Influence of dietary protein levels on growth performance and body composition of African bonytongue fingerlings, Heterotis niloticus (Cuvier, 1829)

Introduction

In west and central Africa, fishing remains the main source for indigenous fish supply to the population. Obviously, these fishing grounds have attained maximum sustainable levels and for the last 15 years, there is even a decline trend. To meet up with the increasing demand for animal protein from the growing human population, without increased fish importation, there is an imperative need to develop aquaculture in the African continent. Apart from Nigeria, aquaculture sector contributes to <5% of local fish demand in most African countries. Despite its aquaculture potential, Sub-Saharan Africa lags far behind. It is therefore indispensable to stimulate the development of this sector through diversification including the culture of indigenous species such as African bonytongue (Heterotis niloticus). This species has a relatively high commercial value and strong aquaculture potential, but very few data are available on its farming.

Heterotis niloticus originated from tropical Africa (Greenwood 1973; Li & Wilson 1996). It is found in big rivers (Senegal, Gambia, Niger and Nile) and in some tropical African lakes like Lake Chad (Aubenton 1955; Levêque et al. 1990). Moreover, it was successfully introduced in the south of Cameroon (Depierre & Vivien 1977), in Ivory Coast (Moreau 1974; Lazard 1980), in the Democratic Republic of Congo, in Gabon (Mbega 2004) and in Madagascar.

Heterotis niloticus is a warm water bony fish well appreciated for human consumption in Sub Saharan Africa. It possesses an important potential market, with a commercial value twice higher than that of tilapia. The rapid growth (3–4 kg in a 12-month cycle), late sexual maturity, short food chain and natural reproduction of this species in small and large ponds make it a good candidate for aquaculture production. Initial research on this species began in the 1950s. Unfortunately, the number of studies diminished considerably just after independence in 1960, before the recent regain of interest. These studies were conducted to better understand the biology (Aubenton 1955; Daget 1957; Omorinkoba et al. 1991; Okoye & Abubakar 1996; Fagbenro 2001; Achionye-Nzeh & Omoniyi 2002; Adite et al. 2006), the ecology (Moreau 1974; Moreau & Moreau 1982; Adite et al. 2005) and the culture (Tillon 1957; Lemasson 1957; Tillon 1959; Olanyan & Zwilling 1963; Reizer 1964) of this species. Heterotis was classified in the omnivorous fish category (Micha 1976; Mbega 2004; Adite et al. 2005).

Protein is a main component in fish feed. Increasing protein level in feed can lead to improved fish production, but excessive dietary protein level is not economical for fish culture. Valid information on the protein requirement of fish is essential for any new aquaculture attempt. It is therefore not surprising that protein requirement studies are usually among the first fish nutrition experiments to be conducted when a new fish species is being considered for aquaculture. Dietary protein requirements has been investigated in a number of fish species (NRC 1993), but no research has been conducted so far on H. niloticus.

This study was designed to investigate the effects of dietary protein levels on growth and carcass proximate composition of Heterotis fingerlings, and to estimate their specific dietary protein requirement. As a consequence, an appropriate supply of dietary protein would allow supporting high growth rate and reducing nitrogen wastes.

Materials and methods

Results

Experiment 1

Growth, survival and feed utilization Weight gain, SGR, FE and survival rate of Heterotis fingerlings during the feeding trial are shown in Table 4. WG and SGR were significantly affected by dietary protein level (P < 0.01). Heterotis fingerlings fed with 300 g protein kg⁻¹ diet displayed the highest growth rate, but values did not differ significantly from those of fish fed with 350 and 400 g protein kg⁻¹ diet (P > 0.05). However, the WG and SGR of this group were significantly (P < 0.01) higher than those of fingerlings fed with 250 g protein kg⁻¹ diet, which displayed the poorest growth.

