Target the LMB when they are spawning. The males are easy to catch when they are guarding the nest, and the females can be caught if they are hanging out around the nest getting ready to spawn. As for the BG, when they are nesting you can catch the males easily, and if you fish out about 2'-3' deeper than the BG nesting colony you can usually catch the female BG.
Easiest way to disrupt the spawning cycle is to catch and remove.
