Again, I respectfully disagree with the idea presented as carrying capacity. At times, my disagreement with this idea was not respectful and for that I apologize. This discussion is not trivial nor is it just a word game. Rather, it goes to our basic understanding of biology and our management of our lakes. In some cases, I will take my examples to an extreme to help illustrate them better.
I grew up on a farm in Nebraska in the 50’s and 60’s. There, I learned about carrying capacity in terms of the number of cattle that we could put on our pasture. Most farmers and ranchers can quickly tell you the carrying capacity of their pastures. This could be called the natural carrying capacity. However, since they must produce more income from this land, they often fertilize, irrigate or manage this land in many different ways so that they can raise more cattle. Sometimes, they take this another step and bring in supplemental feed so they can raise more cattle on this land. We now have an even higher carrying capacity. Taking this idea even further, we bring in all of the feed and have a cattle feedlot. Going even further, we squeeze these cattle into a semi-trailer to take them to market. The extreme of this idea is sardines in a can. This is the maximum carrying capacity of anything, it is its volume.
So far, we have kept the species (cattle) constant. Let’s change the cattle a little bit. Over time, we bought cattle that converted feed to body mass better. Also at this time, I was in college studying ecology and evolutionary biology. This got me thinking. Our pasture and the way we managed it hadn’t changed but the carrying capacity had. If carrying capacity was a constant, this wasn’t possible. I was finally able to figure out that carrying capacity really wasn’t about our pasture but was about our cattle’s ability to use our pasture.
This is the point. Carrying capacity isn’t a measurement of the resource. It is a measurement of different species’ or group of species’ ability to use that resource. To use an extreme example again, cattle could use our pasture very well. Fish could not use it at all. This had nothing to do with the quality of our pasture but rather it has everything to do with the vastly different abilities of these two species to use this particular resource.
This understanding has tremendous implications for managing our ponds. There is a very great difference in different fish species’ ability to use the resources in our lakes.
LMB have a very poor ability to use the resources in our lakes. They are good at using larger animals as food but very poor at using very small animals and plants as food. They are also very poor at converting food to flesh. Let’s illustrate this better by comparing carrying capacity, usually measured in pounds per acre, to a pie. And let’s use your lake without doing anything to it. If you put just LMB into your lake, you will have a very small pie. Gizzard shad are very good at using the resources in your lake (if you are far enough south for them to live). Your lake hasn’t changed but now you have a very large pie. This is also true for any combination of species. Different groups of species will use the resources in your lake better or worse than other groups of species. That means that the particular group of fish species determines the size of the pie. In general (with many exceptions), the more species, the bigger the pie meaning the more total pounds of fish per acre.
Now, let’s say that my understanding of this concept is incorrect (heaven forbid). I have been known to be wrong on more than a few occasions. Your lake supports about 300 pounds of fish per acre. Without fertilizing or feeding, that is your lake’s carrying capacity, maximum carrying capacity or whatever words you want to use. You can divide this 300 pounds between a few species or many species but you still won’t raise more than 300 pounds of fish per acre. Let’s say that your goal is to raise very big LMB. If the normal understanding of carrying capacity is correct, then it is a complete waste of time to add a forage species to your lake. The weight of this forage is taking away from the 300 pounds per acre that would be in the LMB otherwise.
None of this negates any of the research that has been done with fisheries management. Nor am I advocating putting every species we can think of in our lakes.
Seldom does anyone manage their lake to produce the largest pie they can. Rather, they manage to produce a certain crop or experience. Our 4-acre lake is designed for use by kids, handicapped and elderly people in groups. Therefore, I manage it to produce the largest number of good-sized fish with the greatest diversity possible. That means no flathead catfish or northern pike because they would eat everything in my lake and I would end up with a smaller diversity. The point is that I am not just putting different species in this lake to have more species. Rather, I evaluate all of these species to see how they might work together to meet my management goals.
Hopefully, this illustrates my understanding of the concept of carrying capacity better.
