Brad,
Congratulations on your new pond. So with a goal of growing large/larger BG ... you are on the right path thinking about adding some LMB. Also, you've recognized that both the size and number are important to get the BG numbers in control. The optimum number LMB is dependent on the waters productivity per acre and the surface area of your water. The productivity is an unknown but it could be estimated. After stocking ... then judging by the growth of the LMB and the BG ... you determine if it needs to be tweeked (More LMB or take some out).
You want to be careful about the size of LMB that you stock. Were it me, I would stock MINIMUM length 8"- 10" LMB. I think the number I would start with would be 40 of them. They could grow by 1 lb over the next 12 months which with 90% survival would give you a 72 lb/acre standing weight of LMB.
Some things to consider:
Larger LMB from the get go have a higher maintenance ration and so might consume more than LMB stocked in smaller sizes.
Larger LMB from the get go are able to consume larger BG. That means more of the population of BG will be consumable.
I think both of these considerations are important in your particular circumstance. I believe you could go as large as 12" LMB without problems and good success. A good BG pond will have a good number of LMB but there definitely should be some examples that reach a minimum length of 12". What you want is that the 1"-4" BG are mostly consumed where the remaining that continue growing have ample foods to grow to 6". At six inches, you can harvest to prevent overpopulation of mid sized BG.
Something else to consider:
We don't have a good estimate of the BG population structure (#/acre) in the >4" sizes. You must keep in mind, that it is not likely than any which are greater than 4" will be consumed by your LMB. They will be just ahead of them in terms of gape. So you might have trap or harvest some BG that are below your preferred harvest size. Good news. If you act now, you can gain some upper hand on the situation. Please share your decision and keep us updated on your progress.
