Profish, bring the rigger up. Its not the rigger that they hear its the tight line between you and the fish. Just like a guitar string it has a specific frequency which to them sounds like the "dinner bell". I fish in an area with a lot of moochers, no down riggers, and the seals are on the fish immediately. And its definitely not the reels, a few guys have taken off the clickers.
Another good theory. Don’t think it rules out reel sounds down the line as well. They likely key on many factors. Knowing which are the key factors would help us try to minimize them. Perhaps some experiments are in order such as the one Profisher suggested.
Looks like we have managed to hijack a Halibut thread but then I have seen a seal at Constance with a Halibut in its mouth off a line.