Sounds as though it could be the freshwater jellyfish... the freshwater shrimp have been there for years and the perch do actively feed on them but the huge hoards of them don't really resemble jellyfish. A lot of times when you see large schools of perch under the ice and they won't readily bite that is what they are often feeding on (freshwater shrimp).....they sometimes come up puking them and stuffed full.