It's the splash. That noise you hear behind you, up stream, or on the other side of the boat. You know, when it's just getting dark.
Possibly catching a glimpse of the water splashing down, or a tail slipping back into the water. Just when you'd given up hope that there were even fish to be had where you decided to wet a line. That stupid little sound that makes you stay an extra hour.
This particular splash was a bit bigger than normal. What do you think made it?