GENERAL BIOLOGY OF AFRICAN CATFISH
2.3.1 DESCRIPTION
OF THE GENUS AND SPECIES
The catfish genus can be defined as
displaying an eel-like shape, having an elongated cylindrical body with dorsal
and anal fins being extremely long (nearly reaching or reaching the caudal
fin). Both fins containing only soft fin normally has six soft trays. The head
is flattened and highly ossified. The skull bones (above and on the sides)
forming a casque and the body is covered with a smooth scaleless skin. The skin
is generally darkly pigmented on the dorsal and lateral parts of the body. The
colour is uniformly marbled and changes from grayish olive to black according
to the substrate. On exposure to light, the skin colour generally becomes
lighter.
They
have four pairs of unbranched barbells, one nasal, one maxillar (longest and
most mobile) on the vomer and two mandibulars (inner and outer) on the jaw.
Tooth plates are present on the jaws as well as on the vomer. The major
function of the barbles is prey detection. A supra-bronchial or accessory
respiratory organ, composed of a paired pear-shaped air- chamber containing two
arborescent structures is generally present. These arborescent or
cauliflower-like structures located on the secondhand forth bronchial arcs, are
supported by cartilage and covered by highly vascularised tissue which can
absorb oxygen from atmospheric air. The air chamber communicates with the
pharynx and with the gill chamber. The accessory air breathing organ allows the
fish to survive for many hours out of water or for many weeks in muddy marshes.
2.3.2 HABITAT
Clarias spp. Inhabit calm waters from
lakes, streams, rivers, swamps to floodplains, some of which are subject to
seasonal drying. The most common habitats frequented are floodplain swamps and
pools in which the catfish can survive during the dry seasons due to the
presence of the accessory air breathing organs. In nature, the Africa Catfish (Clarias gariepinus) tends to live in
calm waters with vegetation. Living conditions in such an environment can be
harsh. There is often a lot of organic matter in the water. In addition, in
several of these locations (for instance, in flood plains), the water levels
fluctuate seasonally, going up during the rains and receding during the dry
season. In several of the clarias catfish
natural habitats, the water levels drop to the point whereby the water-way
almost dries up. The relatively large amounts of organic matter in water
coupled with the relatively slow water flows through such habitats result in
low levels of dissolved oxygen in the water for prolonged periods and increased
acidity of the water. In order to overcome these environmental challenges, the
fish have adapted in the following ways:
1. Low Oxygen Level – Catfish have developed in addition to gills, an accessory cauliflower
like organ (the arborescent organ) that enables the fish to obtain oxygen from
air when the oxygen levels are too low in water or the swamps have dried out.
However, this organ is largely functional in adults. More than 90% of juvenile
catfish oxygen consumption is from dissolved oxygen in water whereas in adults,
40% to 50% of the oxygen uptake is atmospheric.
2. Body Shape and Features – The fish have no scales, but a relatively thin skin and
protective layer of mucus over the skin. Their long cylindrical shape also
allows them to easily burrow into the mud when water levels drop to keep
themselves moist and cool. They can only survive in burrowing if there is an
air-water interface. In addition, it has barbells
that enable it sense its food even though visibility is poor in the swamp.
Its flattened mouth is designed so that it can ingest food off the pond bottom.
3. Feeding Habits –The fish is also an omnivore, meaning it can literally eat almost
anything although in the wild adults preferably eat other fish, insects or
other forms of aquatic animals. Adults have a diel (twenty four hour cycle) feeding pattern, meaning that they
need to hunt once a day, subject to food availability. Consequently they have a
relatively large stomach capable of holding quite a bit of food, unlike the
Nile tilapia whose stomach is relatively small because it is naturally a
browser and feeds several times a day.
4. Social Behaviour –Catfish are extremely social. They tend to live, hunt in tight groups.
Hunting as a pack is among their natural feeding strategies. They tend to dwell
at the pond bottom.
5. The fish are able to withstand
slightly acidic water.
