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.
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
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 Center and 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
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."
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.
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.
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.
fresh orca scat is no ordinary task. Through the Center's Conservation Canines
program, 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.
hormone levels they calculated from scat include progesterone, testosterone,
glucocorticoid and thyroid hormone. Glucocorticoid and thyroid hormones play
key roles in physiological stress responses -- and determining levels of both
hormones allowed researchers to differentiate between stress due to poor
nutrition and stress due to external responses, such as boat traffic. The
researchers used progesterone and testosterone levels in scat from females to
determine reproductive state. They could even determine whether a pregnant
female was in the early or later stages of the 18-month gestation period for
orcas. They then correlated these data and the date of collection with calf
sightings to determine whether each pregnancy was successful.
total, these hormone data detected 35 unique pregnancies among southern
resident females from 2007 to 2014. In 11 cases, the individual female gave
birth and was seen with a calf thereafter. But in 24 cases -- 69 percent of
total pregnancies -- no live calf was subsequently seen, indicating that these
most cases, the pregnancies likely ended in spontaneous abortion during the
first half of gestation. But in one-third of the failed pregnancies, hormone
levels indicated that the calf was lost in the latter half of pregnancy or moments
after birth -- stages at which the mother has already invested significant
resources and is at higher risk of infection or complications when a pregnancy
fails. These females also showed signs of nutritional stress, with ratios of
thyroid hormone relative to glucocorticoid hormone nearly seven times lower
than females who successfully gave birth.
findings indicate that pregnancy failure -- likely brought on by poor nutrition
-- is the major constraining force on population growth in southern resident
killer whales," said Wasser.
team compared their hormone data to records of Chinook salmon runs in the
Columbia and Fraser rivers, the two most significant sources of Chinook in the
southern residents' natural range. They saw that large runs at those watersheds
coincided with periods of lower nutritional stress in the orcas. But in years
with poor runs at either site, signs of nutritional stress were higher.
Boosting Columbia River and Fraser River salmon runs could help the killer
whales recover, Wasser said.
it stands now, the orca numbers just keep declining with no signs of
recovery," said Wasser. "We're losing a valuable resource here."
are Jessica Lundin, Elizabeth Seely and Rebecca Booth at UW; Deborah Giles and
Kenneth Balcomb at the Center for Whale Research; and Jennifer Hempelmann and
Kim Parsons with the NOAA Northwest Fisheries Science Center. Lundin is now a
postdoctoral researcher at NOAA. The research was funded by Washington
SeaGrant, NOAA's Northwest Fisheries Science Center, the Canadian Consulate
General, the UW Center for Conservation Biology, the Northwest Science
Association and the U.S. Environmental Protection Agency.