While it is true that most trollers have/use black-boxes, they are not always on. For instance, the buddy I fished with this spring has not had the time to install the box on the new (to him) boat. We ran the big commercial sounder ALWAYS, no black box, and were at or near High-Boat each opening. I simply can't believe the sounder was "scaring" those springs away from us given the success rate!
That said, I DO know how they effect some marine mammals! Turned my scanning unit on when approaching a pod of belugas up North, and they literally came out of the water trying to get clear of it! Hilarious!
Any time you want to try the experiment, I'd be game, but, I'll be the fella with the sounder ON thanks!! Would be interesting, but unless something is wrong with your sounder, I cannot see it making any difference beyond coincidence.
Cheers,
Nog