I guess it's certainly "possible" that those are fish .... but, it seems odd that every one of the white marks that you've "red dotted" is directly in front of a branch. It would be easier to determine that those marks are fish, if there were some similar marks not directly in front of a branch. As it is, I suspect those white marks to be nothing more than branches that are on the front edge of the image, and possibly pointing towards the boat ... giving them a slightly brighter coloration.

Sorry, I have no experience with a Mac.

... cp