OBSERVATIONS AND CONCLUSIONS

Completion and maintenance of the experimental farm

Owing to budgetary delays, it took some time before the funds became available for the completion of the construction of the experimental farm. In the meantime the Niger Delta Development Board, at the expert's request, started the construction of sluice gates for individual ponds. Such sluice gates were very necessary to control the supply of tidal water and, in the absence of such gates, it was impossible to conduct any investigations in tidal brackish-water fish culture in the ponds. Although the Board began the construction work in earnest, because of the inherent delays in such construction and owing to scarcity of construction materials, such as cement, and lack of sufficient labour, the progress of work was very slow and the sluice gates were only completed by the end of January 1966 (Figs. 1–4).

It was expected that studies for the selection of species would be started as soon as the sluices for the bigger ponds were completed, but this could not be done as the sluice gate sites of the smaller ponds were inundated when water was taken into the big ponds. For the same reason, thorough washing of the completed ponds could also not be undertaken earlier.

The investigations for selection of species was initiated at a small official function on 19 March 1966. Representatives of the Federal Fisheries Service of Nigeria, United Nations Development Programme and Niger Delta Development Board, as well as the people of Buguma, were present at the fish farm for the occasion and took part in it.

Sulphides and oxygen consumption in ponds

On completion of all the sluice gates, the ponds were washed by taking in tide water to the full during high tides and completely draining them during low tides. In the smaller ponds where sulphides had accumulated for a longer time till the completion of the construction work, mere washing with tide water did not seem adequate for getting rid of the sulphides. In order to assess the sulphide contents in the pond bottom by estimating the quantity of oxygen used up, the dissolved oxygen of the water of these small ponds was studied on 17 February 1966, when the water started flowing into the ponds at high tide. The soil on the pond bottom and sides was then agitated physically before letting out the water at low tide. The water was again examined for dissolved oxygen when it ebbed. The results are given in Table 1, from which it is clear that a fair quantity of oxygen was used up while the water remained in the pond. The process of physically agitating the pond bottom when repeated a few times was helpful in raising the pH.

Table 1

DISSOLVED OXYGEN CONTENT IN THE SMALLER PONDS OF THE EXPERIMENTAL BRACKISH-WATER FISH FARM AT BUGUMA DURING HIGH AND LOW TIDES ON 17 FEBRUARY 1966

Stocking of ponds with grey mullet and pink prawn

According to the programme of initial experiments to determine the most suitable fish or combination of fishes for commercial culture in the brackish-water ponds (see Rep.FAO/EPTA (1973)), two species of grey mullet, namely Mugil grandisquamis and M. falcipinnis, the pink prawn Penaeus duorarum and tilapia, Tilapia melanopleura, were used. As fertilizing of brackish-water ponds which depend on tides for water supply is inconvenient, only liming was attempted and that, too, only in selected ponds. The ponds thus treated were B, F, G, and H, J, L (see Fig.1) in which lime was applied on the bottom. The other ponds were stocked with grey mullet and pink prawn as described below. Pond C was stocked with mullet at the rate of 10,000/ha while pond E was stocked with pink prawn at the same rate. Pond I received grey mullet at 5,000/ha and pond M pink prawn at the same rate. Ponds D and K were stocked with equal numbers of mullet and prawn at the respective rates, while ponds A and N had tilapia.

The two species of grey mullet available around the fish farm look so much alike that it was not very easy to distinguish them at a size of about 2 to 3 cm in length, when they are generally used for stocking ponds. A mixture of the species was, therefore, used for stocking. Pond N was stocked with tilapia of sizes varying from 3 cm to as much as 10 cm in length. The smallest available size of pink prawn (about 2.5 cm in length) was used for stocking.

Fish seedlings were collected from the shallow beaches on the islands around Buguma which were fairly good collection centres. The mullet fry thus collected were transported in canoes as quickly as possible to the ponds with minimum handling. Avoiding direct sun on the fry in the shallow water in the canoe was found to be helpful in reducing mortality.

