Growth performance, nutrient utilization and body indices of Nile tilapia (Oreochromis niloticus) fingerlings fed local feed ingredients

Leon Niyibizi^1,2, Simon Rukera Tabaro² and Aleksandar Vidakovic¹

¹ Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, P O Box 7024, SE-75007 Uppsala, Sweden
leon.niyibizi@slu.se
² Department of Animal Production, School Veterinary Medicine, College of Agriculture Animal Science & Veterinary Medicine (UR-CAVM), Ruhengeri, Rwanda

Abstract

A 10-week trial was conducted to evaluate growth performance, feed utilization, and somatic indices of Nile tilapia (Oreochromis niloticus) fingerlings fed five diets based on local feed protein ingredients (kidney bean leaf meal (KBLM), spent brewer’s grain (SBG), spent brewer’s yeast (SBY), sweet potato leaf meal (SPLM), and wheat middlings (WM) and a fishmeal-based control diet (CD). The experimental diets were formulated to be iso-nitrogenous and iso-energetic, with 27-50% of fishmeal in CD replaced (‘as is’ basis) with test ingredient. Initial average body weight of individual fish was 28.9±1.88 g and final body weight (FBW) was 60.2±2.81 g. Weight gain (WG) and FBW were highest (p<0.05) for fish fed CD, followed by SPLM, SBY, SBG, WM, and KBLM in that order. Specific growth rate (SGR) was highest in fish fed CD and SPLM, followed by SBY and SBG, and lowest in fish fed WM and KBLM. Feed conversion ratio (FCR) was highest in fish fed KBLM and lowest in fish fed CD and SPLM. Survival was 75-87% and did not differ between the groups. Hepato-somatic index (HSI) and viscera-somatic index (VSI) also did not differ across dietary treatments. These results indicate that SPLM, SBY, and SBG protein can efficiently replace fishmeal in Nile tilapia diets without adverse effects on growth, feed utilization, or body indices, acting as a valuable protein source for sustainable tilapia production.

Key words: agro-industrial by-products, aquaculture, fishmeal, Rwanda, vegetable ingredients

Introduction

Aquaculture is the fastest growing animal food-producing sector. It currently produces over 50% of all fish consumed worldwide and has high potential to meet the increasing global demand for aquatic foods created by global population growth (FAO 2018; Stevens et al 2018). Seafood is a balanced and nutritious foodstuff, and over 3 billion people worldwide consume fish protein as an essential part of their diet (FAO 2020). Aquaculture in Africa currently represents only 2.7% of global aquaculture, but this proportion is expected to increase by 48% in the near future, driven by additional aquaculture capacity introduced in recent years (FAO 2020). Most fish production in Africa is based on freshwater systems (99%) with tilapia (Oreochromis niloticus) and African catfish ( Clarias gariepinus) being the major cultured fish species (Adeleke et al 2021).

In Rwanda, aquaculture started in the 1940s as small-scale extensive tilapia pond farming, while from 1948 the nascent fish farming sector was promoted by the Belgian colonial authorities (Schmidt and Vincke 1981; Dadzie 1992). According to the latest annual report by Rwanda’s Ministry of Agriculture and Animal Resources (MINAGRI 2021) fish production reached an estimated 41,664 tonnes, with 7059 in 2020 tonnes from aquaculture.

Rwanda’s burgeoning aquaculture sector is predicted to expand further and to become an important source of high-value animal protein for the population. The country is endowed with important untapped aquaculture potential, including sufficient water resources, but future expansion will require use of locally available ingredients to make aquafeed, and robust fish species suitable for culture (such as tilapia). Local fish farmers are already struggling to obtain fish feed of good quality, which constitutes a major challenge to successful growth and intensification of aquaculture production in Rwanda. Feed constraints are most critical for Nile tilapia, which is by far the most commonly cultured fish species (89%) in Rwandan aquaculture (Niyibizi et al 2022).

Tilapia is a warm-water omnivorous fish species capable of utilizing nutrients from animal and plant feedstuffs (Felix et al. 2020; El-Sayed 1999) and agro-industrial by-products (Agboola et al 2021; Nhi et al 2018). It is the second most cultured and economically important fish species worldwide (FAO 2020), mainly due to its adaptation to a wide range of environmental conditions, tolerance against stressful conditions, acceptance of artificial diets for all stages of production, and high nutritional value (El-Sayed 2006). The future supply of fish for human diets will largely depend on the availability of farmed fish (FAO 2020). Due to its robustness, tilapia is likely to be an important fish species for sustainable aquaculture (Yue et al 2016).

