Killer whales along the U.S. West Coast were listed as endangered in 2005. Their number is now down to just 78 from 140 some decades ago. The dwindling population of Chinook salmon appears to affect the ability of the orcas to increase their numbers.

Shortage Of Food, Toxins And Vessel Traffic

Findings of a new study to be published in the journal PLOS One have suggested that the shortage of food is a bigger factor that limits the recovery of resident killer whales when this is compared with toxins and vessel traffic.

In the new study, researchers looked at the nutritional, physiological and reproductive health of southern resident orcas and found a link between the low reproductive success of the marine animals to stress caused by low supply of the Chinook salmon, their favorite prey.

High Rate Of Unsuccessful Killer Whale Pregnancies

Study researcher Sam Wasser, of the Center for Conservation Biology at the University of Washington, and colleagues found that only 11 out of the 35 pregnancies among the orcas they tracked between 2007 and 2014 produced a live calf.

The researchers think that the number of failed pregnancies may even be higher since they were not able to detect the earliest months of pregnancy, the period when failed pregnancies typically happen. They attribute the high rate of unsuccessful pregnancies of the endangered species to the shortage of the Chinook salmon.

Abundance Of Chinook Salmon, Nutritional Stress And Failed Orca Pregnancies

The female orcas, whose pregnancies failed, were found to have levels of hormone that hint of nutritional stress. These levels are about seven times higher compared with the females that successfully gave birth.

Study researcher Deborah Giles, from the Center for Whale Research, acknowledged that vessel traffic and toxins are bad for the whales as well but when the marine animals are well-nourished, other problems do not affect them as much. She noted that whales that are well-fed do not have a strong signature for stress hormones that are linked to vessels.

The researchers also compared the hormone data of the marine animals with records of Chinook salmon runs to determine if the abundance of the fish was correlated with the nutrition of the orcas. They found that the killer whales tend to have lower nutritional stress during large salmon runs. They also noticed that the killer whales suffered from more nutritional stress when the runs were poor.

"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," Wasser said.

Save The Salmon To Save The Orcas

The researchers said that the take-home message of the findings is that there really is a need to start looking for ways to restore the population of the salmon, which makes up majority of the orcas' diet. Restoring the population of the orcas' favorite prey may help boost the sea mammal's dwindling population.

"A large number of whales are conceiving, but when nutrition is poor, they don't sustain those pregnancies," Wasser said.