The problem is, you're not going to see any small swarms of fishes if they're to be added to the campaign layer as independent objects... unless you put around 100.000 and even way more of them. Put only like a hundred or two and imagine your chance of crossing one on the external cam... 0.01% maybe?
Swarms will have to be spawn by some kind of particle generator around the sub, so you can often have a chance to see them. For whales and dolphins, it's another story. You don't want to see one of those every 2 minutes and everywhere in the world, would you?