Feed input is the single largest operating cost in aquaculture, representing 40-75% of total production costs (FAO 2018; Rumsey et al 1990) Approximately 70% of all commercial fish species produced in aquaculture rely on fishmeal-based feeds (Tacon 2020). Fishmeal is a major protein source in commercial fish feeds and is predominantly used in aquafeeds for high-trophic finfish and crustaceans. Its use in diets for low-trophic finfish like tilapia is limited (incorporated at 3-10%), where it is valued mostly for its ability to enhance growth (Tacon et al 2011). High inclusion rates of fishmeal are not sustainable, particularly in landlocked countries such as Rwanda where it is rarely available and where the fish species used in fishmeal are also used as food for humans (Niyibizi et al 2022). Identification of accessible, highly digestible proteins to reduce reliance upon expensive and unsustainable marine ingredients is essential, and numerous studies have examined the effect of replacing marine ingredients (meals and oils) in tilapia feeds with alternative ingredients such as low-cost, indigenous plant and animal ingredients (Lim et al. 2011; El-Saidy and Gaber 2003). Animal raw materials as an alternative to fishmeal have been tested in tilapia diets, with varying results. Poultry waste products can be included at rates up to 40%, but only limited amounts of blood meal (10%) can be used without affecting average weight gain (Felix et al 2020; El-Sayed 1998).

A recent study identified 31 ingredients in Rwanda worth investigating as tilapia feed alternatives (Niyibizi et al 2022). These included agro-industrial by-products such as spent brewer’s yeast (SBY), spent brewer’s grain (SBG), and wheat middlings (WM), and plant materials such as sweet potato leaves (SPL) and kidney bean leaves (KBL), which are all readily available ingredients. These ingredients could be suitable feed resources to increase the protein and energy content in fish diets in general and in tilapia diets in particular. Tilapia is an omnivorous species known to efficiently utilize high levels (30-70%) of dietary carbohydrates as a primary energy source, which allows protein to be saved for growth (Kamalam et al 2017; FAO 2018).

Spent brewer’s grain is the major by-product of the beer industry, representing ~ 85% of total by-products generated when producing beer (Mussatto 2014), and is thus an abundant feed resource. Brewing removes the soluble part of the grain, concentrating insoluble compounds as lignocellulosic residues (SBG) that are still rich in protein. This makes SBG potentially valuable as a high-volume and low-cost source of protein in the diet of farmed tilapia and e.g., striped catfish ( Pangasianodon hypophthalmus) (Jayant et al 2018; Zerai et al 2008; Mussatto et al 2006). Spent brewer’s yeast biomass is the second major by-product from the brewing industry is another potentially interesting feed ingredient for use in tilapia feeds (Marson et al 2020). In Rwanda, SBY and SBG have never been used in fish feed formulation, although their good availability and high crude protein content (380 and 266 g kg^-1 dry matter (DM) in SBY and SBG, respectively) indicate good potential as a feed ingredient for use in aquaculture (Niyibizi et al 2022).

Plant-derived ingredients are a readily available alternative that can potentially be used in fish feed, provided that they do not compromise the nutritional quality of the feed (Dorothy et al 2018; El-Sayed 1999). Plant protein sources, including soy and other legumes, are currently key alternatives to fishmeal in most commercial fish diets (Gatlin et al 2007), while little use has been made of sweet potato leaves and kidney bean leaves as feed ingredients in animal and fish diets (Adewolu 2008). Such plant-derived ingredients are relatively low-cost and locally available, which are advantages for sustainable aquaculture (Bergamin et al 2013).

Fishmeal and fish oil (currently widely used in aquafeed) are expensive and the supply is gradually becoming limited as the world’s fish stocks are either fully exploited or seriously depleted (FAO 2018; Tacon and Metian 2008). The sustainability of future commercial aquaculture will depend on reduced use of wild fish inputs in fish feed (Naylor et al 2000). A shift to alternative protein sources, especially by-products and other ingredients not used by humans, will be important and possibly cost-saving (Agboola et al 2021; Montoya-Camacho et al 2019; Gasco et al 2018). The challenge facing the aquaculture industry is to identify alternatives to fish oil that are economically viable and environmentally friendly, achieving least-cost and sustainable aquaculture production. The potential of alternative feed ingredients in fish diets can be established based on their proximate chemical composition (Mzengereza et al 2014). In Rwanda, there have been no attempts yet to replace fishmeal with plant and agro-industrial by-products in formulated tilapia feeds and there is a lack of empirical data on the feed potential of these materials. Thus the present study assessed growth performance of Nile tilapia fed experimental diets in which fishmeal was replaced by plant-derived ingredients and agro-industrial by-products.

