Optimal folks have conducted many feed studies and are very solid with trials producing 1.15 to 1. Could we say that .15 is "leaving the fish"? would this be a safe assumption? I know it's not that simple but 1.15 to 1 leaves little unusable content unless eating-the process of.. uses "X" energy regardless forage/feed present.
Kenny, this is wrong on a couple of levels but first is the one where you have been unable to repeat their results in single pass tests with YP and BG. I'm not saying they didn't get 1.15 to 1 but we do not have context. Perhaps this is with the fry food with fry to fingerling sized fish which have high growth rates and feed low on the feed chain even able to re-consume waste feed and gain. There are several in pond studies which findings indicate that feed begins to benefit the growth of BG once a length of 4" or greater is attained. Growth of YOY year BG was otherwise not statistically different and the same goes for the energy content of YOY (something which suggests they have similar nutrient profiles). Furthermore, in pond FCRs regularly are below 1 when growing fry to fingerlings (eg CC fingerlings on 35% feed). This is not the case when growing juvenile and adult fish. This is primarily because they don't have as high a specific consumption ... something which greatly reduces feed efficiency.
Feed is relatively dry with "as fed" water content somewhere around 10%. BG water content is 7.5 to 8.2 times that of feed. Since the water comes from the pond, it doesn't come from the feed. In order to understand the retention of nutrients one must understand the dry weight conversion (dry/dry). So even at 1 to 1 with the generous assumption that BG are 75% water the FCR is 3.6 to 1. But this doesn't really tells us the nutrients entering the pond. To understand this we need to understand the nutrient profile of the feed and the nutrient profile of the fish. Fish vary but the dry weight of BG is about ~72% protein. Assuming a fry feed at 48% and assuming 10% water out of bag for the feed, the protein assimilation efficiency of feed in the dry weight of the fish would be around 41.66%. Lots of ifs there and I have some ocean front property in Arizona for any willing to buy that this example will happen on a broad scale in his pond. Conversions on the order of 1 (dry/wet) result by secondary utilization of 1st pass nutrients. The good news is that feed waste nutrients will work their way up the food chain and put weight on fish, in some cases (like fry) even waste feed will be consumed and additional gain made. The bad news is that one can't rely on all the rosiest assumptions for his pond. The first pass will be much less efficient in the larger fish eating it and unlike water managed for the production of food fish, there will competition for waste nutrients by macrophytes which do not participate meaningfully to a BG's food chain.
The inefficiencies of conversion applies to everything any animal eats. A general rule of thumb is that each consumer trophic level requires 10 fold from the trophic level below. With feed it is more along the lines of 5 fold (dry/dry). If the fish feed were as nutritious as natural foods, this number would probably be much closer to 2.5 where one is eliminating the need to chase down prey and replacing it with the most lower energy expenditure of competing for feed. So a food made from 100% fish meal is probably around twice as efficient as the feed you are buying. Yet even if you feed 100% fish meal feed more than half of the nutrients would still exit the fish.
