jpsdad, given that there is P in the food. how much of that P in the food is retained in the fish? (to grow the bones, scales, etc.)
That's a great a great question esshup and its one that I have been seeking. Its complicated but I will share some things that I am learning from available research documents.
First, it in part it depends on the fish. Some fish put more investment into the structures that require P. For example, BG and LMB put more investment into scales, fins, and bones than to rainbow trout.
Second, it "might" partly depend on the concentration of P in the feed. One reference stated findings that a reduction from 1.2%P to 0.8%P led to a 33% decrease of P in effluent in a rainbow trout facility. Interesting thing is 0.8 is 1/3 less than 1.2% and so it would seem the amount retained is to some degree dependent on the amount consumed.
We can make a ball park estimate of P retained from FCR. For example, for 1% P feed with FCR of 2 and a fish whose dry weight is 20% and whose dry weight concentration of P is 1.5% the P Consumed and retained is:
Pcon = .01 per lb feed consumed
Pret = .2*.015/FCR = .0015 per lb feed consumed
So the proportion retained is:
PropPret= .0015/.01 = .15 or 15% is retained, 85% discharged in feces
This proportion of retention keeps pretty well with the 10% assimilation rule. In the example above, the proportion of P in feed is less than that of fish and so the calculation is greater than 10% retention. It is probably more accurately somewhere between 10% and 15% retained and 85% to 90% discharged.
Some other interesting facts. Increasing P in feeds creates leaner longer fish and reduces conversion efficiency (increases FCR). Decreasing P in feeds creates fatter (higher lipid content) shorter fish and increases conversion efficiency (lowers FCR). Above we a assume a minimum P is present in the feed below which the metabolic needs for P are not met. This particular circumstance is very interesting to me. A low P feed might produce slower growing ... more energy dense prey for predators than a high P feed. A high P feed might produce fish more capable of reaching trophy potential because it better builds the frames trophy fish have.
There is a lot more to learn here. But some of the answers to questions I have be exploring are coming to light. One of things I have learned, for example, is that harvesting fish cannot meaningfully control P additions unless the P additions are pretty much the same as P in the harvested animals. For example, feeding 100 lbs of feed will add 50 lbs of wet weight fish to your pond but to remove the P of 100 lbs of 1% P feed one must remove more than 300 lbs of wet weight fish (whose dry weight P is 1.5%). So I retract any prior statements that fish harvest is part of adequate control of nutrient accumulation. It hardly makes a difference at all. One of the keys to science is to listen to evidence and to allow evidence to reshape one's thinking. One must be ready to abandon a preconceived notion ... no matter how much sense it seemed when first conceived.
The evidence clearly demonstrates there is no way to manage P introduction with harvest unless other organisms convert the P in feed excrement into food for the harvested fish. In the example above, this would require a whole pond FCR of .333 where 250 lbs of the gain from the necessary harvest came from pond organisms utilizing the waste P. What this tells us is that P addition requirements are very low in a harvested system where Sun and native nutrients are providing the food for growth each year. Most systems naturally accumulate more P than required to replace the P in harvests that are appropriate for balanced populations.