SEATTLE (AP) — The endangered killer whales that frequent the inland waters of Washington state are having pregnancy problems because they can't find enough to eat.

That's according to a new study by researchers who analyzed hormones in excrement collected at sea and found more than two-thirds of orca pregnancies failed over a seven-year period. They linked it to nutritional stress brought on by a low supply of Chinook salmon, the preferred diet of southern resident killer whales.

University of Washington professor Sam Wasser, the paper's lead author, says whales are conceiving. But when nutrition is low, they don't sustain pregnancies.

The study published in the journal PLOS ONE says 35 orca pregnancies occurred between 2008 and 2014. Eleven calves were spotted. But in 24 cases, no calves were seen with the closely monitored population, indicating the whales lost the babies or the calves died shortly after birth.