NUTRITION AND FEEDINGSTUFF

(Nutrition and Feedstuffs) Nutrients essential to fish are the
same as those required by most
other animals. These include water,
proteins (amino acids), lipids (fats,
oils, fatty acids), carbohydrates
(sugars, starch), vitamins and minerals. In addition, pigments
(carotenoids) are commonly added
to the diet of salmonid and
ornamental “aquarium” fishes
to enhance their flesh and skin
coloration, respectively. The general proportions of various
nutrients included in a standard
fish diet are given in Table 1. One of the best descriptions of the
essential nutrients for fish and the
nutrient content of various
ingredients is Nutrient
Requirements of Fish, a publication
by the National Research Council available free on the Internet at . Table 1. General amounts of nutrients incorporated into diets for growing fish. In their natural environment fish
have developed a wide variety of
feeding specializations (behavioral,
morphological, and physiological)
to acquire essential nutrients and
utilize varied food sources. Based on their primary diet fish are
classified as carnivorous
(consuming largely animal
material), herbivorous (consuming
primarily plant and algae), or
omnivorous (having a diet based on both plant and animal
materials). However, regardless of
their feeding classification, in
captivity fish can be taught to
readily accept various prepared
foods which contain the necessary nutrients. Increased understanding of the
nutritional requirements for
various fish species and
technological advances in feed
manufacturing, have allowed the
development and use of manufactured or artificial diets
(formulated feeds) to supplement
or to replace natural feeds in the
aquaculture industry. An abundant
supply of feedstuffs are available,
and farmers and hobbyists are now able to prepare their own fish
feeds from locally available
ingredients. Proteins and Amino Acids. Fish
meal, soybean meal, fish
hydrosylate, skim milk powder,
legumes, and wheat gluten are
excellent sources of protein.
Additionally, the building blocks of proteins (free amino acids) such as
lysine and methionine are
commercially available to
supplement the diet. Utilizing raw fish as a main
ingredient in fish feeds has long
been recognized to be harmful to
the health and growth of fish due
primarily to the presence of the
anti-nutrient, thiaminase. Thiaminase, an enzyme that
destroys thiamine (vitamin B-1),
one of the essential water-soluble
vitamins, is mostly found in
fresh water fish and is destroyed by
heat (i.e., cooking). Other concerns related to using raw fish in diets
include the spread of infectious
diseases such as mycobacterium
and botulism. In preparing diets,
preferential use of marine fish is
suggested to minimize thiaminase activity, and raw fish could be
steamed or poached. Lipids. Oils from marine fish, such
as menhaden, and vegetable oils
from canola, sunflower and linseed,
are common sources of lipids in
fish feeds. Carbohydrates. Cooked
carbohydrates, from flours of corn,
wheat or other “breakfast”
cereals, are relatively inexpensive
sources of energy that may spare
protein (which is more expensive) from being used as an energy
source. Vitamins and Minerals. The variety
and amount of vitamins and
minerals are so complex that they
are usually prepared synthetically
and are available commercially as
a balanced and pre-measured mixture known as a vitamin or
mineral premix. This premix is
added to the diet in generous
amounts to ensure that adequate
levels of vitamins and minerals are
supplied to meet dietary requirements. Pigments. A variety of natural and
synthetic pigments or carotenoids
are available to enhance coloration
in the flesh of salmonid fish and the
skin of freshwater and marine
ornamental fish. The pigments most frequently used supply the
colors red and yellow. The
synthetically produced pigment,
as taxanthin (obtained from
companies such as Cyanotech and
F. Hoffmann-La Roche Ltd.), is the most commonly used additive
(100-400 mg/kg). Cyanobacteria
(blue-green algae such as
Spirulina), dried shrimp meal,
shrimp and palm oils, and extracts
from marigold, red peppers and Phaffia yeast are excellent natural
sources of pigments. Binding Agents. Another important
ingredient in fish diets is a binding
agent to provide stability to the
pellet and reduce leaching of
nutrients into the water. Beef heart
has traditionally been used both as a source of protein and as an
effective binder in farm-made
feeds. Carbohydrates (starch,
cellulose, pectin) and various other
polysaccharides, such as extracts or
derivatives from animals (gelatin), plants (gum arabic, locust bean),
and seaweeds (agar, carageenin,
and other alginates) are also
popular binding agents. Preservatives. Preservatives, such
as antimicrobials and antioxidants,
are often added to extend the
shelf-life of fish diets and reduce
the rancidity of the fats. Vitamin E
is an effective, but expensive, antioxidant that can be used in
laboratory prepared formulations.
Commonly available commercial
antioxidants are butylated
hydroxyanisole (BHA), or butylated
hydroxytoluene (BHT), and ethoxyquin. BHA and BHT are
added at 0.005% of dry weight of
the diet or no more than 0.02% of
the fat content in the diet, while
ethoxyquin is added at 150 mg/kg
of the diet. Sodium and potassium salts of propionic, benzoic or sorbic
acids, are commonly available
antimicrobials added at less than
0.1% in the manufacture of fish
feeds. Attractants. Other common
additives incorporated into fish
feeds are chemoattractants and
flavorings, such as fish
hydrosylates and condensed fish
solubles (typically added at 5% of the diet). The amino acids glycine
and alanine, and the chemical
betaine are also known to
stimulate strong feeding behavior
in fish. Basically, attract ants
enhance feed palatability and its intake. Other Feedstuffs. Fiber and ash
(minerals) are a group of mixed
materials found in most feedstuffs.
In experimental diets, fiber is used
as a filler, and ash as a source of
calcium and phosphorus. In practical diets, both should be no
higher than 8-12% of the
formulation. A high fiber and ash
content reduces the digestibility of
other ingredients in the diet
resulting in poor growth of the fish. Other common feedstuffs used in
ornamental fish diets include live,
frozen or dried algae, brine
shrimp, rotifers or other
zooplankton. The addition of fish
or squid meal will enhance the nutritional value of the diet and
increase its acceptance by the fish.
Fresh leafy or cooked green
vegetables are often used.
Although vegetables are composed
mainly of water, they contain some ash, carbohydrates and certain
vitamins. Kale, dandelion greens,
parsley and turnip greens are
examples of relatively nutritious
vegetables.
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