Table 4. Feed utilization, growth, survival performance and body composition of Heterotis niloticus fingerlings fed with diets containing graded levels of protein (initial average weight 3.96 g)

	Diets (g protein kg−1 diet)
	250	300	350	400
Final weight (g)	11.2 ± 0.5a	14.8 ± 1.1b	14.6 ± 1.0b	14.1 ± 0.9b
Weight gain (%)	183 ± 2a	272 ± 14b	269 ± 20b	254 ± 10b
SGR (% day−1)	2.5 ± 0.0a	3.1 ± 0.1b	3.1 ± 0.1b	3.0 ± 0.1b
Feed intake1	50 ± 3a	46 ± 1a	34 ± 2b	33 ± 2b
FE	0.4 ± 0.0a	0.6 ± 0.1a	0.6 ± 0.1a	0.8 ± 0.0b
PER	1.52 ± 0.16a	1.86 ± 0.34a	1.61 ± 0.18a	2.12 ± 0.11a
PD	22.4 ± 2.7a	28.2 ± 4.8a	24.0 ± 2.1a	30.1 ± 1.6a
Survival (%)	76 ± 2a	68 ± 6ab	59 ± 3b	76 ± 2a
Moisture2	80.5 ± 0.1	79.6 ± 0.3	80.3 ± 0.5	80.5 ± 0.5
Protein2	13.6 ± 0.2	14.0 ± 0.2	13.6 ± 0.2	13.5 ± 0.2
Total lipid2	1.1 ± 0.1	1.3 ± 0.2	1.1 ± 0.2	1.1 ± 0.2
Ash2	4.0 ± 0.1	4.1 ± 0.3	3.5 ± 0.1	3.8 ± 0.1

• SGR, specific growth rate; FE, feed efficiency; PER, protein efficiency ratio; PD, protein deposition.
• Values are mean ± SE of three replicates. Mean values in a row with different superscript letters are significantly different (P < 0.05).
• ¹ Feed intake (g week⁻¹).
• ² Values are mean ± SE of three replicates and expressed in (%). Initial whole body composition was 79.5% moisture, 12.2% protein, 0.87% lipid and 5.1% ash.

Survival in all treatments ranged from 59% to 76% and was affected by dietary protein level. The survival rate of fish fed with 300 and 350 g protein kg⁻¹ diet did not differ from each other, but survival of fingerlings fed with 350 g protein kg⁻¹ diet was significantly (P < 0.05) lower than that of the fish fed with 250–400 g protein kg⁻¹ diet. Feed intake decreased progressively with graded dietary protein level and was found to differ significantly (P < 0.001) between groups fed with 250–300 g kg⁻¹ and those receiving 350–400 g protein kg⁻¹ diet. FE increased progressively with graded dietary protein level and was found to differ significantly (P < 0.05) between fingerlings fed with 250–350 and 400 g protein kg⁻¹diet. On the contrary, PER and PD did not differ significantly between treatments (P > 0.05) (Table 4).

Body composition The whole body composition of Heterotis fingerlings is presented in Table 4. Moisture, ash content, whole-body protein and lipid content were not significantly affected by dietary protein level (P > 0.05). Initial whole body composition of fish contained less moisture, protein and lipid than final body composition of fish, regardless of experimental diets.

Estimation of protein requirement The influence of increased dietary protein levels on the growth response was examined by fitting dose–response data to a polynomial curve (Espinós et al. 2003). In that way, the polynomial equation obtained between WG and dietary protein level was:

The maximum WG was observed when the dietary protein level was 340 g kg⁻¹ diet. Using the broken line model of Robbins et al. (1979), the dietary protein requirement for the Heterotis fingerlings (3–15 g) based on percentage WG was estimated to be 306 g protein kg⁻¹ diet (Fig. 1).

Experiment 2

Growth, survival and feed utilization Weight gain and SGR were significantly affected by dietary protein level (P < 0.05) (Table 5). Fish fed with 250 g protein kg⁻¹ diet displayed the lowest growth rate. The WG and SGR of fingerlings fed with 300 and 400 g protein kg⁻¹ diet did not differ from each other, but were significantly (P < 0.05) lower than those of fish receiving 350 g protein kg⁻¹ diet.

