Good question, Ewest. A partial answer, certainly not complete, is that the largemouths and smallmouths tend to spatially segregate in larger waters. Smallmouths gravitate toward rock when available, and largemouths to aquatic plants (or at least the upper, more productive waters), in a reservoir. Overlap between the species occurs in ponds because of small size. Heck, you probably know the literature better on habitat use by largemouths, smallmouths, and spots in reservoirs than I do!
