Like any other fish, migrating or otherwise, smelts like structure. That is to say, anything "different" from the immediate surrounding environment. In the case Retroboy mentioned, a trench can be the only thing different in the immediate area, and so that is where the smelts congregate (albeit temporarily before continuing on with their migration upriver) as opposed to twenty feet or even less away. This is pretty much scientific fact. As for the smelts leaving a scent when they bite which deters other smelts from following suit, this I cannot comment on as it is the first I hear of it. However, I can confirm that smelts are primarily scent hunters and secondarily visual hunters, therefore that is why it is important to change your bait often. You will find that bait like shrimp, which gives off a lot of scent, will give you a hit almost instantaneously if smelt are in the midst, however they tend to not stay on your hook very long. Prawns give off less scent than shrimp, so even though they stay on your hook much longer, you still have to replace or imbue them with an attractant like Retroboy suggested after ten minutes or so as they have little scent left to them. Somebody mentioned using red yarn somewhere. This can work reasonably well if used on the same line with bait, but not so much on its own as needless to say yarn does not give off much scent, if any at all.
