。
Rice production has been declining.
Literal
Rice's production-amount [topic-は] decreasing has-come.
減ってきた uses ~てくる to mark a change that has been unfolding over time and reaching the speaker's present — 'has been declining (and the decline continues).' 生産高 is 'production volume.' The ~てくる adds temporal directionality that a simple 減った wouldn't convey.