Experimental studies for species selection

Compatibility of tilapia and grey mullet in ponds.

The number of mullet fry and prawns stocked in the ponds gradually decreased and it was found to be due to the tilapia that attacked them. Certain species of tilapia are known to change their food and feeding habits in the absence of their natural food. After observing a large number of tilapia preying upon the mullet and prawn of the ponds, experiments were conducted with a view to confirming the predatory propensities of the local tilapia. In aquarium experiments, Tilapia heudeloti and T. zillii were seen to kill the mullets and prawns when the latter were introduced into the aquarium. Repeated experiments confirmed the observation. The local species of tilapia, therefore, appeared to be incompatible for culture in ponds with other fishes.

Possibility of using tilapia as a forage fish.

The possibilities of using the prolific tilapia as a forage fish to rear the red snapper, Lutjanus sp., were then studied. A total of 256 red snappers varying from 7 to 12 cm in length were liberated in pond D. These were healthy and active at the time of stocking but after a few days started to die in large numbers. Individuals with damaged fins were seen struggling on their side. In the course of a few days all the snappers died. At the same time a tilapia of 12 cm in length was seen swimming about in the pond with its brood. Although this tilapia was not seen actually killing the snapper, it was, on occasions, seen chasing it. It is believed that tilapia with its brood became pugnacious and attacked the red snapper.

Eradication of tilapia from fish ponds.

As tilapia had proved itself to be incompatible for culture in ponds with other fishes, it had to be eradicated from the ponds where they occurred. All attempts at eliminating them by draining the ponds or hand-picking, etc., proved futile. The ponds could not be dried completely because of tidal action. A few centimetres of water always used to percolate into the ponds as soon as the tide started to flow, making it possible for the tilapia to escape deep into the soft mud where it could remain for hours without harm.

Two aspects of the problem were to prevent tilapia from entering the pond with tide water and to eliminate those that were already in the pond. A sand and gravel filter for installation in the sluice gate was then designed with a view to preventing tilapia from entering the pond. This filter is a rectangular wooden box like structure which would fit in the sluice. A vertical column of sand in between two vertical columns of gravel allowed only water to pass through. Water entered through a large series of holes made on two sides of the filter.

For the eradication of tilapia already existing in the ponds, Aldrex 40 (Aldrin diluted 40 percent) was used. It is a chlorinated hydrocarbon and is a contact poison, widely used as an insecticide. It releases hydrochloric acid on hydrolysis, and this acid effects the kill. However, as the acid soil of the brackish-water ponds contains sulphides which are ultimately oxidized to sulphuric acid, the combined effect of the two mineral acids can be too strong and prevent fish life in the water for an unduly long time.

The chemical was first used in pond B, which was de-watered and fitted with the filter when the tide was at its lowest. The sluice gate was then blocked watertight at the outer end so as not to allow any water into the pond at high tide. The pond was then treated with Aldrex 40 (18 ppm). Within half-an-hour a total kill occurred. The pond was left like that expecting that the chemical would work itself out. A fortnight later the sluice was opened for washing the pond. Three weeks washing could not raise the pH of the water beyond 3.6. Lime was used at 340 kg/ha, and a fortnight later at 680 kg/ha and even the last dose did not raise the pH sufficiently high for maintaining fish life. By that time the filter, which in the beginning had allowed a fairly good flow of water, almost ceased to be effective, presumably because of the settling and clogging of the sand and gravel, making the filter almost watertight. Pond C was also fitted with a filter and treated with Aldrex 40 with the same results.

It was clear that the two ponds first used for Aldrex treatment were not the best ones for the experiments as ‘chikoko’¹ which often contained a large quantity of accumulated sulphides, had been used for levelling the bottom and it became highly acidic when in contact with Aldrex. The offending ‘chikoko’ was removed after draining the pond and lime was applied at the rate of 5,000 kg/ha. The outer end of the sluice was blocked to prevent water from getting into the pond. ‘Chikoko’ is evidently very unsuitable for levelling the pond bottom and, instead, the clay found beneath the ‘chikoko’ is recommended for that purpose. In order to wash away the sulphides and the acidity created by those, it would appear advisable to expose ponds constructed in the area to free tidal flooding and draining for a sufficiently long period of time before stocking.