Table 5. Feed utilization, growth and survival performance of Heterotis niloticus fingerlings fed diets containing graded levels of protein (initial average weight 26.40 g)

	Diets (g protein kg−1 diet)
	250	300	350	400
Final weight (g)	51.7 ± 1.8a	58.6 ± 2.6b	61.6 ± 2.2b	58.9 ± 1.6b
Weight gain (%)	96 ± 0a	122 ± 4b	133 ± 1c	123 ± 0b
SGR (% day−1)	2.4 ± 0.1a	2.9 ± 0.1b	3.0 ± 0.0c	2.9 ± 0.0b
Feed intake1	230 ± 7	208 ± 18	152 ± 16	97 ± 1
FE	0.7 ± 0.1a	0.7 ± 0.1a	0.8 ± 0.0b	1.0 ± 0.0c
PER	2.53 ± 0.03a	2.26 ± 0.03b	2.18 ± 0.05c	2.43 ± 0.02a
Survival (%)	88 ± 2a	84 ± 4ab	76 ± 4bd	72 ± 2cd

• SGR, specific growth rate; FE, feed efficiency; PER, protein efficiency ratio.
• Values are mean ± SE of two replicates. Mean values in a row with different superscript letters are significantly different (P < 0.05).
• ¹ Feed intake (g week⁻¹).

Similarly, survival was affected by dietary protein level. The survival rate generally showed a decreasing trend with increasing dietary protein level and was significantly higher (P < 0.05) in fingerlings fed with 250–300 g protein kg⁻¹ diet than in fish receiving 350–400 g protein kg⁻¹ diet (Table 5). FE increased progressively with graded dietary protein level and was found to differ significantly (P < 0.01) between fingerlings fed 250–300 and 350–400 g protein kg⁻¹ diet. There was no significant (P > 0.05) difference between fish receiving 250 and 300 g protein kg⁻¹ diet, but fingerlings fed with 350 g protein kg⁻¹ diet had significantly (P < 0.01) lower FE than those of fish receiving 400 g protein kg⁻¹ diet. The highest PER was obtained with diets containing 250 and 400 g protein kg⁻¹ diet whereas the lowest PER was obtained with diet containing 350 g protein kg⁻¹ diet.

Estimation of protein requirement As in experiment 1, the effect of increased dietary protein levels on the growth response was examined by fitting dose–response data to a polynomial curve. The polynomial equation obtained between WG and dietary protein level was:

The maximum WG was observed when the dietary protein level was 349 g kg⁻¹ diet. Using the broken line model of Robbins et al. (1979), the dietary protein requirement for the Heterotis fingerlings (26–62 g) based on percentage WG was estimated to be 311 g protein kg⁻¹ diet (Fig. 2).

Discussion

In this study, an estimation of the dietary protein requirements of H. niloticus fingerlings was performed by formulating diets incorporating readily available and cheap local ingredients. In a majority of similar studies, researchers employed various purified and semi purified diets with high-quality protein sources such as casein, gelatin or synthetic amino acids that yielded more precise values. These protein sources being relatively expensive and not readily accessible for an average fish nutritionist in developing countries, our choice was guided by the amino acid composition of proteins (experimental ingredients) and their digestibilities in the one hand; in the other hand by similar estimations of dietary protein requirements of tropical bagrid catfish, Mystus nemurus fingerlings demonstrated by Khan et al. (1993) and by Ng et al. (2001). These authors reported the requirements of 420 and 440 g kg⁻¹ diet in this species when using practical and semi purified diets respectively. Identical results were obtained in other studies involving tilapia juveniles (Jauncey 1982; El-Sayed & Teshima 1992; Gunasekera et al. 1995; Al Hafedh 1999). Despite the slight difference, the similarity observed permits the use of practical diets in the determination of fish protein requirements. Moreover, in our study, experimental diets were formulated by adjusting levels of fish and soybean meals with maize meal and wheat bran; the part of protein which comes from fish meal was highly digestible and its amino acid composition was close to requirements; the digestibility of other protein which comes from practical ingredients depends on digestibility of dietary amino acids which varies among feed ingredients. So, it is crucial to underline that the amount of non-protein energy in the feed and the quality of the protein influence the growth response of fish fed with diet containing different levels of protein.

