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By: Ajuzie C.C. and S.N. Danjuma *
This article developed by researchers of the University of Jos in Nigeria, investigates the relative importance of aeration in incubation units of eggs of C. gariepinus, as one of the criteria for meeting optimal hatchery conditions in African catfish hatcheries.
The African catfish, Clarias gariepinus is a highly adaptable and hardy catfish species. They are found in both lentic and lotic freshwater ecosystems. They are known to tolerate a wide gradient of environmental variables. For example, they can withstand water temperatures in the range of 8 to 35°C and tolerate 0 to 100% dissolved oxygen concentration (Bruton, 1988).
They can also survive outside water for a considerable long time, especially under moist conditions, by using their epibranchial structures to breathe. The epibranchial, or accessory respiratory organ, is composed of a paired pear-shaped air-chamber containing two arborescent structures.
These arborescent or cauliflower-like structures located on the second and forth branchial arches, are supported by cartilage and covered by highly vascularised tissue which can absorb oxygen from atmospheric air (Moussa, 1956). Nigeria is the leading country in Africa in the farming of African catfish (see Figure 1).
All the qualities mentioned above make C. gariepinus a long recognized popular fish in the aquaculture sector (see Hecht et al., 1988). However, those could be attributes of the adults fish and may not apply to the young, including eggs and larvae.
In aquaculture, it is often necessary to provide fish of specific sizes at certain times of the year for stocking ponds (Dwyer, 1987; Ajuzie and Appelbaum, 1996). However, the shortage of juvenile fish for regular pond stocking led to the development of artificial propagation and juvenile rearing techniques for most farmed fish (Britz and Hecht, 1988).
Concerning C. gariepinus, the novel aquaculture techniques became imperative because the traditional method of collecting juveniles in the wild for culturing purposes proved highly unreliable for the fish farmer (see Britz and Hecht, 1988).
Artificial breeding may either involve hypophysation of C. gariepinus (via the use of carp pituitary extracts) or injecting the fish with hormones to get milt and eggs readily (e.g. Ajuzie and Appelbaum, 1996). The breeder mixes the milt and eggs in a recipient for fertilization to occur.
The fertilized eggs are then incubated under controlled conditions. But the creation of optimal artificial environments in C. gariepinus hatcheries (Hecht and Britz, 1988) is of significant importance for the survival of eggs and larvae. Hecht (1982) successfully reared larvae of C. gariepinus at high densities (250 to 300 larvae per liter) with mortality as low as 2.7%, using relatively high water exchange rates (200L per hour), in plastic bins maintained at 290L capacity.
We investigated the relative importance of aeration in incubation units of eggs of C. gariepinus, as one of the criteria for meeting optimal hatchery conditions in African catfish hatcheries.
Broodfish and Treatments
Mature male and female Clarias gariepinus were bought from a small scale fish farmer in Jos, Plateau State, Nigeria. The fish were taken to the hatchery and kept for almost a day, without feeding, before they were injected with Ovaprim® hormone for induced spawning.
“The hormone was administered intramuscularly on one of the sides of the dorsal fin. The injections were based on a dose of 0.5ml/kg. The fish were injected twice. The first injection, usually referred to as a loading dose, was 10 % of the total dose. The second injection (90 % of the total dose) was administered six hours later.”
Both the male and female fish had no injuries on their bodies and were in good shape. So, they were kept together in a plastic basin with minimal water, just enough to cover the head when submerged.
After about 12 hours, following the second dose of injections, eggs and milt were collected from the fish. Eggs were hand-stripped into a plastic bowl from the female fish by gently pressing the abdominal area, starting from the anterior end and towards the posteriorly-located vent.
Testes from the sacrificed male fish were cut open with a clean blade, and the squeezed out milt was dropped on the eggs. Both the eggs and milt were gently mixed with a plastic spoon. The treated eggs were then evenly spread on a nylon net frame (20x20cm) with a mesh size of 0.5mm, and placed into incubation units, as described in Ajuzie and Applebaum (1996).
There were a total of six incubation units. These units were assigned to three different treatments. Each treatment, thus, had a replicate. In the first treatment (Treatment 1), eggs were incubated in units that were connected to a water recirculatory system, and with aeration.
In the second treatment (Treatment 2), eggs were incubated in units where the water was recirculated, but with no aeration. In the third treatment (Treatment 3), the incubation units were neither served with recirculating water nor with aerators.
In the water recirculatory systems, the water exchange rate was 100ml/min. Room and water temperatures were determined using a mercury-inthermometer, and these ranged from 25 to 27.5°C for room temperature and 23.5 to 25°C for water temperature.
The proportion of larvae that hatched from the incubated eggs was expressed as a visual percentage of the total area the eggs occupied on the screens (whereas areas, where hatched eggs had lodged, became vacant following the migration of the hatched larvae into the water column, unhatched eggs remained attached to the incubation screens).
Results
Two days after hatching occurred, the tanks were monitored for larvae survival. Hatching of eggs and larvae survival were best in aerated incubation units where eggs started hatching after about 1.5 days. The percent hatching success of eggs in the three different experimental systems (treatments) can be seen in Table 1.
No eggs hatched in incubation units where there was no aeration, and where water was not recirculated. But, whereas tanks with aeration and water recirculation had a 40% hatching success, tanks where water was recirculated but with no aeration recorded a paltry 10% hatching success.
Larvae survived, two days after hatching, only in tanks where the water was aerated. Some of the larvae were seen swimming freely in the water column. Others were at the bottom where they hung unto balls of debris containing dead eggs. All larvae in tanks without aeration died by the second day following hatching (Table 1).
Discussion and conclusions
Temperatures recorded during this study are not optimal for African catfish hatchery operations. The optimum incubation temperature range for eggs and larvae of this fish, as determined by Ajuzie and Appelbaum (1996), is 30-31°C.
The temperatures were low because the study was carried out in November, a month within the cold Harmattan season, which has a strong weather effect in Jos, Nigeria. As observed by Ajuzie and Appelbaum (1996), the temperature at which eggs are incubated is critical for egg survival.
Relative decreases or increases in water temperature can significantly affect C. gariepinus egg survival/ development and larval growth. Therefore, under cold conditions, thermostatically-controlled immersion heater(s) should be used in catfish hatcheries to keep both eggs and hatched larvae optimally warm (see Ajuzie and Appelbaum, 1993, 1996).
Comparatively, more eggs hatched in the tanks where aerators were installed than in those without aerators. This shows that aeration is very necessary in catfish hatcheries. Aerators have the capacity to increase dissolved oxygen levels in hatchery tanks for optimal hatchery operations.
Studies by Ajuzie and Appelbaum (1986) showed that by constant aeration, oxygen saturation levels are maintained above 80% in incubation units.
“Similarly, water recirculation has a positive effect on dissolved oxygen content in fish tanks. Water recirculation helps to moderately stir the water and cause water movement, which could cause some atmospheric oxygen to be dissolved in the incubation unit.”
This may have been why eggs hatched in incubation units where water was recirculated, but with no aeration. However, in these incubation units, the hatched larvae quickly consumed the available dissolved oxygen, causing their death. Further work is needed to ascertain these postulations. However, we strongly advise for the use of aerators in catfish hatcheries.
*Ajuzie C.C. and S.N. Danjuma. Aquaculture, Freshwater and Marine Ecology Research Lab.
Fisheries and Aquaculture Unit.
Department of Animal Production, University of Jos, Nigeria.
Correspondence email: efulecy@yahoo.com
Note: references made by the authors within the text are available under the previous request to our editorial team
