"has been" indicates an action that is still occurring. Therefore, since there is no endpoint the value 16% for a percent change doesn't make mathematical sense.
Using past tense "declined" indicates that the end point is now, so a % change can be calculated.
Additionally if the percent reported was smaller (or a reasonable value) it could be falsely, interpreted as a yearly decrease:
... has been declining 1% in the last 10 years.
To unambiguously indicate an average yearly decrease, something like the following could be used:
... has been declining 1% each year in/for the past 10 years.
My mind prefers to use the preposition "for" but either could work I think.
Additionally, "by" could be inserted before the percent in every sentence. This seems to make it less academic/news-y but conveys exactly the same meaning.