RAH asks - "Can well-fed full-grown SMB eat stunted 6" YP? When are yellow perch too big for a full grown SMB to target?"
It depends on your definition of full grown SMB. I would start buy looking at the mouth gape of smallies and the body depth of 6" long YP. What then is the shortest SMB that could eat a 6" YP?
From my personal data: a 10" SMB died eating a 5.5"; YP average body depth (1.2). Thus I conclude that 12" smallies can eat a 6" YP. Let's look at some numbers. My body depth measurements for 6" YP are from numerous direct measurements. Some of my gape sizes for smallies are calculated estimates. We on the PB Forum need those with SMB to provide more gape sizes or measurements for smallmouth bass. Get busy you guys with smallies and make some body length and mouth width measurements for your smallmouth bass. Your measurements will help us verify how close my numbers are for the mouth size of pond raised smallmouth bass. This has been done for largemoth bass by thoughtful PB forum members in this link.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=453084Here are my data for gape width of smallmouth bass. The body depth of 6 Yellow perch ranges from 34-38mm (1.4). Note there is some natural variability (size width) in the gape for each length of SMB. For example the gape for a 10 SMB could be 30-34mm (1.25).
10 SMB Gape 32mm (1.25)
12 SMB Gape 36mm (1.4)
14 SMB Gape 46mm (1.8)
15 SMB Gape 49mm (1.9)
16 SMB Gape 53mm (2.0)
18 SMB Gape 59mm (2.3)
Thus IMO a 14 to 15 smallmouth could easily eat a 6 long yellow perch. When yellow perch grow to 9.5" long their body depth is 55 to 60mm (2.26").
Smallmouth bass were studied; 214 smallmouth bass ranging from 48 mm (1.9) to 486 mm (19) were measured for the study. A graph in this research paper shows the relationship between total body length and mouth gape for smallmouth bass in Lake Sharpe, South Dakota.
In this research: For all smallmouth bass collected, we measured total bodylength TL and maximum horizontal gape GW to the nearest mm. Maximum horizontal GW was measured by stretching the mouth open and measuring the distance between the outside edges of the maxillary bone (Lawrence 1958, Hill et al. 2004, Slaughter and Jacobson 2008).
https://openprairie.sdstate.edu/cgi/viewcontent.cgi?article=1112&context=nrm_pubsBased on your goals for growing big SMB, I think your pond's current conditions and management are very good to achieve that goal, but not necessarily a goal for growing big yellow perch. Adding more predators will reduce the abundance or numbers of good optimum forage items for the smallies. As the pond ages the fishery will naturally change by having more SMB. Thus expect the growth rate of the SMB to gradually slow down as the pond ages and more predators are present. You could counteract this by not allowing too many bass to survive, cull the smaller bass, and do your best to maintain a high density of smaller fish. As yellow perch become larger and more numerous they too will be predatious on the smaller fish. I've caught 10"-12" YP using 4" YP as bait.