6. The catfish also grows fast and does
not become sexually mature until it is about a year old (about 600g) depending
on feeding.
Females
become mature earlier than males. The above mentioned attributes, make the
catfish a good candidate for aquaculture. However, these biological
characteristics affect the fish’s production requirements and potential in the
following manner:
1.
Low Dissolved Oxygen Levels –Where the water has adequate levels
of dissolved oxygen, catfish obtain their oxygen from the water through their
gills. Aerial respiration in catfish is largely a compensatory mechanism for
the periods when the dissolved levels of oxygen in the pond are low. However,
in order to survive the periods of low dissolved oxygen, the fish must have
access to air otherwise they will die. In addition, the fish spend more energy
obtaining oxygen from the air than they would do, when they can obtain it from
the water.
In fish farming, the initial limiting
factor to production as far as water quality is concerned is oxygen. Because
adult catfish have the ability to overcome this by breathing air, higher
carrying capacities and feed input level can be accommodated in catfish
grow-out ponds as long as the buildup of metabolic waters (ammonia) in the
water are kept under control and fish can access the water surface.
However, because more energy is spent
when the fish obtain air from the atmosphere, Feed Conversion Rations (FCRs) tend to increase, which in turn
affects the profitability of the enterprise. Therefore, just because clarias catfish have the ability to
withstand situations of low dissolved oxygen is not a reason to compromise
water quality. For profitable production, water quality parameters should as
much as possible be maintained within the recommended ranges. A minimum amount
of oxygen within the water is required for the breakdown of metabolic waters.
Having more oxygen dissolved in water improves the efficiency with which the
gills function which is more energy-efficient for the fish and results in
better growth rates.
2.
Body Shape and Features – Because the catfish have no scales, the thin layer of mucus is the only
first line of defense against infections whose port of entry into the fish is
the skin. Consequently, removal of this layer of mucus through poor handled
with care during routine production operations. Because catfish are bottom
dwellers, most of the time within the pond they will be at the bottom unless
there is a reason for them to come up, for example, to feed or gulp air to
obtain oxygen.
Consequently, they tend to stir up
the pond bottom which makes the water in catfish grow-out ponds muddly (i.e. turbid). Coupled with their burrowing
behavior, they also dig into side of the pond, creating what is termed as the ‘catfish highway’. These result in
breakdown of pond levees, especially
when they are not constructed as recommended and in addition increase levels of
pond turbidity (see sections 4.1.1, 3.1.2 and 5.2.5. for more details
respectively). Their body shape and the catfish highway they create in the pond
enable them to easily escape seining when nets and relate techniques are poor.
They are able to pass under and around the nets. When there is a hole at the
bottom of the seine net and one catfish finds it, because they move n hordes
like sheep unlike tilapia, all the others shall be informed and they will all
escape in a stream through the hole. Therefore, seine net specifications and
seining technique are important; otherwise one can easily come out with an
empty net.
3.
Feeding Habits – The clarias catfish is omnivorous.
This means it can consume a wide selection of food items that allows for a
range of options in culture to provide for its nutritional needs. Because of
their social hunting behaviour, it is preferable to feed them in ponds from a
single place as doing so induces a feeding frenzy that results in the complete
consumption of the feed, improved FCRs and
reduced feed wastage. This is an important fact as above 60% of production
costs are the feed cost.
4.
Social Behaviour – Pack hunting is a natural feeding strategy in
Clarias catfish. Keeping the fish at high densities, consequently results in
reduced stress and aggression while stocking at low densities results in the
such that:
(i)
They are high enough to the point whereby territories are not
established, aggressive behaviour is reduced, feeding response is high and feed consumption time is reduced.
(ii)
Management requirements to sustain the biomass in the pond not reach the pond’s carrying capacity limits
for the specified management level (see sections 5.3. and 9.2.6. for more
details).
5.
Most healthy fish tend to swim against a current. Therefore, they will
tend to aggregate and swim out of the inlet water is flowing into the pond
during the production cycle. Thus, most escapes of catfish from ponds actually
occur through the inlet.
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