SEATTLE (AP) — Over the years, scientists have identified dams, pollution and vessel noise as causes of the troubling decline of the Pacific Northwest’s resident killer whales. Now, they may have found a new and more surprising culprit: pink salmon.

Four salmon researchers were perusing orca data several months ago when they noticed a startling trend: that for the past two decades, significantly more of the whales have died in even-numbered years than in odd years.

In a newly published paper, they speculate that the pattern is related to pink salmon, which return to the Salish Sea between Washington state and Canada in enormous numbers every other year. They suspect the huge runs of pink salmon, which have boomed under conservation efforts and changes in ocean conditions in the past two decades, interfere with the whales’ ability to hunt their preferred prey, Chinook salmon.