The nutrition of fishes isn't that alien.
After all, fishes are our own conservative
stay-at-home cousins. Their main digestive
enzyme, like ours, is pepsin. Like all vertebrates
they can't manufacture all the amino acids
they require. They have to collect them here
and there in their diet. Yet all the amino
acids must be present for protein synthesis;
lack of any one becomes a "limiting
factor" that puts a cap on the usefulness
of all the others. This is the fundamental
reason for varying your fishes' diet.
Growing fry require more protein than adult
fish. And small fish require more protein
than large fish. In natural conditions, fish
diet is largely protein and lipids (fats
and oils). Though fishes require lipids,
there is little carbohydrate in a natural
fish diet. Any cereals and cereal by-products
in flake feeds are essentially fillers--
the cheapest ingredients in processed feeds.
Comparisons of the actual caloric energy
that fish realize from metabolism to the
theoretical possible maximum suggests that
the efficiency of fish metabolism lies slightly
under 50%. The rest is lost, mostly as muscle
heat dissipated in the water. Factors that
influence the nutrient requirements of fish
include low energy requirements, which are
strongly affected by temperature.
Adult carnivorous fishes require 40 to 55%
dietary protein, while omnivores need only
35 to 45%. As fishes mature, they utilize
less of their available dietary protein.
And at all stages of their lives, high-intensity
feeding leads to less efficient protein conversion.
In other words, though excess protein goes
in, it can't be assimilated and passes right
through. Two-thirds of the phosphate found
in plants and half the animal phosphate passes
right through a fishes' system.
Ordinarily fish get all the calcium they
need in their carnivorous diets, but when
they were experimentally deprived of calcium,
channel catfish proved efficient at absorbing
it across their gills, even from water that
was as low in calcium as 5ppm.
Vitamins are absorbed from the fishes' intestine.
Water-soluble vitamins are constantly excreted,
but fat-soluble vitamins may be stored, and
excesses can cause trouble.
Fishkeepers often feed
some lightly boiled
liver or beef heart to
larger, carnivorous
fish. Beef heart provides
concentrated protein
to maximize the growth
of fry, so Discus
breeders use it. But for
the average adult
fish in captivity, there's
too much protein
in heart and liver to make
it a steady diet.
Healthy mammal liver tissue and heart muscle
don't have fat in them. Mammal and bird fats
tend to be unsaturated fats. In the warm-blooded
creature they remain soft, but at cool fish
temperatures they harden and go waxy. So,
though you may feed some beef heart or liver,
muscle meat from any bird or mammal is not
recommended as fish food. The essential unsaturated
fatty acids that must be supplied in the
fishes' diet, are not the same series that
warm-blooded creatures require. "Wild
Discovery" photographers might throw
the carcass of a capybara to the piranhas,
but the piranhas' more usual prey are other
fish.
Fish aren't geared to deal
with much fat
and lipids anyway. Surplus
fats that can't
be burned get stored in
the fishes' liver.
Eventually the liver tends
to enlarge, finally
badly enough to make the
fish look bloated.
Liver disfunction results
in retained water,
a condition we still might
be calling "dropsy"
or attributing to constipation.
When you read about fish nutrition in the
documents linked to below, keep in mind a
basic difference in focus. Fish nutritionists
are essentially writing for aquaculturists.
They are trying to maximize rapid weight
gain for food fish. Though nutritionists
will recommend carbohydrates as a cheap source
of energy, they won't be recommending those
carbohydrates that aren't easily digestible
by fish, which include cellulose and hemicellulose,
gums and pectins-- all forms of dietary fiber.
Your objectives are somewhat different: you
are feeding for fish that remain lean and
active into a ripe old age.
Links.
"The Introduction to Fish Nutrition" by C.D. Webster and C. E. Lim, part of the
"Nutrient requirements and Feeding for
Finfish in Aquaculture," is a little
technical, but it discusses proteins, carbohydrates
and lipids in fish nutrition and all the
vitamins and trace minerals required. It
takes the place of what was the best single
web resource on tropical fish nutrition:
Shim Kim Fah's document "Nutrient requirements
of tropical aquarium fish" now withdrawn
from the National University of Singapore
website.
The FAO document "Fish feed technology" is a U.N. handbook covering every aspect
of nutrition and digestion in teleost fishes,
slanted towards aquaculture of food fishes.
Compare it with Webster and Lim to get an
idea of general agreements about nutrition.
D.P. Bureau and C.Y. Cho, "Introduction to nutrition and feeding
of fish," U. of Guelph, Ontario, Canada is also slanted
to fishfarmers' concerns. Based on aquaculturing
salmonids (trout etc), it starts with general
fish nutrition and gets increasingly technical.
Read the opening sections.
