Our experience with grilse is that the ones that jump completely out of the water are those that are just moving through the pool up river. Whereas the fish that "porpoise" like the one in picture two are hanging around the pool, and are active. I can't tell you how many fish I've hooked, and seen hooked over an area where a fish was just seen porpoising. In the tea dark peat stained waters in my area, those two presentations are about the only indicators of fish in the pools, because you can't see them. Hope this was of some help.