Do you have another water source flowing into your pond?

BH can come from anywhere. However, in my experience I have never seen BH come to the top for feed. I unfortuantely have 1000's in my 1.5 acre pond and I have never seen them come up to the top other than during a period of extremely low oxygen a couple weeks ago. My Black BH have more of a white belly than the Brown BH with more of a yellow belly though.

Try some shrimp this evening, if it's a cat of any type, I bet you can catch it on a shrimp either on the bottom or a few inches off the bottom. Good luck.