NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH,

LAGOS, NIGERIA

Fingerlings of African catfish (Clarias gariepinus x Heterobranchus longfilis) were reared in concrete tanks under three dietary treatments for 60 days. Tadpole meal produced the highest fingerling survival rate (90%), with a mean weight value of 2.68g at harvest. The best growth results were obtained among the fish that were fed on fresh tilapia meal, but this treatment also had the lowest fish survival rate (52%). An equal mixture of tadpole and tilapia meal gave fish mean weight value of 2.65g and a survival rate of 80%.

Keywords: Hybrid, Catfish, Tadpole, Tilapia

The demand for African hybrid catfish fingerlings for culture by commercial fish farmers far exceeds the supply in Nigeria. Studies on African catfish have been extensively carried out as reported by Ayinla and Nwadukwe (1998, 1999); Legendre (1992); Nwadukwe (1993, 1995). Observations have also revealed that in high-density culture tanks where water quality conditions have been favourably controlled, the culture of African catfish fingerling always results in stunted populations in the absence of protein-rich diets. Like most predators African catfish, Clarias gariepinus and Heterobranchus spp required high protein content in their diets. This condition is not always met in Nigeria because of the high cost of protein-rich, balanced artificial diet in commercial quantities. Consequently, the common practice involved the use of locally available feed ingredients as fish feed.

In tilapia ponds, the presence of several thousand tadpoles is common occurrence during the rainy season, constituting a nuisance condition. Usually, the process of eradicating these tadpoles has posed a serious problem to aquaculturist and commercial catfish breeders. At the African Regional Aquaculture Centre (ARAC), studies have been carried out with the aim of putting tadpoles and their parent frogs and toads into positive use (Ayinla et al, 1992; Nwadukwe, 1993; Bekibele et al, 1995). In view of the fact that the hybrid catfish has been widely cultured successfully due to its high market price, fast growth rate, high resistance to disease and poor water quality (Adeogun et al, 1999) it is important to investigate the methods of reducing its production costs by developing a cost-effective diet.

To this effect this study investigates the growth performance and survival of the African hybrid catfish in tanks, using separate rations of tadpoles, Tilapia flesh and a mixture of both materials as feed.

Feeding trials were carried out at the in-door fish hatchery tanks at the African Regional Aquaculture Centre (ARAC), Aluu near Port Harcourt. The experiments were conducted during the rainy season of 1996 when there was an abundance of tadpoles in Tilapia ponds.

Three-week-old fingerlings of African hybrid catfish (C. gariepinus x H. longifilis) were used as test fish; these were obtained by artificial propagation of female C. gariepinus and male H. longifilis as described by Nwadukwe (1995). The test fingerlings ranged from 3.0cm (0.20g) to 4.6cm (0.70g).

Six concrete tanks (1 x 1 x 1.3m each) were used for this study. Each tank was filled to the 50cm level with clean borehole water (well water) before the onset of each rearing trial.

Three dietary treatments were tested during this study, which lasted for 60days. Two concrete tanks (in duplicate) were used for each treatment and stocking density was 50 fingerlings in each tank.

During each trial, two tanks were stocked with African catfish hybrids. These were fed with crushed (minced) freshly collected tadpole of Bufo sp.

During each trial, two tanks were also stocked with catfish hybrids and fed with a mixture of minced tilapia, Oreochromis niloticus flesh and minced tadpole meal. Both feed materials were mixed at a 1:1 ratio (wet weight) to make up the required weight of feed.

Under this treatment, two tanks were stocked with hybrid catfish fingerlings and fed exclusively on minced Tilapia flesh.

The fingerlings in each tank were collectively weighed in order to determine the initial fish total weight at stocking. Feeding was carried out daily in two split doses for six days in each week. During the first month, feeding was a rate of 7% of the total biomass in each tank. This was reduced to 5% during the remaining rearing period in each trial. Left-over feed was removed from each tank at the beginning of each day with the aid if a fry net with a long handle before fresh feed was administered. Water renewal was carried out once every two days by exchanging all the water in each tank with fresh clean borehole water.

Water quality was determined forth nightly for Dissolved Oxygen (DO), Ammonia-Nitrogen, Nitrite-Nitrogen, pH and temperature before water renewal, using the (DO meter (YSI model 51B), pH meter, (Aquatic Ecosystems Inc. Florida).

At the end of 30 days, a random sample of 25 fingerlings was collected from each tank for individual length and weight measurements. Mean weight and weight gain were calculated for fish under each treatment Specific Growth Rate (SGR) was determined for each growth period as follows:

SGR = Ln W₁ – Ln W_{0 X} 100%

T

Where W₀ is initial mean weight, W₁ is the final mean weight and T is the time interval during growth determination, as described by Ernst et al (1989). Food conversion ratio was also calculated for each treatment. At the end of 60 days, each tank was drained and the fingerlings were collected and counted in order to determine the survival rate.

Analysis of variance (ANOVA) at P=0.05 was used to determine any significant difference in survival rate, based on arcsine converted values of pooled data as described by Nwadukwe (1995). Anova was also used to determine any differences in fish growth between the three dietary treatments.

The mean results of the two feeding trials are shown in Fig 1. while Table 1 shows the composition of the tadpole as well on the tilapia meal that were used during the experiments.

