Read the full article in UW Today