SEATTLE — 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 data on the website of the Center for Whale Research, which studies the orcas, 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 and Canada in enormous numbers every other year — though they’re not sure how.