So everyone knows that I like to use known principles and then apply the theoretical math in spreadsheet form in order to formulate strategy to achieve preconceived goals. To be sure, reality doesn't always match the plan but it is useful to understand the limitations and so a preconceived plan working at optimum efficiency provides the rosiest possible outcome given a set of reasonably assumed constraints. In as much as this is true, we can arrive at understandings of what will not happen under a given strategy which is even more useful than understanding what is going to happen. I say this is more useful because sometimes it is difficult to understand that management methods/actions cannot produce our preconceptions in the real world.
We all know that predator growth and ultimate weight are determined by the population of predators and the production of forage. It takes so much to maintain the metabolism of a predator (let's assume 5 lbs/annum) and so much to grow a predator (let's assume 10lbs per pound gained for forage consumed above the maintenance requirement). Let's use LMB as the predator and lets us assume, as with a put and take system, we have complete control of the recruitment of predators via stocking 0 year females in the fall. As a sustainable management system, let us assume that the stocking regimen of x number of fish/acre has been followed for a sufficient length of time that the population reflects x number of LMB in each year class with harvest in the y year. So the y year represents the limit of time the predator is allowed to live in our pond. So under the strategy we envision either a natural mortality at the end of expected life (completely catch and release) or at a predetermined time where trophies are harvested and killed by the angler. We can play with this number depending on goals and learn the limitations and requirements of each in the rosiest potential outcomes. If we find we cannot reach goals in the rosiest potential outcome, then we know under what population or forage production conditions a particular method of management will fail ... and why it will fail.
So let's begin with an 8 year cycle where LMB of 8 years are harvested at completion of the 8th year depicted above. The growth per year is determined by the carrying capacity and the population. In the SS it is assumed that the magnitude of growth is the same in each year but you may notice that the percentage gained early in life is greater than later in life. This can be expected because the prey are YOY or BG that have not yet reached their second year. The smaller a fish is, the better its access to prey because new crops must grow past the smaller predators first. In the example, only one predator is recruited (stocked female) annually. To support the scenario, approximately 8000 BG must be produced annually in the appropriate sizes. Here I assume 18% the length of the LMB as the mode of the distribution of sizes consumed. To grow by 100% an LMB must consume 9.2 BG/day across the 180 day growing season ... while to grow by 11% requires the consumption of 4.4 BG/day. It takes a fair number of BG per LMB and it is the number they eat that determines how much they grow. One of the things I noticed is that limiting the number of LMB, limits the number of BG that are required. We can see this particularly by looking at a scenario that allows very little growth where the LMB are consuming just a little more than a maintenance ration.
So in the scenario above there are just too many LMB (64/acre) to support large LMB in a pond that carries 49 lbs/acre of LMB. One of the more interesting things though is what I previously mentioned. The number of BG required to sustain the LMB is dramatically increased. It now takes 5 times as many forage BG to support a pond with many small bass as it does to support the same weight of trophies. Are you surprised by that? Most people think that a stunted LMB pond doesn't produce enough BG. But the reality is that many more BG are produced because the standing weight of BG adults are relatively low as compared to the trophy pond. The BG standing weight is held below the carrying capacity and the BG adults grow large, are in good condition, and produce prodigious quantities of YOY. Were this large number of BG not produced the large number of LMB simply would not be maintained. So we KNOW this must be the case, the production of BG YOY is very large its just that there are so many LMB that few make it past 3".
So the next scenario is a trophy pond that one never keeps a trophy. We will allow the LMB to reach the ripe old age of 11 years at which point they die of natural causes and are no longer supported by the pond or consume prey.
Notice that in this scenario more of the prey consumption goes into maintenance and less goes into growth. So by not harvesting at the end of year 8 we are requiring a much larger proportion of prey to be used in maintenance of the greater number of predators. Allowing the LMB to reach their age limit would not increase their size even though the annual stocking rate is the same as our original example. It would take a pond with a higher carrying capacity to grow LMB as large as the original scenario if they are allowed to die in year 11 (as opposed to year 8).
So I ask you, what happens if we harvest in year 5? Is it possible grow LMB to the same size as the 8 year trophy pond? The answer is yes provided the annualized average growth is attainable. So the example below may require Florida genetics but at a stocking rate of 1.5/acre (3 per 2 acres) the LMB could achieve >10 lbs in the 5th year with annual growth of 1.84 lbs/year. What is interesting is the annual harvest of >10 LMB is 50% greater than the 8 year scenario even with a decrease in required forage consumption. The population is also almost as great with a reduction in the population of only 1 LMB per 2 acres. The needed number of BG forage individuals is practically the same. So why is the 5 year system better than the 8? The primary reason is that it is more efficient. There are three maintenance years absent relative to the 8 year system and so a higher proportion of an equivalent amount of prey can be used for growth instead of maintenance. It is very difficult to convince one that trophies can be harvested. I guess where a pond is doomed due to over recruitment this may be the case. No new trophies are going to produced anyway. But if recruitment can be controlled as here modeled ... new trophies will be grown annually. It is worth noting that the 5 year example REQUIRES harvest of 5 year LMB. This is because the annually stocking rate is 50% greater and so if they are not harvest after completion of year 5 ... they can not achieve as great an ultimate weight as presented in the scenario.