... is that because fish turn it into flesh more efficiently?
Frank, the assimilation of feed is just a formula. When we are talking flesh ... then we are talking protein assimilation and so its a simple matter of calculating how much protein the fish gains in dry weight and then dividing it by the protein in the feed consumed. So we can take the tack that we will judge assimulation efficiency by protein assimulation.
The protein assimilated from 1 lb of feed can be calculated as below
P_a =1/FCR * (DW/WW)*DW%P
where FCR is the unit weight of dry feed consumed per wet weigh gain
and (DW/WW) is dry weight proportion of the fish species
and DW%P is the percentage of a fish's dry weight that is protein.
For TP in cage culture an FCR of 1.75 is considered good conversion and this is achievable with 28% protein feed. So we can calculate the Protein weight assimilated at this conversion as follows.
P_a= 1/1.75 * (.235)*.72 = .097 lbs protein per 1 lb feed consumed
If we divide this number by the weight of protein in 1 lb of 28% feed we get the proportion assimilated.
A= P_a/P_f = .097/.28 = .346 OR 34.6% assimilation of feed protein.
This is pretty close to the limit of protein assimilation efficiency where usually a smaller proportion is actually achieved from the consumption of feed. This formula is just one of others that can be used to root out false claims and to test the honesty and integrity of "so-called scientific findings".
One of the key components of scientific integrity is the "conflict of interest" disclosure. When there are financial interests involved reviewers and readers want to know. Generally, when this happens, the authors also address steps and measures they took reduce conflict of interest influence in their experiment and/or data collection etc.
The example above reflects a goal of conversion in a natural foods limited culture. In cages, most of the gain will be acquired by feed alone. But in a pond setting where TP are free to roam and where the density permits, FCR can be much better. So 65% of the nutrition passes through fish into the pond environment stimulating the food chain which produces additional food. So it is possible to get the FCR below 1 under such conditions where secondary effects may also contribute to gain.
Though the initial equation above applies to all fish ... not all fish are equal. For example (DW/WW) isn't the same for all fish. Some are much more dry matter dense than others. Like the Koi that the Kansas DOW use to feed their LMB the (DW/WW) exceeds 0.30. There is less percentage of water in them. For LMB and BG the (DW/WW) is ~ 0.195.
Now at first glance we might suspect it would be easier for LMB and BG to gain than TP or Koi but we have found that this isn't the case. Their ability to assimilate feed is not nearly as efficient. Their nutritional requirements are also more specific. So to achieve similar FCR the protein content of feed must be increased and protein profile needs to be more similar to fish protein than it is required for TP. Fishmeal is a very good source of the protein that predator fish need. So the source of protein can influence efficiency. Let's take a look at BG particularly at 1.5 FCR with a 42% protein feed.
P_a= 1/1.5 * (.195)*.72 = .0936 lbs protein per 1 lb feed consumed
Where assimilation efficiency is:
A= P_a/P_f = .0936/.42 = .223 OR 22.8% assimilation of feed protein.
So the situation is that not all fish are equal and not all feeds are equal.
An interesting take from this analysis is that one can test the proposition that feeding one feed over another reduces nutrient loading. For a 42% protein feed with an FCR of 1.5. 77.2% of the protein passes through and since nearly all of the Nitrogen in feed is in the protein we can make an estimate of the weight of N from feed that passes through the fish for each pound of feed eaten. It can be calculated according to the following equation:
N =( .42 * .772)/6.25 = .052 lbs Nitrogen per lb feed
Now lets assume BG are fed 28% feed intended for catfish or TP and they don't gain anything from it. All 28% of the weight of the protein goes into the pond. Let's calculate the Nitrogen loading from 1 lb of feed.
N= .28/6.25 = .0448 lbs Nitrogen per lb feed
So from this it is clear, for a fish with a dry weight percentage of 19.5% fed a feed with 42% protein and an FCR of 1.5 in cage culture, the only way to reduce nutrient loading relative to a 28% feed that produces no fish gain ... is to reduce the weight of the 42% feed to less than the 28% feed. There is NO WAY around this.
So clearly, no 28% feed produces no gain in BG, unless the BG aren't eating it (which could be a problem for caged BG). But this doesn't take into account the organic fertilization of the pond's food web by the 28% feed. Just like in the case of Tilapia FCRs that can be less than one in pond culture due to food chain effects, all the waste that isn't assimilated by the fish eating 28% feed is introduced as nutrients to the food web. So this will decrease FCR (an increase conversion efficiency) to levels below that of direct conversion (as observed in cages). An analysis of cost/fish produced in a pond system will reflect the secondary effects. For the organisms at the bottom of the food chain, the difference between the wastes of 42% feed and 28% feed is the proportions of Carbon, Nitrogen, and Phosphorus. 28% feed has a higher CN ratio than 42% feed for obvious reasons.
There are many nuances one should consider when reviewing pseudo-scientific tests between feeds. One of first is the size of feed and the size of fish. In aquaculture, one sizes feed to sizes appropriate for the size of fish and so the sizes of pellet increase with fish size. So for example, Sportman's Choice, which is big pellet much larger than the mouths of 2" fish, it is clear to me that its treatment was disadvantaged by pellet size. The pellets would have to soften in order for the 2" BG to pick them apart into bite sized chucks. If no provision was made to keep the pellets in the cage, they may have drifted from the cage by wave and wind actions before the 2" BG could have consumed them. They certainly look to have been starved in the photo. The BG Jr being of the appropriate size for juvenile BG provided advantage to the Optimal over the competing feeds allowing the 2" BG of the Optimal treatment to get off to a much better start. A better comparison would have been a larger Optimal pellet, one consistent with the sizes of the feeds under evaluation. This is how a scientist would compare different formulations in order to gain a true metric of direct conversion for each. A scientist would ensure that all the feed were consumed also ... something which I doubt happened.