Damn this was mathy and realistically nobody's gonna care, but it got the better of me to try it out:

C = Chicken
D = Common Bottlenose Dolphin
H = Harp Seal
M = Eurasian Eagle-Owl

I: Given that 2D+2H = 8, then D+H = 4
II: Given that D+H+2M = 18, then 2M = 18-4 and thus M = 7
III: Given that 3C+M = 13, then 3C = 13-7 and thus C = 2
IV: Given that C+2H+M = 11, then 2H = 2 and thus H = 1
V: Given that D+H = 4, then D = 3

So Mimi had 7 candies, Doiruka 3, Chicken 2 and Harp Seal 1. So in total they all gathered 13 candies (which was the question).