About all you know is that people stop catching them. I’ve seen major arctic blasts with not much shad kill and then smaller fronts lead to massive ones. My theory is that when we usually get a big shad kill it’s when the weather has lulled and they move a bit shallower, and then a very cold front comes in and cools the shallower water down too fast for them to retreat. There was one year that we had a really cold front come through and it wiped pretty much all of the threadfins out on our local lakes and texoma. Interestingly enough, I don’t remember having a massive die off when we had that ridiculously cold weather about 5 years ago that caused all those blackouts. It was cold enough that the trinity in Fort Worth was frozen over and I don’t recall it wiping them out like that. If I remember right the last time we had a really bad shad kill the lakes were low.