I posed the question to biologist Daryl Bauer what qualifies as an old bluegill, and asked if he had sampled some really ancient specimens in his years as a fisheries professional.
Here was my question.
"Actually, I'd like to know from Daryl if he's seen any truly ancient bluegill in his sampling days. You know, like 12 or 13 year old fish. I've probably aged, or assisted in aging somewhere around or over 200 bluegill, but haven't run across any real oldies like that yet.
An interesting point is that in a given population, a ten year old fish isn't necessarily going to weigh more than an eight year old fish. Sometimes the fish begin to regress, or go "geriatric" and the weights drop back a little".
His answer was as follows.
"I do not know how many fish, bluegills included, I have aged. I am sure it would be in the thousands. In addition we have an age and growth database of thousands and thousands of fish we have aged in Nebraska. We have documented bluegills up to age 13 from Nebraska waters. Ancient bluegills are possible, but not at all common. In fact in many waters it is rare to see bluegills older than age-6.
Now let me say something about aging fish, especially aging old fish. Fish to some extent have indeterminate growth--they continue to grow throughout their lives. Yes, there are some genetic limits, you are not going to have a 50-pound bluegill, sorry to you and "OldBaldGuy" about that, but fish can grow throughout their lives. As they grow their bony structures grow and the growth patterns in those bony structures are how we age fish. Scales are the most commonly used bony structure used to age fish. However, the older fish get the more difficult it is to achieve an accurate age determination from scales. Scales can erode over time and the older a fish gets the slower it grows; those are a couple of reasons it is difficult to determine the exact age of old fish from scale samples. For most bluegill populations, populations with relatively fast growth and few ancient fish, "reading scales" would be accurate enough to determine age and growth rates needed to make management decisions.
But if you rely solely on scales for aging you might underestimate the age of the oldest fish. If you want to know maximum age, then otoliths (i.e. ear "bones") are probably the best bony structure to use. The thing with aging fish by using otoliths is the fish have to be sacrificed and there is some prep. work and additional techniques to be mastered to "read" otoliths. "Reading" scales is relatively quick and easy compared to "reading" otoliths and the fish aged from a scale sample can still be swimming. I said all of that to say this, we may underestimate the oldest bluegills out there because we do not look at as many otoliths as we do scales.
You are absolutely right on your last point--the oldest fish may not necessarily be the biggest. In fact if I wanted to find the oldest bluegill I could, I would look for a population of relatively slow-growing fish. A population of small, stunted bluegills might be more likely to have some "ancient" fish than a population of fat & happy fast-growers. As a general rule in fish populations, fish from populations with slower growth rates will live longer than fish from populations with fast growth rates. That is one reason I continue to insist that big fish are not necessarily as old as one might believe. Big fish might get that way because they are fast-growers and not necessarily just old. That is why catch & release of big fish, big bluegills especially, can be so important--those are likely fast-growers and some of the best genetic stock.
You are also right that the very oldest fish may actually "regress" towards the end of their life. Fish can grow throughout their life, but growth rates slow the older a fish gets. They may "plateau" at some point where they really do not grow any longer for the rest of their life. Big fish that reach that "plateau" may continue to be healthy and may put on weight, but their lengths may increase very little if at all. Naturally as fish age they eventually reach a point where they just wear out, their condition and weight begins to decline, but their length does not shrink; eventually they die."