I came across this
little gem of a 55 year old paper some time ago but only recently have had the time to do a thorough review. I am posting in part to answer esshup's question with regard to assimilation of foods eaten. In the case of this paper, the foods are live food where the predator is LMB and the prey is GAM. All the same, the principles of bioenergetics are laid out very well and I have learned a lot from this study. Below is a graph of the bioenergetic use of consumed energy. They converted fish mass into energy content and so presumably we could substitute prey of varying energy density to arrive at estimates of conversion for these prey as well.
I guess we can't upload images any more through the document manager? (OK so I was able to share a link to the image at Google Drive but the image address doesn't work with the UBB image tags. If anyone has learned how to embed Google Drive images with UBB forums, please share how and thank you in advance!)
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https://drive.google.com/file/d/1tK33-qBfrUcRsgJgxNS6KcMwtE9zi5yr/view?usp=share_link[/img]
In their analysis, assimilation is the amount of energy LMB can extract from consumption for both growth and metabolism (respiration). In earlier posts, I spoke of assimilation as the growth piece treating assimilation as the nutrients retained (not expelled as waste and/or used for metabolism). Since the authors are speaking of assimilation in the accepted definition of bioenergetics, I will correct my earlier mis-definition. Another way of describing assimilation is the digestibility of food. So in the case of GAMs, LMB are able to digest between 73 to 80% of the energy they consume. The assimilation/digestibility depends on the rate of consumption. This may deal with retention time in the gut. The assimilation efficiency is limited to 80% but will decline the more an LMB consumes. The curve of respiration is the metabolic use of consumed energy. If the respiration is subtracted from the assimilation what remains is the growth of LMB.
I digitized the curves of assimilation and respiration so they could be tabulated in excel. From this information I was able to calculate wet weight and dry weight conversion of GAM consumption to LMB wet weight growth. There were some interesting numbers that arose from the analysis. The curves demonstrate that there are two things working against gain conversion. The first is respiration. The metabolic requirement must be met before any gain can be made. The metabolic requirement is not large but as consumption increases, metabolic demand also increases. Of interest, it is a geometric relationship that diminishes growth conversion efficiency at higher consumption rates. The second is assimilation. Assimilation also follows a geometric relationship at higher consumption but the relationship is inverse (IOWs assimilation as a proportion of consumption declines as consumption increases). In combination, growth conversion efficiency is most efficient at a wet weight consumption of 6% of body weight per day of GAMs. This translates to dry weight consumption of 1.6% of body weight per day. At higher wet consumption rates, conversion efficiency worsens. Now although efficiency declines above 6% wet weight the conversion is still excellent. At peak conversion efficiency, LMB add 74% of the wet weight GAMs consumed to their own wet weight. It speaks to the high nutritional properties of GAMs which are substantially more energy dense than the LMB are on a wet weight basis. Here are few take-aways.
1. LMB convert fish extremely well.
2. The wet weight consumption for maintenance is around 1% of body weight per day. Adjusted for the energy density of BG, LMB require 1.36% of their body each day to meet the respiration/metabolism requirement. Multiplied by 365 days this translates to 3.65 lbs/year of GAM or 4.97 lbs/year of BG.
3. The wet weight FCR for GAMs at 6% wet weight consumption is 1.35. This is better than the conversion of many feeds which are fed dry. If the we calculate FCR on a dry weight basis (1.6% of body weight), the FCR is .36. This is just an amazing number that speaks to the digestibility of fish meal. Consider for example a formulated feed with an FCR of 2 at the dry weight consumption of 1.6 % of body weight per day. Such a feed need only contain 18% fish meal. Everything else could be inert providing no additional nutrition. Such a feed would only contain around 12% protein by weight. Most of the protein in typical formulated feeds is definitely not fish based or the conversion would be better. The exception to this would be feed formulated for fry. Now there is a lot more to feed than just fish meal. For example, it has to float, be thrown from a feeder, be affordable, and be economical for production. All of these will diminish how much fish meal goes into a feed and so a 100% fish meal feed is not practical and I would not want this to be construed in this way. Even so, IMHO, the quality of any formulated feed as it relates to LMB and possibly BG may well be a function of the percentage of its weight that is fish meal or other animal by products.
4. GAMs are a high quality forage for fish up to about 1 lb. Very energy dense and comparable to trout and shad. The dry weight percentage of GAM is 26.58%. The dry weight of GAM is divided 56.5% protein, 25.27 % lipid, 18.53% ash. In the wild GAMs average 10% growth by weight every day and have attained growth rates as high as 20% of body weigh per day under laboratory conditions of ad libitum natural foods. The high rate of growth and reproduction explains how small standing weights of GAMs can produce substantial forage contributions. It is not clear what weight of GAMs were produced in Swingle's treatments. But the difference in the gain of BG could be explained by an increase in forage equivalent to 200 lbs of GAM/acre (Additional consumed energy that would be derived from 200 lbs of GAMs). Given that GAMs compete with BG, at least this much GAM forage was produced and this fact is made all the more remarkable in that they were produced from an introduction of only 1 lb/acre. Even a small standing weight of GAMs can produce substantial forage. If a pond were to support a 40 lb/acre of GAM by early July, for example, it would produce a minimum of 4 lbs/acre of GAM per day for the remainder of the reproductive/growing season. This is equivalent to 1.06 dry weight of Gam every day. Relative to a feed that converts at 2, over a 90 day period, this has an equivalent feed value for growth that translates to feeding rate of 5 lbs/acre/day. Over 90 days, this translates to $450 of feed at a feed cost of $1/lb.
5. The maintenance requirement for fish need not come entirely from protein. One of the things that spoke to me in the analysis is that formulated feed does a better job of maintaining LMB and BG than it does in helping them gain (relative to dried fish). After some thought, the only explanation I could deduce is that LMB and BG can assimilate lipids better than they can the proteins in formulated feeds. Protein is essential for growth and the mix of proteins in formulated feed is ... overall ... inferior to fish meal. But lipids, whether fish based or not, must possess proportionally higher digestibility than plant based protein. Bio-modification of plant proteins however may change that. But for now it is clear the quality for maintenance in relation to the quality for growth are two different things in formulated feeds where growth quality is correlated to the content of fish/animal based proteins.
6. After the first year, most of the forage production in recreational ponds goes to maintenance and then trends toward being unable to provide maintenance. Also any day a fish goes without consuming maintenance is a day the fish lost some weight.