In further experiments all the ponds were treated with Aldrex 40. The gravel filter was not found satisfactory and was, therefore, replaced by a three-layered fine-meshed velon screen. When maintained carefully this screen appeared to be quite effective as it allowed a free flow of water and, at the same time, prevented any fish from entering the pond. Careless handling of the screen while cleaning would, as happened a few times in the farm, allow undesirable fish into the pond, thereby making it necessary that the pond be treated again. The screen should, therefore, be cleaned only at low tide, when there is little chance of extraneous fish gaining access to the ponds.

¹ Peaty “fibrous” mud of saline swamps, containing large quantities of organic matter mainly in the form of undecomposed roots of the mangrove.

Mortality of grey mullet in ponds.

There was mortality of mullet twice in the pilot pond. Both mortalities occurred while the expert was away and the water was not available for study. It was reported that on both occasions there had been heavy and continuous rains in the delta area. Laboratory experiments of lowering the salinity of water at different rates confirmed the previous observations [Rep.FAO/EPTA (1973), p.26] that mullet could tolerate waters of very low salinity. The mortality seemed to have been brought about by the change in water conditions caused by the washing of the dykes after heavy rains, as suggested in the previous report [Rep.FAO/EPTA (1973)]. As it was felt that mortality might have resulted due to oxygen depletion, the dissolved oxygen of the pond water was estimated both when it came in with the high tide and at the last stages of the following low tide. Table 2 shows that a certain amount of oxygen was definitely being used up while the water remained in most of the ponds. The ponds were then washed during the tides continuously for a fortnight and it was then observed that there was no such pronounced demand for dissolved oxygen afterwards (see Table 2).

Table 2

AMOUNT OF DISSOLVED OXYGEN USED UP WHILE THE WATER REMAINED IN THE LARGE PONDS

Culture of catfish, Chrysichthys nigrodigitatus.

This species is well-liked in Nigeria and is highly priced. It is known to grow to a fairly large size and specimens 50 cm long are fairly common. This fish, being quite hardy, was believed to be relatively more tolerant of changes in water conditions. The young ones were available in the creeks around the fish farm, especially at night. Because of these particular reasons, this species was also used for experimental studies. Between 15 and 18 February 1967, pond G was stocked with 200 specimens of Chrysichthys nigrodigitatus ranging from 5 to 12 cm in length. They grew well in the ponds where they were fed with ‘gari’ (processed cassava) soaked in palm oil. It was later reported that the catfish from pond G early in June 1968, before the liberation of Buguma by the Nigerian Army, ranged from 25 to 30 cm. This growth does not appear too bad, especially since no particular care was taken of the pond or any artificial feeding done since the end of 1968.

Acclimatization of catfish Clarias lazera to waters of low salinity.

It appeared that there were three species of catfish in the genus Clarias known locally as ‘are’. Clarias lazera is in great demand, especially in the southern region of Nigeria and fetches more than double the price of most other fish. Adult fish of about 1 metre in length are not rare in the fresh-water swamps. Clarias is hardy and possesses accessory respiratory organs. It is known to thrive on artificial food. In view of these it was decided to study the possibility of culturing this species in brackish-water environments.

Two aquaria (I and II) of 120 × 270 × 60 cm size were used for conducting experiments on acclimatizing Clarias to brackish-water conditions. The aquaria had an 8-cm layer of mud at the bottom and was filled with fresh water up to the 18-cm mark. Fifteen specimens of an average length of 17 cm were introduced into aquarium I while II received only four specimens of the same size.

