Water temperature is a major factor in the distribution of different fish species. The reason you don’t see trout in the river isn’t because it’s murky or doesn’t contain enough oxygen- it’s because the river reaches temperatures above their preferred range for survival. This is why you’ll occasionally catch brown trout in the MIssissippi during Winter months. The larger fish will travel greater distances to find bigger prey and in the winter, the river temps are well within their range for survival. With shad its a similar, yet different scenario. Shad are adapted for living in southern lakes and reservoirs. They’re not adapted to live in our temperate climate of the midwest. The summer months bring warm enough temperatures for the shad to reproduce and thrive, but many of them perish when the water temps dip below 45 degrees. And I’m not talking about surface temperature, you have to factor in that the temperature below the surface stays warmer deeper into the fall. You’ll rarely see the big breeder shad, but they’re there. I’ve accidentally snagged shad as long as 18″ before in the Mississippi. They usually come out of deep water. As someone else said, its hard to know just how many of them die off. From my own observation, i say with some degree of confidence that it’s over 50% of young of the year shad, if not more. I very rarely see 2nd year shad in the spring and summer months on the surface like you would when the YOY group comes on strong later in the year.