Heybud often asks questions that result in some of the best discussions here. As a result I try to help them along by being sure my first answer is short and without too much detail. I wait to see how things develop to encourage everyone to get in on the discussion. I will post a few items from a new study (saving some of it for upcoming PB Mag.“Cutting Edge” articles) . This study looked at BG across 23 impoundments with “ extensive latitudinal (and therefore, ecological) variation and encompassing a broad range of factors known to be important to bluegill growth rates and population size structure “ within Ill. Keep in mind this is focused on the big picture not individual ponds. I will look for the data on metabolism ,and growth rate at temp. for BG to post. What I provided in the first post above is a generalization and you should remember that local adaptation exists (in one population of BG in Ohio growth may continue at 45 F temps while in another in TX it may slow to almost none at 48 F while in south Fla all BG growth may stop in one lake at 52 F ).

Examining Interpopulation Variation in Bluegill Growth Rates and Size Structure: Effects of Harvest, Maturation, and Environmental Variables
R. JOHN H. HOXMEIER, D. DEREK ADAY, AND DAVID H. WAHL Transactions of the American Fisheries Society 138:423–432, 2009


“Consistent with previous investigations of bluegills and other species, temperature had a pronounced effect on growth and body size; based on our models, temperature may be the most important environmental factor for explaining variation in bluegill size structure. The effects of Secchi depth and prey abundance were also generally important and were consistent across metrics; good growth and large body size were associated with warm, clear lakes and abundant prey resources. An exception to this pattern was observed in juvenile bluegills, for which larger body size was associated with relatively low Secchi depth (i.e., turbid water). Size at maturation was important in determining the body size of adult males but was relatively
unimportant to female body size. Conversely, bluegill density and harvest mortality did not contribute much explanatory power beyond the effects of prey abundance and temperature in any of our models.”

“However, the relative importance of temperature compared with other abiotic and life history factors is interesting and indicates the value of this approach when considering the multivariate nature of influences on population size structure. Latitudinal clines in growth resulting from changes in temperature have previously been demonstrated for a number of freshwater fishes. “

“ In a test of Bergmann’s rule for freshwater fishes, almost all of the fish species reviewed had smaller lengths at age in northern latitudes than in southern latitudes (Belk and Houston 2002). In a review of largemouth bass growth rates across North America, McCauley and Kilgour (1990) found that half of the variability in growth of largemouth bass was related to temperature. Similarly, bluegill growth was positively correlated with air temperature in Minnesota lakes . Our study provides a relatively unique perspective because we were able to quantify the importance of temperature relative to other environmental factors in its effect on variation in population size structure.”