I think I would try to get as many as possible out with the net. GSD have some very bad possible consequences in a pond with out very large LMB. In studies GSD have the tendency to grow fast as a population until pond carrying capacity is reached then they repress all fish reproduction so they can grow large and there are few new fish offspring. A pond with 50%+ of its carrying capacity in GSD is not a good option IMO unless you have good numbers of 20+ in. LMB.