In this study, the relationship between dietary protein levels and WG was fitted using a second-order polynomial equation (Espinós et al. 2003). The maximum WG was observed when the dietary protein level was 340 g kg⁻¹ diet (initial average weight 3.96 g) and 349 g kg⁻¹ diet (initial average weight 26.40 g). The dietary protein requirement of H. niloticus fingerlings between 3 and 15 g was estimated to be 306 and 311 g kg⁻¹ diet for fish between 26 and 62 g when fish meal and soybean were the major protein sources. These values are slightly lower than those obtained for other omnivorous fish species (Oreochromis niloticus and Cyprinus carpio) of similar body weight (350–420 g protein kg⁻¹ diet), as recommended by Tacon (1987). These low dietary protein requirements of Heterotis fingerlings can be correlated to the natural feeding of this species. In fact, juveniles of this species naturally ingest zooplankton, phytoplankton, seeds, aquatic insect and other small benthic organism (Daget 1957; Lauzanne 1976; Micha 1976; Adite et al. 2005).

Protein requirements between fish species are complicated by the difference in species, size and age of fish, diet formulation, stocking density, protein quality, hygiene and experimental conditions between studies (NRC 1993). However, the dietary protein requirement of young Heterotis (3–62 g), as estimated in this study, was slightly lower or very close to the range determined for fingerlings of other omnivorous species, such as cyprinid fish Spinibarbus hollandi (327 g protein kg⁻¹ diet; Yang et al. 2003), common carp Cyprinus carpio (310 g protein kg⁻¹ diet; Takeuchi et al. 1979), jundia Rhamdia quelen (326 g protein kg⁻¹ diet; Meyer & Fracalossi 2004), channel catfish Ictalurus punctatus (280–320 g protein kg⁻¹ diet; Robinson et al. 2000) and Nile tilapia (280–300 g protein kg⁻¹ diet; De Silva et al. 1989; 300–360 g protein kg⁻¹ diet; Shiau 2002).

Dietary energy has a major impact on the utilization of dietary protein in fish, and energy affects quantitative requirements for protein (Wilson 2002). In this study, isoenergetic experimental diets were formulated by adjusting levels of fish and soybean meals with maize meal, wheat bran and fish oil. The protein to energy ratio also may influence the dietary protein requirements. Therefore, further study is needed to evaluate the response of Heterotis fingerlings to diets containing various protein and lipid levels, to establish the optimal dietary protein to energy ratio and to determine the maximum inclusion level of dietary lipid to spare protein for growth. Numerous investigations were conducted to elucidate the protein-sparing effect of many fish species (Hillestad & Johnsen 1994; Thoman et al. 1999; De Silva et al. 2002; Lee et al. 2002; Espinós et al. 2003; Meyer & Fracalossi 2004; Kim & Lee 2005).

From 250 to 300–350 g protein kg⁻¹ diet, statistical analysis on juveniles of 3–15 g indicated a significant increase in WG and SGR with increasing levels of dietary protein. Above this value, we observed a plateau in the growth rate. A similar trend was revealed in juveniles of 26–62 g, except that growth decreased above diet containing 350 g protein kg⁻¹ diet. Investigations with other fish species have reported the same kind of growth profile, fish growth either attaining a maximum output (Shiau & Huang 1989; Al Hafedh 1999; Lee et al. 2001; Kim et al. 2001; Yang et al. 2002; Giri et al. 2003; Islam & Tanaka 2004; Meyer & Fracalossi 2004) or decreasing (Henken et al. 1986; Yang et al. 2003; Ng et al. 2001) above a plateau varying from species to species.

