Have put huge amounts of sour grain out before then fished with chicken hearts. Have caught cats with HUGE bellies that when cleaned, were full of grain and several hearts. Once in a feeding frenzy, its hard for them to stop. Sometimes when a large school of fish is feeding, they will gorge themselves until unable to put anymore in. At that point, they will regurgitate the contents of their stomachs in order to feed more. The stomach contents and oils and enzymes within will rise to the surface forming a "slick" patch on the surface. It tends to have a sweet smell, kinda like watermelon. Many species of fish do this.