During the first few days after liberation into a cement tank used for storing experimental fish or the aquaria, it was noticed that the fish refrained from feeding. Once they started feeding, they ate voraciously whatever feed was supplied. Once the conditions settled and the fish started feeding, small quantities of water in aquarium I were replaced with equal quantities of sea water for raising the salinity. This replacement was effected twice in a week at regular intervals and the result in salinity recorded. In order to ascertain the degree of salinity the fish could tolerate on sudden transference, a fresh marked specimen of Clarias was also introduced into aquarium I immediately after each addition of sea water. It was seen that this fish when directly introduced from fresh water thrived well in waters of salinities up to 10‰. However, they died within 4 to 12 hours in salinities over 10‰ and up to 19‰. In salinities higher than 19‰ mortality occurred immediately after transfer.

When salinity was raised gradually, the fish tolerated salinities up to 29.2‰. They started dying when the salinity was further increased. Although these laboratory observations have yet to be confirmed by further field experiments, it appears clear that the species can be acclimatized to most areas in the Niger Delta.

Acclimatization of common carp, Cyprinus carpio, to brackish water.

As common carp, a widely used pond fish, is known to grow well in brackish waters with low salinities, it was also used in the experiments. Sixteen small carp fry about 4 cm in length were released into fresh water in a cement tank 2.4 m in diameter. A week later brackish water at the rate of 36 litres per day was added from a nearby creek at high tide. In the course of about 12 days the salinity of the tank water reached that of the creek (see Table 3). The carp were found to thrive well in the saline water. As a result of heavy rains that occurred at this time, the tank water became almost fresh overnight, but this sudden change did not cause any apparent harm to the fish. Again, the salinity of the tank water was raised by the addition of brackish water from the creek. The carp were intended to be released into one of the experimental ponds after the acclimatization, but this could not be done as the expert had to be evacuated from the area at that time because of conditions created by the civil war.

Table 3

SALINITY, DISSOLVED OXYGEN AND PH OF THE WATERS OF THE BUGUMA CREEK, PILOT POND AT THE FARM AND THE CEMENT TANK USED FOR EXPERIMENTS IN ACCLIMATIZATION OF COMMON CARP TO BRACKISH WATER

Growth of the sardine, Sardinella aurita, in ponds.

While collecting grey mullet fry for stocking ponds, a few specimens of Sardinella aurita of about 4 cm in length were caught accidently. These were also released in pond M with the mullet fry on 10 May 1967. Later, on 27 June 1967, when these fish were caught, they measured 9 cm in length which is almost the adult size of those caught in the Delta. This observation indicated the possibility of raising a short-term crop of sardine in brackish water ponds.

Spatfall and growth of the mangrove oyster

Studies showed that oysters (Ostrea tulipa) feeding on mangrove roots had an average growth of about 4 mm during the month of August but for some reason or other they all died soon after. Old oyster shells, as recommended in FAO/EPTA Report No.1973, were then used as spat collectors and placed in the creek at Port Harcourt. There was a good fall of oyster spat on these collectors and the growth of the oyster was studied until October 1965, by which time they had grown to an average size of 13 mm. This group of oysters, and many other groups laid out there afterwards, were consistently stolen. It was then decided to continue this study at the Buguma experimental farm when the pond construction was completed instead of in open creeks where theft could not easily be prevented.

Due to the evacuation of the expert to Lagos in December, he was unable to pursue this work at Buguma. Further observations on spat fall and relative efficiency of spat collectors were carried out in Lagos in 1968.

In March 1968, old oyster shells and asbestos plates were laid as spat collectors in the Curamo creek. Better spat fall occurred on the asbestos plates. The oyster shells became covered with the Serpulid worm, Mercierella enigmatica, in a very short time and the spat that settled on the shells died after a few days. The average measurements of the oysters on the asbestos plates are given in Table 4.

A comparative study of collectors made of other materials such as plywood, bamboo, tiles, etc., was initiated, but could not be completed due to the termination of the project.

Table 4

MEASUREMENTS OF MANGROVE OYSTER CULTURED IN CURAMO CREEK, LAGOS

NOTE: From July, the oysters had to be cleaned regularly by scraping, to remove infestations of Serpulids. This often caused the breaking off of the thin margins of the shells. The apparent reduction in average measurements in certain weeks from July onwards is on account of this.