The values of the physico-chemical parameters of water in the experimental tanks are presented in Table 2. Temperature values were similar in the tanks among the treatments. pH values were also similar in the treatment tanks. Dissolved oxygen values were found to be within catfish tolerant limits, but the lowest value of 3.0mg/l was recorded in tanks under treatment III. Ammonia-nitrogen ranged from 0.6mg/l to 1.0mg/l in the three treatments, which received artificial feed, but was lower in treatment I (Table II). Nitrite-nitrogen level was similar in all the treatments. The control treatment had the best water quality during the experimental period.

Mean weight increased from 0.52g in August to 2.03g in October and daily weight gain also increased from 0.02g in September to 0.03g in October. At harvest, the fingerlings ranged between 3.1cm and 10.2cm (0.98 - 6.87g). Food conversion ratio was 1:0.4, while survival rate was 90%. Specific growth rate over the first 30 days was 1.43%/day and this dropped to 0.93% in the final rearing month.

Under this treatment, total fingerling weight increased from a range of 3.0cm (0.28g) – 4.2cm (0.66g) to a range 3.5 (0.55g) – 11.8cm (7.00g) at the end of the rearing period. Fingerling mean weight also increased from 0.56g in the first month to 2.65g at harvest (Fig. 1). Specific growth rate ranged from 1.50% to 0.73% during the period and fingerling survival rate was found to be 80% harvest.

Size range of harvested fingerlings was from 3.5cm (0.49g) to 12.0cm (18.00g). Fingerling mean weight increase from 0.53g in the first month to 3.99g at harvest, with a corresponding daily weight gain from 0.02g (1^st month) to 0.07g (harvested). Survival rate was 52% at the end of the rearing period. Specific growth rate decreased from 1.94% in the first month to 0.98% in the last month of rearing.

ANOVA did not show any significant difference in fingerling mean weight after the first month of rearing, when the three treatment were considered together (P>0.05). However, a significant difference in fingerling mean weight occurred at harvest when comparing the treatment (P<0.05). ANOVA also showed no significant difference in survival rate between harvested hybrid fingerlings among the three dietary treatment (P<0.05).

The similarity in the results of physico-chemical parameters among the treatments in this study seems to be subject to the fact that water renewal was carried out two days in a week in all the tanks. In addition, water temperature was constant (room temperature) because the rearing tanks were situated inside the in-door hatchery. The frequent water renewal led to the fact that no long-term adverse effect of either uneaten feed or fish excrement was obvious during the study period. This explains the low levels of ammonia-N and nitrite-N. However, treatment III had slightly higher values of ammonia and nitrite probably because of the use of crushed fresh live fish. Generally, these results were within acceptable values for adequate fish growth and survival (Boyd, 1982).

The present results clearly revealed that tadpole meal sustained the growth of African hybrid catfish fingerlings during the 60-day rearing period. Analysis had earlier shown that toad tadpole meal contains 43.5% protein as presented in Table 1. Degani et al (1989) had earlier recommended a diet of 40% protein content for C. gariepinus, while Fagbenro et al (1992) obtained good growth results for H. bidorsalis, using a diet of 42.5% protein level. These diets have similar protein content as the tadpole meal that was used in the present study. Bekibele et al (1995) reported the replacement of fish meal with toad meal in diets of C. gariepinus with favourable growth results. This is an important trend of development in catfish feed production, directed towards cost effectiveness. Ayinla et al (1992) had earlier demonstrated the replacement of fish meal with tadpole meal in a diet for H. bidorsalis with positive cost effective results at the inclusion levels. This shows that tadpoles have a positive place in catfish feed technology as also indicated by the present study. The growth performance of the hybrids are demonstrated in the present results could have also been enhanced because of heterosis.

Dry Tilapia flesh (fish meal) contains over 60% protein and this further explained the accelerated growth pattern of hybrid fingerlings that were reared under Treatment III. Webb and Gatlin (2003) stated that certain fish species that provide dietary protein above minimum requirement levels may promote enhanced growth and reduced lipid deposition in the body. In the present study, fish weight of 180g was attained after 2 months of rearing in tanks, using a diet of crushed Tilapia flesh.

The results also indicated that survival rate was inversely related to growth rate when all the treatments were considered together. The cannibalistic nature of the African catfish had earlier been discussed by Legendre et al (1992) and Nwadukwe and Nawa (2000). This phenomenon was largely responsible for the relatively low survival rate that was recorded in Treatment III where there was marked differential growth among the catfish fingerlings as against the Treatment II. The bigger-size fish preyed on the smaller ones, thereby reducing survival rate, increasing available space within the rearing tanks, thus creating less competition for food, thereby enhancing further growth.

Based on the present results, the use of tadpole meal should be encouraged as feed for African hybrid catfish for good growth results; this should be mixed with fish flesh or used to partially replace expensive fishmeal. The trial of these diets on a larger scope is the next line of action by the investigators.

During the study, daily weight gain directly increased with rearing period. This was probably because the hybrids slowly accepted the tadpole diet as rearing progressed. This is also in agreement with the findings of Ayinla and Bekibele (1992). Specific growth rate was found to decrease as fish size increased in all the treatments. This was also similar to the report of Ernst et al (1989) who reared red Tilapia in sea water pools.

African catfish are known to be cannibalistic and so a diet of fish flesh is expected to be appropriate for their growth and survival. This also explains why the diet containing mixture of crushed tadpole and tilapia flesh gave improved growth results under Treatment II, while the best results were obtained under Treatment III using fish (Tilapia) flesh only.