Why the size difference?

I've always been interested in why the size differences between the average panfish in southern waters as opposed to those hugging the Canadian border for instance. I know they'd have a longer season down south where their metabolism is in higher gear due to warmer water on average. But does that alone account for the larger fish? Last I read, the subspecies of Largemouth Bass from Florida was just inclined to grow bigger than a strain from say, New England. Put them both in the same water from minnowhood, and the Florida Bass is going to top out larger in adulthood simply from genetic differences.

I also know that in northern ponds, sunfish (and perch, crappie, bass, etc.) often stunt from overcrowding if there are too many fish/not enough predators, but that shouldn't be any different with southern waters. I've fished ponds at both extremes of north and south, and I didn't see any obvious differences in the amount of food in the water. Any biologists out there, amateur or pro who has a handle on it?

From what I've read northern sub-species of fish grow slower and with a short growing season tend to be older for size. But they also live longer according to what was in print? Hopefully one of the biologists on here can give more details.

In the South you have a Longer growing season, more fertile water, larger food source, and no I am not a biologist....I just read a lot.

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Originally Posted by NIMROD

From what I've read northern sub-species of fish grow slower and with a short growing season tend to be older for size. But they also live longer according to what was in print? Hopefully one of the biologists on here can give more details.

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Part of the reason northern strains grow slower is the much shorter growing season, especially true of the sunfish, not all of which are even hardy up here. Even in those species that are, the southern strains don't do as well up here anyway. There is also a different predator community that the panfish need to adapt to, besides the climate. Some of them like pike and muskies actually gain weight over the winter rather than just survive it. They lose weight if the summer is too long or the water gets too warm.

One needs to be careful about crappies. They do not grow as fast up here, but they do live longer and can reach the same ultimate sizes. Just check the state records. Sunfish are another matter though. Ours in the north just do not get as big and definitely take a lot longer to do it. In Minnesota and Wisconsin studies have shown that it can take 10 years to produce a 10" bluegill under the best of conditions and many waters cannot even do that. Most of the Lepomis species are not even hardy this far north.

no1son, Bluegill seldom grow to 10'' in alot of southern lakes like Nimrod in Arkansas.

Then final size is not much difference in most cases, but I really doubt that there are any northern waters that can produce the maximum size potential of the south. The same can probably be said for largemouth bass, but crappies seem to have the ability to make maximum size in the north (if one can go by state records at all); so they are a bit of a different case.

Most folks don't know that bluegills actually gain very little size after they mature; so the biggest stayed juvenile a very long time for something that size, and that period is what determines the largest sizes. No question that bluegills can mature in a very small amount of time, but the best do not. I think that up here even with extended life expectancy there is simply not enough of it to reach the top southern potential even if most southern bluegills will never make it any more than most northern ones.

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Originally Posted by no1son

bluegills actually gain very little size after they mature; so the biggest stayed juvenile a very long time for something that size, and that period is what determines the largest sizes.

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So with faster maturing fish being smaller, they're going to be breeding for at least a few seasons more and earlier than the potentially larger and slower maturing fish. They'll have many more offspring hatching and maybe even breeding again before the slower maturing/larger fish even spawn. Makes a compelling argument for returning the biggest fish back to the water to ensure their genetic strain gets the best opportunity to continue in that body of water. It would also make a good argument for thinning out the smaller fish if you could be sure you were taking home 'early maturing' genetically smaller fish for supper and not the immature 'slow maturing' potentially larger fish. Any way to tell the difference without a portable lab? :confused:

I don't think it quite works that way. My impression is that once they mature they have about the same amount of life expectancy left and so about the same number of breeding seasons regardless of the size. Two things determine the size at which the fish mature. Pretty obviously not all lakes are created equal; so the potential of the body of water itself is very important.

But one other thing happens. Male bluegills delay maturity until they are of sufficient size to have a competitive chance at nesting in the center of the bed, which is where the biggest local males will be found. That is also where the most fry survive to produce recruitment. Quite literally none of the fry from the nests on the edges of the bed usually make it past the first few days. So you end up having a pair of related factors that push bluegills toward the maximum size possible in the waters where they are found. If the top sizes have been harvested off the size spread, the juveniles will naturally mature earlier and smaller. They are often said to be stunted, and while that may be the case in some waters, it is far more likely in bodies of water of larger than private pond size, that there are few to no larger males for the juveniles to measure against, especially up here in the north where the process takes quite a bit longer.

Females are also a bit picky, preferring to spawn towards the middle of the beds themselves, although they spread out their favors some and generally spawn a bit with each of several males.

Makes me wonder if just introducing some large males to a small population is enough to stimulate some of the young 'uns to delay maturing and grow larger? It sounds like the maximum size of the fish remaining in any given spawining area is more a determining factor to either stunting or having a range of sizes, including the big 'uns, than the numbers of fish in the pond or lake. I assume there also has to be a tipping point, though, where food supply is going to begin to affect growth when competition gets intense.

At any rate, some of the largest fish being left in the water is apparently still important to getting more large fish, regardless of the mechanism. But then....got any info on the cannibalism rate on fry? Do the larger fish tend to be more/less canibalistic?

There are lakes up here in MN that can grow 12 inch bluegill, but the key is the biomass of the lake has to be perfect in order to do so. Perfect cover, water clarity, forage types and quantities, and the ratio of predators to prey fish has to be in balance. A 'perfect storm' of all these conditions can produce big fish. These bodies of water are not the majority of lakes here, however. 10-12 in. sunfish are older bruisers that have been around awhile. A co-worker recently caught a 1 lb. pumpkinseed sunfish in a east central MN lake, so I know it is possible. Pumpkinseeds, on the average, do not usually grow that large. The MN DNR implementing limit restrictions on sunfish on a number of lakes in MN has had some success, but it is too early to tell if that is the answer to growing larger panfish on the average, here. Our growing season is obviously more limited than down south, so numbers, and size are reflected accordingly when comparing north vs. south.