According to tamu, phosphorous comprises up to 8.5% of the weight of aquaculture feeds. I presume the highest quality feeds contain also the highest quantity of phosphorous due to the use of higher proportions of fish meal to source the protein content of the feed. It's not entirely clear if we could limit feeds like Optimal and Purina AM to this number. These are essentially pet feeds that are too expensive for fish producers to give any credible consideration to. Not dissing these feeds, its just they don't compete in any meaningful way for the production of food fish. They are pet feeds where the purpose isn't optimizing profits. Their higher prices justify the use of more fish meal (and phosphorous) and the consumer should expect its there (otherwise why pay extra for it?). In any event, it would be a nonsensical argument to claim than the recreational pet food is lower in phosphorous than say relevant feeds used in the production of food fish which are much cheaper and presumably of lower quality. Soooo ... I will use 8.5 % of dry weight to estimate the phosphorous added when feeding a high quality feed like Purina AM or Optimal.
So a common fertilization technique where free phosphorous is limiting the productivity is to add 1/2 to 1 gallon per acre of 37% phosphorous liquid fertilizer. This is sufficient to cause blooms of <20" secchi depth. Water that maintains such blooms is defined as hypereutrophic and of poor quality. Now such conditions are normal and even required to maximize production of food fish and this is also what many recreational fisheries are trying to maintain. But make no mistake ... the risk of fish kill is elevated under such strategies and efforts and expense should be undertaken to mediate the risk. Aeration for example. But in a pond with high water quality and low fertility, the effect of such a treatment appears to be temporary. The phosphorous is sequestered in living things and is only recycled when these creatures die. So the living creatures store it away in a greater food limited carrying capacity. A balance is reached by annual mortality which recycles phosphorous. The phosphorous freed by mortality stimulates production of the autotrophs which are sufficient food to fuel the replacement of that annual mortality .. something we call production. Its a beautiful cycle really ... a cycle of renewal.
As it pertains to us, feed inputs load nutrients into our ponds and this expands the carrying capacity (and yes even the annual mortality). It seems a rather widespread notion that feeding is necessary and can be carried on indefinitely. With the exception of myself and one other member, I have read absolutely nothing that encourages the curtailment of feed use when sustained eutrophic conditions are achieved. I would like to say I do not understand why that is but I the fact is ... anyone with any common sense can figure it out. As my grandpa use to say, "Just follow the money, there is your answer"
Pond Star, we can calculate the phosphorous addition to your pond from feed simply by dividing the dry weight of feed by the proportion of phosphorous in the feed. We can also relate it to a fertilization treatment. For example, if we fertilize with 3/4 gallon of 37% P fertilizer per acre ... we are adding ~2.25 lbs of phosphorous. An equivalent quantity is introduced with ~26.5 lbs of feed. Your 70x60 pond is 9.64% of an acre and so you would provide a treatment of phosphorous fertilizer for every 2.5 lbs of feed that your introduce into your pond. At this point it is unclear what quantity of feed your are feeding but to mimic monthly fertilization of 3/4 gallons of 37% phosphorous fertilizer you only need to add 1.33 ounces of feed per day or 4 ounces every 3 days. (this ignores the addition of other nutrients like protein, lipids, and carbs).
Pond Star, your situation could be exacerbated if your pond doesn't have much flow through. If it is an evaporation basin ... it retains nutrients except for those you move to your other ponds or what the cormorants might take. Even without feed, the river water could be very fertile and you wouldn't have to feed to have nutrients to accumulate to ridiculously high levels. So every pond is different in how it deals with nutrients and some can take more additions than others due to flushing of diluted water. But I think your pond could be more sensitive than most if it rarely runs over. I just wanted to give you an idea of how little feed it takes to make a pond hyper-eutrophic.
Feed is one of the cheapest ways you can grow fish if the density of fish is higher than can occur under natural fertility. Its highly concentrated nutrition. But its costs are not limited to feed. Aeration, herbicides, nutrient flocking, pond dyes, etc. are all costs of feeding and/or fertilization. Add all these costs to the cost of feed and/or fertilizer and then divide that by the weight of feed and this is the true cost of feed/fertilizer. The optimum efficiency occurs where the fertility of the water does not need these additional interventions and a person will never grow fish more cost effectively than when his pond supports all its fish with just its native nutrients and the Sun. If one will focus on working keeping his water's fertility limited to less than hyper-eutrophic conditions, there will be other benefits. Better water quality leads to longer life and faster growth. The key to working with water of any fertility is maintaining appropriate populations of fish. This is only made more difficult when pushing water into hyper-eutrophic conditions.
