Stocking YP pre or post spawn primarily depends on if you want to have lots of young YP this summer or next summer. If you don't have predators present to consume the small YP then I would stock post spawn. If you are trying stock a new pond with only a few adult perch to save money then stock pre-spawn. There are various reasons for stocking pre or post spawn. Each pond situation can be unique.
aka Pond Doctor & Dr. Perca Read Pond Boss Magazine - America's Journal of Pond Management
