" 's " or "of something", without an idea of ownership

I've seen this question asked many times but there are so many particularities that I did not find an anwser to my case. I'm a french student writting a science speech, and I hesitate between writing :

1. ... used to map the dolphin's absence...
2. ... used to map the absence of the dolphin...

and I have the same problem with :

3. ... recorded the size of the groups of dolphins
4. ... recorded the size of the dolphins'groups.

Thank you very much, and have a nice day !

As Kate Bunting said in a comment to the OP, 3. ("the size of the groups of dolphins") is better than 4. ("size of the dolphins'groups"). But, considering that the sentence talks about some property of the groups, I guess you've already mentioned the groups themselves earlier? You say you feel that "the size of the groups of dolphins" is too heavy.

So perhaps you could say "the sizes of the groups" (if "groups" would be clear in the context).