See seine surveys here - a method of finding out pond balance . You do it with a short seine. You don't seine the whole pond only a small area. Then look at results to see what is there.
I bet your LMB are there and stocking more will lead to quicker LMB overcrowding.
Here is a link to the MSU Pond Mgt. Book. Look at the text and charts on pages 16-19 on population assessments.
http://msucares.com/pubs/publications/p1428.pdf