In juveniles with mean body weight varying from 9 to 44 g, SGR values obtained in this study are 3.13. These growth rates are higher than those reported in other young omnivorous species such as Nile tilapia [0.49–0.75 according to Al Hafedh (1999) and 1.90–2.20 according to Abou et al. (2007)] and jundia (1.84–2.61) according to Meyer & Fracalossi (2004). However, these values are in the same order of magnitude as those reported for juveniles of African catfish Clarias gariepinus (2.17–3.10) according to Nyina-wamwiza et al. (2007). The SGR values obtained in this study are very much higher than those reported on the same species by different authors. Actually, SGR values of between 1.36 and 2.77 were reported for juveniles of Heterotis (30–50 g) fed with groundnut cake and brewer’s grain and 0.87 according to Bard (1960) under unfavourable feeding conditions; values of 0.47 (Okoye & Abubakar 1996) and 1.27 (Omorinkoba et al. 1991) have been reported for Heterotis fingerlings of 5 g reared in fertilized ponds and in polyculture with Nile tilapia and African catfish irregularly fed with pellets of 250 g protein kg⁻¹ diet. The good result recorded in this study is explained by the use of fish meal and soybean meal as major protein sources. Nevertheless, Tillon (1959) recorded an SGR value of 3.48 for Heterotis fingerlings (3–6 g) fed with cotton seed at 100 kg ha⁻¹ day⁻¹.

Feed efficiency values obtained in this study are 0.96, which are higher than those reported for young tilapia of similar sizes (Al Hafedh 1999). However, these values are in the same order of magnitude as those reported for juveniles of jundia R. quelen (0.47–0.87) according to Meyer & Fracalossi (2004). On the other hand, FE values obtained in this study are lower than those reported for fingerlings of African catfish C. gariepinus of similar sizes (0.85–1.29) according to Nyina-wamwiza et al. (2007). From the two experiments, FE values obtained with Heterotis fingerlings (3–15 g) were slightly lower than those obtained for juveniles (26–62 g). These data suggest that FE increased with the size of fingerlings in H. niloticus. Nonetheless, this hypothesis has to be further studied at other ontogenetic stages of this species.

From 250 to 300–350 g protein kg⁻¹ diet, statistical analysis on juveniles of 26–62 g indicated a significant increase in WG and FE with increasing levels of dietary protein. Above this value the WG does not significantly decrease, while FE still increases significantly. A similar trend was observed in fingerlings of 3–15 g. This kind of growth and FE profile obtained in this study can be explained by the decreasing feed intake in fish receiving dietary protein (from 250 to 400 g kg⁻¹ diet). Moreover, energy-balanced experimental diets were obtained by adjusting levels of fish and soybean meals with maize meal and wheat bran. These data suggest that the amount of non-protein energy in the feed probably affects intake in Heterotis fingerlings; and then dietary carbohydrate affects diet digestibility and growth in Heterotis fingerlings (3–62 g). In omnivorous fish such as common carp (Cyprinus carpio), Nile tilapia (Oreochromis niloticus) and channel catfish (Ictalurus punctatus), dietary carbohydrate utilization is more important (Wilson 1994). Therefore, more research is needed to study carbohydrate utilization in Heterotis fingerlings, to estimate the maximum dietary starch content acceptable without significant reduction of growth.

The whole-body protein, lipid, moisture and ash contents were not significantly affected by dietary protein levels. Similar results were observed in the carcass composition of Nile tilapia (El-Saidy & Gaber 2005). Other studies in which fish meal was used as main protein source for investigating the dietary protein requirement of fish showed that body ash content was not influenced by the dietary protein level (Shiau & Huang 1989; Yang et al. 2002, 2003).

Conclusion

This study is the first controlled nutritional research in H. niloticus, a promising species for Sub Saharan Africa aquaculture exploitation. The results of this study indicate that the maximum growth of Heterotis fingerlings (3–62 g) was achieved at about 345 g protein kg⁻¹ diet when fish and soybean meals were used as the major sources of protein. Using the broken line model, the dietary protein requirement for H. niloticus fingerlings (3–62 g) was estimated to be 310 g kg⁻¹ diet.