I don't have any personal experience with that trap, but from the specs it seems kind of small to me, especially in light of removing tons of smaller BG. I highly doubt that a 6" BG would be able to enter, but you're right...more info on the opening size would be beneficial.

Is it possible to run a seine through the pond, or at least through some areas/coves?

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.