|
Act early to ensure a good offspring
The nutrition of broodfish has major affects on it’s
sexual maturation and reproductive success as well as on the
subsequent development of offspring. Lourens de Wet explains
why broodfish feeds should aim to achieve the following objectives:
- Producing large parent fish:
Large fish tend to produce more milt and larger eggs, and
hence improved rates of fertilization, whereas larger offspring
also show improved survivability. Any reduction in food
supply during the final stages of egg development can lead
to re-absorption and thus reduced fecundity (eggs/female).
- Supplying balanced nutrients:
Sexual maturation puts high demand on body reserves - the
size of this body store is dependent on how the fish has
been fed prior to this. The nutrient provision for the eggs
must therefore be sufficient in order to obtain viable hatched
fish capable of realizing its genetic potential. An oversupply
of energy increases fat accumulation in the gut with subsequent
reduced ova quality. The amount of energy provided in the
daily diet must therefore not be excessive in order to avoid
obesity and its consequences. Great controversy exists regarding
the effect of egg colour on hatching success. Carotenoids
are compounds that are responsible for the colour of salmonid
eggs, however, colourless carotenoids do exist, and egg
colour is not necessarily a good indicator of carotenoid
presence. These compounds provide pigment to the skin of
alevins and may possibly assist respiration under low oxygen
conditions. The latter may occur while the eggs sit in the
abdomen after ovulation and the carotenoids may therefore
retard over-ripening.
- Minimize diet derived hazards:
One of the most common known nutritional problems in broodstock
stems from feeds where lipids have undergone oxidation (rancidity)
or when unsuitable fishmeal or lipids were incorporated
into aquafeeds. Rancidity alters the protein and vitamin
content of the diet, creating deficiencies that is usually
more apparent in very young fish Furthermore rancidity can
lead to lipoid liver degeneration. Care must therefore be
taken to ensure that the feed does not become rancid through
improper or prolonged storage.
- Minimizing spawning stress and mortalities:
One aspect that clearly has much effect on reproduction
is physical stress, induced by acute handling and transfer.
In addition to increased mortalities, stress reduces reproductive
performance by delaying ovulation and causing fertilized
eggs to be smaller and poorer in quality. In stressed males
reduced sperm counts are observed. Exposure to disease-causing
agents such as viruses or bacteria does not always result
in illness. The ultimate determining factor is the condition
of the immune system. The more active and powerful the immune
system, the less likely the fish is to contract disease.
When the body is pushed to the limit by regular intense
activity, the levels of adrenaline, cortisol and other immunosuppressive
substances increase substantially. These substances are
known to decrease the effectiveness of the immune system
and result in lowered resistance and increased susceptibility
to disease in the period following stress. Because these
infections are usually opportunistic, it is sometimes possible
to control the spread of infection by correcting housing
problems that precede disease outbreak (e.g. poor water
quality). In severe cases, control of these infections often
requires the use of medicated feed such as feed supplemented
with antibiotics. Alternatively, the feed can be their first
medicine: in stead of giving a drug to cure the invasion
of pathogens, “functional” aquafeeds can be
fed to minimize or prevent it. These feeds are enriched
with specific natural feed ingredients with properties to
reinforce the fish’s natural ability to fight the
invasion of pathogens. These ingredients are called neutraceuticals
and specifically aid body-protecting functions, thereby
offering a health benefit over and above simple nutrition
or basic fortification. The use of nutraceuticals in aquafeeds
helps control the potential pathogenic bacteria and fungal
growth and digestive problems commonly occurring after prolonged
stress conditions and antibiotic treatment. It is therefore
often successfully being used as prophylactic “treatment”,
but it does not replace the therapeutic value of antibiotics
- however it shows future potential to minimise its use.
Practical considerations in feed management
The correct diet composition will result in a good status
of the body stores at the start of gonadal growth and to achieve
this, fish should be fed a special broodstock diet from 6-8
months prior to spawning or when trout broodfish are selected
at around 500g. This is related to the development of the
eggs which starts early in the gonadal growth phase. As trout
may also be used in multiple spawnings, they should be fed
special diets between spawnings in order to bring the fish
back into condition as soon as possible. On the question as
to how often fish should be fed, farmers are usually referred
to the prescribed tables of the feed producers. These tables
should however not be followed rigidly. It should be adjusted
to changing production circumstances. Special care should
be taken not to over-feed the fish as it may lead to obesity
problems. A sample of fish should be gutted and the fat quantity
and liver appearance noted. If fish appear too obese, manifested
by excess fat around the internal organs, feeding frequency
and/or the amount should be decreased and maintained at a
lower level. The feeding regime should be modified according
to findings. Recommended feeding frequency for broodstock
is once or twice a day and fish should be fed to satiation.
Feeding should be done by hand so that fish can be observed
for abnormal behavior. Feeding can continue until 2 weeks
prior to spawning. In repeat spawners, feeding after spawning
should be gradually increased over a 3-4 week period. Floating
pellets or slow-sinking pellets of the largest size (9-11mm)
should be fed to bring the fish to the water surface so that
feeding behavior and any other abnormalities can be observed.
Although broodfish nutrition greatly influences reproduction
performance it should be stressed that problems with egg and
larval quality can also be attributed to poor management and
environmental factors as well as to genetic variations in
individual broodstock. Broodstock fish should be maintained
under as close to optimal conditions as possible, and fed
unrestricted rations of feeds containing high-quality ingredients
Lourens de Wet
|
