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

FishingRod,

The thread and questions have taken a lot of twists and turns but with regard to the food chain I think the mix of vegetables is of critical importance and the single most limiting factor in terms of the carrying capacity for fish. Varying populations of plants are the main contributor to variability in standing weights for waters of otherwise equal nutrient composition. Supported standing weights of fish are indeed highly variable and can be manipulated by controlling what veggies are growing.

Most (though not all) plants are beneficial when their proportion of the veggie biome is ideal. It's finding the right balance that is difficult part.

A couple of years ago I wanted to understand nutrient recycling (both native nutrients and introduced ... eg feed). What I learned from my study is that 3% organic composition of the pond soil is limiting. This is HYPER eutrophic nutrient loading. Organic composition of some prairie soils exceed this organic content and so any inundated native soil which is shallow enough to get sunlight will produce abundant plant matter. 3% is limiting because organics are mobile in pond soils. They tend to accumulate to this concentration but no higher. Rather what happens is they migrate deeper expanding the region of the 3% organic limit. This has been demonstrated in fish production ponds which are many decades old being fed at rates measured in tons per acre. So for a recreational pond where one wants good water quality ... creating an initial basin with the desired nutrient reserve ... I think ... is a very important consideration.

In the image below, I was attempting to model trophic nutrient cycling where understanding native nutrient reserves could provide a reasonable estimate of supported fish standing weights. The first tier from mobilization onward are average expectation of nutrient mobilization (of the standing organic pool) and the % organics of the soil is a variable of the spread sheet. The maintenance% is assumed where the approximate standing weight of LMB & BG is limited to 600 lbs per acre for a 3% pond soil. Though probably not exact ... it is not that far off when the lion's share of nutrients are being utilized by phytoplankton (as opposed to naiad, coontail, cattails, and such). Now I don't mean that you don't want plants other than phytoplankton ... what I mean is that you want these only in moderation at densities that support fish standing weight (as habitat/cover).



Now one of the things I will mention is that "having no nutrients" can be dealt with by feeding or fertilization. The 40% assumption of dry protein % is reasonable for dried phytoplankton but also for a high quality feed. So imagine the feeding as a mobilization of nutrients. Now two things.

1. Feeding bypasses the food chain and so could feed LMB (or BG) directly but we are also interested in its contribution to the food chain.

2. The second part is maintenance and growth (of the standing weight). If the standing weight is maintained annually with no harvest of fish then all of that is assimilated is recycled (in other words none of the feed nutrients are sequestered in fish). What is assimilated is determined by the digestibility of the feed. If 100% fish meal ... the digestibility will be between 70% and 80%. Digestibility of most feeds are much less than that (it really depends on how much high quality animal protein is in the feed). Most all of what binds the feed or spikes the protein % (think spikers like soybean meal) are not going to be assimilated by BG or LMB. So these kinds of additives are going to be residue that will decay as a declining percent each year. In the first year, probably around 50% of this residue nitrogen/protein can be recycled as mobilized nitrogen. This declines to around13% annually by the 5th year. After about 5 years about 22% of the original waste remains and is decaying more slowly with each year.

Now this is important. Of the amount assimilated ... all that is used as maintenance will be recycled as ammonia and this is readily available to plants. Also natural morts are volatile in terms of their nutrients and quickly recycled. So it depends on the quality of the feed just how much of the feed will be recycled to grow plants when the feed is supporting the maintenance of fish (higher quality feeds have higher potential for mobility because they have higher digestibility). When a fish grows, the nutrients are sequestered until those periods where the nutrients are mobilized by the fish for metabolism (think periods where prey consumption is insufficient to cover metabolism). When the fish mobilizes nutrients in its flesh for metabolism then the nutrients are mobilized for plants in the pond as waste products ... also all natural morts return nutrients to the pond for plant use. For nutrients sequestered in the growth of fish, the only way to prevent their eventual return for use by plants is to harvest fish. This increases the amount feed going into growth and reduces the amount of feed going into maintenance. Harvest also improves the production of fish flesh and the growth rates of remaining fish.

With fish meal, it takes almost exactly the same amount to grow a weight of fish as it does to maintain the same weight of fish for a year. When feed only maintains that as a limiting standing weight there is no conversion and no gain in the standing weight. All of the digested nutrients return to water via maintenance and natural mortality. The greater the percentage of standing weight harvested (at end of years growth) ... the better the conversion. Weight loss of the standing weight for maintenance and natural mortality will reduce the conversion and remobilize the nutrients.

Modest rates of feeding don't contribute a whole lot to mobilized nutrients. IOWs, the level of mobilized nutrients will be more driven by other sources of nutrients. A ponds mobilized nutrients primarily come from pond soils, leaching of nutrients from the watershed, and deposition of organics (think topsoil erosion, cattle and birds, and atmospheric deposition ... in that order). 3 to 10 pounds/acre of nitrogen are deposited by rain and snow every year. Feeding a 40% protein feed at the rate of 100 lbs/year will contribute ~ 6.5 lbs of Nitrogen and if fully mobilized for use by phytoplankton this would only support 26 lbs of BG and LMB. So you can imagine where 100 lbs of feed will maintain a 100 lb standing weight of fish (without a food chain) that feeding 100 lbs per acre year would support less than 126 lbs of BG & LMB(with recycled nutrients supporting a food chain). So how much one should feed really depends on non-feed mobilization of nutrients. In a normal year, the deposition of nitrogen from the atmosphere could be comparable to 100 lbs per acre of a 40% feed. All ponds accumulate nutrients and so have the potential to grow carrying capacity over time. Eventually, they have too much. There are a couple of ways to approach this to maximize the life of the pond. You could design the nutrient reservoir to support the standing weight desired (for me a mesotropic trophic level) and allow natural accumulation to eutrophy the pond OR you could start with less than the nutrients needed and supplement nutrients until the pond supports that standing weight (for me a mesotropic trophic level) on its own. At this stage controlling the mix of plants to maximize fish carrying capacity.

To reduce the accumulation of nutrients I think the following is important.

1. A good buffer zone around a pond denying direct access by cattle.

2. A sediment pond if possible to catch eroded organic laden soils before they enter the pond. Maintain it also on a reasonable schedule ensuring that the deposits cannot leach back into the pond (IOWS. take the deposits out of the watershed)

P.S.

There was a paper I read regarding standing weights supported at differing trophic levels (in Florida lakes). A number of variables were analyzed (eg secchi depth, nitrogen, phosphorous) and the results definitely showed correlation of fish standing weights with increased nutrient levels ... BUT ... there was a whole lot of variability. For example, for a secchi of 5 ft the standing weight supported could be as low as 32 lbs per acre or as high as 253 lbs/acre (averaging 80 lbs/acre). One of the things I was interested in understanding is what gives some waters higher standing weights over others. One is the mix of fish where trash fish significantly increase standing weight. The other trend I noted was the average depth relative to secchi depth. There were definitely higher standing weights where lake average depths exceeded the secchi depth considerably. Where the average depth was less than secchi depth, these were consistently the least productive water. Having depth in a pond gives greater space, more potential for oxygen storage, and inhibits macrophytes over a large portion of the water body. I think this is important for designing BOWs to have higher natural carrying capacity.