High pregnancy failure in southern resident killer whales caused by nutritional stress

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Southern Resident Killer Whales

A new study has shed much needed light on the possible reasons behind the high pregnancy failure rate in souther resident killer whale.

According to the study published in journal PLOS ONE reveals that high pregnancy failure in southern resident killer whales has links to nutritional stress and low salmon abundance. The study includes data from a multi-year survey of the nutritional, physiological and reproductive health of endangered southern resident killer whales.

Data indicates that up to two-thirds of pregnancies in these resident whales failed in this population from 2007 to 2014. The reason behind this seems to be the stress brought on by low or variable abundance of their most nutrient-rich prey, Chinook salmon.

Researchers from the Center for Conservation Biology at the University of Washington, and colleagues say their findings help resolve debate about which environmental stressors – food supply, pollutants or boat traffic – are most responsible for this struggling population’s ongoing decline. Scientists say that low abundance of salmon is the primary factor for low reproductive success.

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. To gather data about orca health and reproduction, scientists 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.

The 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.

In 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 pregnancies failed.

In 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.

“These findings indicate that pregnancy failure — likely brought on by poor nutrition — is the major constraining force on population growth in southern resident killer whales,” authors note.

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