Pond Boss Forums STOCKING A NEW POND Creating the food chain Food Chain in a 100% Clay Pond

Hey FishingRod,

Since this seemed to have interested you, I will share some other findings. The model above suggests that ~4.165 lbs of LMB & BG can be supported for every 1 lb of Nitrogen mobilized. This should be thought of as a maximum number and that other factors could reduce the weight carried. So one of the questions I wanted to answer was how can the full effect of feeding be modeled where feed wastes are utilized as fertilizer for the pond food chain. This paper is study of the production of BG/LMB where the feed formulation (Auburn #2) can be found in in this publication. Essentially the feed is 32% protein catfish feed. In the first reference paper, after feeding for 4 years there was 2307 lbs of fish produced, of which 1129 lbs were standing at draining and 1178 lbs were harvested. So I worked up a simulation to model this realized scenario which is depicted in the image below.



You will notice that by year 4 the mobilization of nutrients for a constant feed rate is relatively stable and only grows marginally thereafter. By year 4 nitrogen mobilization from the feed input is supporting a food chain that can support 423 lbs of fish. But the carrying capacity is 1129 lbs and so feed is supporting 706 lbs of fish on its first pass. So another way of stating the same thing is that food chain effects from the utilization of feed wastes increased the carrying capacity by roughly 60% as a secondary effect. An additional way of stating the same thing is that feeding increases carrying capacity by 166% over the food chain effect. So in a very lean pond (one with essentially no nutrients) it is possible to custom design the food chain nutrients and carry more fish than its mobilized nutrients could otherwise carry. Another way of saying the same thing is that one could design a meso-eutrophic trophic status that supports a carrying capacity that would otherwise require a eutrophic or possible hyper-eutrophic trophic status if it were solely dependent on the food chain. The carrying capacity is maximized by a basin that starts essentially devoid of nutrients allowing one feed more without exceeding the desired nutrient mobilization.

So whether one wants native nutrients or introduced nutrients to support his carrying capacity depends partly on budget and partly on how far one wants to push the trophic status of his water. The combination of native nutrient mobilization and introduced nutrient mobilization should be below this manager imposed limit. Hyper-eutrophy will occur and water quality will suffer when nitrogen mobilization exceeds 144 lbs of N per acre-year. Carrying the greatest weight of fish at the best water quality is facilitated by having a nutrient poor basin, preventing deposition of non feed nutrients, and feeding to fill out the trophic goal. In this sense, water quality drives the usage of feed where carrying capacity is maximized in the most nutrient poor basins which allow greater feeding rates for a particular goaled nutrient mobilization.

The simulation in the image above ties with the earlier simulation on native nutrients and also ties precisely with the results in the paper on feeding LMB-BG ponds with the Auburn #2 feed. I will mention that the simulation is caused to tie with the paper by adjusting the protein sequestration rate in the spreadsheet. Protein sequestration is the amount of protein sequestered within the gain of fish biomass. From this we can calculate the Gross FCR of the Auburn #2 feed. Sequestering 13.44% of the protein of a 32% protein feed produced 706 wet pounds of fish when the fish is 80% water and 66.7% of its dry weight is protein. This yields an FCR of 3.1. Not great ... which is what should be expected of BG when fed a feed formulated for catfish. We can further tie this to energetics of fish meal conversion. I earlier posted a thread on the conversion of GAM to LMB and in the reference paper it was found that LMB can convert 50% of the energy in GAM to its own energy content and that it can digest 80% of the energy in GAM. If we assume that plant proteins are not digestible by BG and LMB and that BG and LMB can similarly convert fish meal (a good assumption in that both LMB and BG have equivalent wet weight energy density) then we can predict the amount of fish meal protein (or equivalent) that would be required in the feed to produce the observed results. So the proportionate weight of protein sequestered is (.32 * .1344 = 4.3 % the weight of feed). At a conversion of 50% the required amount of protein in the fish meal is (4.3% / .50 = 8.6% the weight of the feed). Since the fish meal they used was 60% protein the proportion weight of fish meal required is (8.6% / .6 = 14.33% the weight of feed). As it turns out, the #2 feed is 12% by weight fish meal but it is also 5% by weight blood meal. So animal proteins only exceed the 14.33% required by 2.67%. Its pretty dang close ... don't you think? I suspect that most of difference lies in the lower energy content of the proteins in blood meal and modestly less than the 50% conversion efficiency I earlier assumed.

Now I want to share one other thing. Let's remove the harvest of 35.5% of the standing weight. This is shown in the image below. Notice how it hardly affects the year 4 carrying capacity? IOWs, abstaining from the 403 lb annual harvest would only add 33 lbs of fish. There is a very good reason for this. The harvest reduces the quantity of feed/forage used for maintenance allowing that feed/forage to convert as gain. When there is no harvest, the feed/forage goes to maintenance unless natural mortality occurs. The reference's experiment demonstrated that 35.5% of the carrying capacity can be harvested and the weight regrown each year. Swingle had such great intuition in postulating that fish will grow into maintenance ... something that is now corroborated by the findings of energetics. Abstaining from harvest CANNOT help one grow a significantly greater weight of fish. Abstaining from harvest can at best increase the number of fish (which of course means they must be smaller than they would otherwise be without harvest).