being a cash crop farmer myself and relying on corn, soybeans and wheat as my only income,,,, I also believe that they have no place in
fish food of any kinda quality. hard to get around it though. its cost effective for the manufactures.
Personally I wouldn't go so far as to say they have no place...... I have no issue with some grain content, if it is used in a limited quantity, and only if its use is as a binding agent. It's when manufacturers use excessive amounts that I take issue. If you look at the "algae" discs that Cardeater posted, it is a classic example of ingredient splitting. I have used that exact formula many times over the years to illustrate how ingredient splitting works.
Spirulina algae meal,
corn gluten feed,
corn gluten meal,
corn distillers dried grains,
wheat flour,
wheat germ,
wheat gluten,
wheat middlings,
linseed meal,
canola meal, dehulled
soybean meal,
soy protein concentrate,
pea protein, brewers dried yeast,
corn flour,
rice flour, dehydrated
alfalfa meal,
soy protein isolate, ground
barley, calcium carbonate,
soybean oil,
How in the hell anyone had the stones to market that formula as an "algae" product, is beyond me - yet clearly millions of consumers bought into their marketing, and millions apparently still do. As cardeater pointed out, if one was to add up the corn & wheat in that formula, it would make up the bulk of the food. Not to mention all of the other fillers added - and they even went so far as to use the oil from the soybean as their main source of lipid in this food. Disgusting!
But through years of science we understand that many/most fish can assimilate and utilize some carbs in their diet. The following is from a post I made many years ago. Not sure if the links work, some of the following is my wording, other sections taken from various credible sources with cited papers/studies. If ones views our captive fish and carbs, across the board, I think that a 10-15% inclusion rate would be reasonable. Even if some of the science points in another direction, their goal is obviously quite different than a hobbyists.
http://www.reeis.usda.gov/web/crisprojectpages/196810.html
At the same time, we also raise new questions about the upper limit of feed levels of carbohydrate in this species. Depending on the source and quality of dietary carbohydrate, the aquaculture industry standard of 20% carbohydrate represents a "conservative" value. We documented outstanding growth performance of trout receiving 24% (mostly wheat flour) or even 30% (mostly purified starch) fed aquaculture rations or to satiety, respectively.
From the NRC's Nutrient Requirements of Fish, which is pretty much the bible for every commercial fish food manufacturer. Not the be all to end all, but a great reference source to work off of for tropical species.
The nutritional value of carbohydrates varies among fish. Warm-water fish can use much greater amounts of dietary carbohydrate than cold-water and marine fish. No dietary requirement for carbohydrates has been demonstrated in fish; however, if carbohydrates are not provided in the diet, other compounds, such as protein and lipids, are catabolized for energy and for the synthesis of various biologically important compounds usually derived from carbohydrates. Thus, it is important to provide the appropriate concentration of carbohydrate in the diet of the fish species being cultured.
The problem with carbs is that unless they are used immediately as an energy source, they get stored as glycogen, and eventually converted to sugar, and then fat. This is exactly why one can find some VERY large obese specimens that have been fed generous amounts of lower quality high starch food. No different than a human that eats a large amount of white starch. (bread) You will still grow, and get big, but eventually become obese.
These types of foods do not make for a healthy fish, especially those species that are more on the carnivorous side of the equation. They are made with commercial fish farms in mind, where the fish are not expected to live long term.
The ability of carnivorous fish species to hydrolyze or digest complex carbohydrates is limited due to the weak amylotic activity in their digestive tract (Spannhof and Plantikow, 1983). Thus, for fish species such as trout, as the proportion of dietary starch is increased, starch digestibility decreases accordingly (Singh and Nose, 1967; Bergot and Breque, 1983). Furthermore, in long term feeding trials with carnivorous fish species (ie. salmonids) it has been shown that high dietary carbohydrate levels depress growth, elevate liver glycogen levels, and cause eventual mortality (Phillips
et. al., 1948; Austreng
et al., 1977). By contrast, warmwater omnivorous or herbivorous fish speceis such as carp (
C. carpio), channel catfish (
I. punctatus), tilapia (
O. niloticus), and eel (
A. japonica) have been found to be more tolerant of high dietary carbohydrate levels; the dietary carbohydrate being effectively utilized as a dietary energy source or excess stored in the form of body lipid (Chiou and Ogino, 1975; Robinson and Wilson, 1985; Anderson
et al., 1984; Degani, Viola and Levanon, 1986).
The utilization of dietary carbohydrate has also been found to vary with the complexity or chemical structure of the carbohydrate source used (digestible polysaccharides and disaccharides having a more beneficial effect on growth than monosaccharides: fish - Pieper and Pfeffer, 1980; Robinson and Wilson, 1985; Anderson
et al., 1984; shrimp - Alava and Pascual, 1984; Deshimaru, 1981;
1 Kanazawa, 1983), the physical state of the carbohydrate source used (cooked or gelatinized starches having a higher digestibility and beneficial effect on growth than native or raw starches: fish - Spannhof and Plantikow, 1983; Bergot and Breque, 1983; Robinson and Lovell, 1984), and daily feed intake (a restricted feeding regime having a beneficial effect on starch digestibility: fish - Bergot and Breque, 1983). From the above discussion it appears that the ability of fish or shrimp to adapt to high carbohydrate diets depends on their ability to convert excess energy (ie. glucose) into lipids or non-essential amino acids.
1Furuichi, Taira and Yone (1986): availability of glucose in yellowtail (
S. quinqueradiata) is lower than that of alpha starch.
Since most farmed fish species have a relatively short gastro-intestinal tract that does not lend its self to the development of an extensive bacterial flora (as in a ruminant animal), the intestinal cellulase activity of fish from resident bacteria is weak or absent (Stickney and Shumway, 1974). It follows therefore that dietary cellulose or ‘crude fibre’ (ie. dietary carbohydrates which are resistant to chemical treatment with dilute acid or alkali, including cellulose and hemicellulose) has no utilizable energy value to fish, and in dietary excess has a deleterious effect on growth and feed efficiency (Anderson
et al., 1984; Poston, 1986; Hilton, Atkinson and Slinger, 1983; Bromley and Adkins, 1984).
