grammar - 5% are left or 5% is left; which is better or correct?

So here is the sentence written both ways. Which one is correct and maybe a description why so I can understand why.
Thanks!
/R

Leatherbacks need your help.

They’re endangered and only 5% of the population are left.

OR,

Leatherbacks need your help.
They’re endangered and only 5% of the population is left.