Materials and methods

Study area and facilities

The study was carried out at the hatchery of University of Rwanda Fish Farming and Research Station (UR-FFRS), Rwasave, Huye campus, southern Rwanda (2^o40´S; 29^o45E), during the period August-December 2019. The experiment was conducted in a recirculating aquaculture system comprising 18 fiberglass tanks, each 100 L in volume, installed above 4480-L concrete tanks equipped with a mechanical and biological water filtration system. The recirculating system was continuously supplemented with fresh well water at a flow of 2 L min ^-1. The water was constantly aerated using an electric air pump connected to stone diffusers supplied in each tank to ensure adequate oxygen supply.

Fish and rearing conditions

A total of 360 mixed-sex Nile tilapia fingerlings bred at UR-FFRS were used in the feeding experiments. Before acclimatization, the fish were subjected a five-minute bathing treatment with NaCl (5 g L^-1) to remove potential ectoparasites, bacteria, or fungi (Barker et al 2002). All fish were weighed and measured (average weight 28.9 ±1.9 g, average length 11.8 ± 0.4 cm) and then 20 fish were randomly distributed to each of the 18 experimental tanks, with three replicate tanks per diet. The experimental tanks had a plastic mesh top cover to prevent fish from jumping out. The fish were acclimatized to the experimental conditions for one week, during which they were fed a commercial diet from Premier Animal Feed Industry (PAFI Ltd), Rwanda, and kept at a natural photoperiod of 12 hours light: 12 hours dark. All fish were weighed individually at the beginning and end of the experiment, using an electronic balance (Mettler PM4000, Hampton, NH 03842, USA). The three replicate tanks per dietary treatment were arranged in a completely randomized design.

Experimental diets

Ingredients used in this study (see Table 1) were purchased from local markets, obtained from food and beverage industries, or freshly harvested in local fields. Dried soybean grain was first rinsed in clean cool water and then autoclaved at 110 °C for 30 minutes to reduce anti-nutritional factors. Fresh cattle blood was cooked for 30 min. Treated soybean grain and blood were then sundried for 2-3 days. All 12 dried feed ingredients (Table 1) were ground to flour using a Grain Hammer Mill Crusher (GMEC-280 Zhengzhou Runxiang Machinery Equipment Co., Ltd, Zhengzhou, Henan, China) and then mechanically mixed (Santos 10Ltr Dough Mixer, Lyon, France) with vitamin and mineral premix for 5 minutes. Sunflower oil was then added and mixed in for an additional 5 minutes. Finally, a small amount of clean water was added and mixing continued for 10 minutes to form a homogenous dough, which was pelleted using a meat grinder (FAMA FTS107, Brugnera, Italy). The pellets (2 mm diameter) were sun-dried for 2-3 days and stored at -20 °C until use. A small portion (enough to be offered within five days) was regularly taken from main diet batch and kept at 5 °C in sealed food-grade plastic bags.

Six different diets were produced by replacing fishmeal in the control diet with locally produced ingredients, and formulated to meet the nutritional needs of Nile tilapia (NRC 2011). The control diet (CD) was fishmeal-based and the five test diets were formulated with maximum possible fishmeal replacement rate on an ‘as is’ basis without affecting crude protein and energy content of the diet. Proximate composition of test ingredients used to formulate the diets is presented in Table 2. The fish were hand-fed to satiation (three portions, at 09.00 h, 12.00 h, and 15.00 h) over the 70-day study period, starting with a pre-determined ration (approximately 4.5% of body weight per day) that was adjusted according to measured growth.

Sampling and measurements

On the day before the feeding experiment started, all 20 fish per tank were weighed, giving the initial weight per tank (W_i, sample zero). During the experiment, six fish were randomly netted from each tank every 14 days and weighed, to monitor intermediate body weight gain (BWG). Total number of fish remaining in each tank was also calculated on these occasions, for diet adjustment and mortality evaluation. At the end of the experiment, the remaining fish were counted and weighed (W_f, final biomass), after which they were anesthetized with 100 mg L^-1 of MS-222 and weighed again (BW, body weight) (Mettler PM4000, Hampton, NH 03842, USA). Three fish per tank were randomly collected and dissected for determination of hepato-somatic index (HSI, %) and viscero-somatic index (VSI, %).

Chemical analyses

Prior to experimental diet production, all ingredients were analyzed for their proximate chemical content at the Food Science Laboratory, College of Agriculture, Animal Sciences and Veterinary Medicine, Busogo campus, University of Rwanda. Moisture content was determined by oven-drying at 105 °C to constant weight. Ash content was determined by incineration of samples at 550 °C for 4 h. Total nitrogen (N) content was determined by the Kjeldahl method (KEL PLUS, Pelican Equipment, Chennai, Tamil Nadu, India) and crude protein (CP) was calculated as N x 6.25. Crude lipid (CL) was determined by Soxhlet ether extraction (ALCON.51, Alcon Scientific Industries, Ambala Cantt, Haryana, India), and crude fiber (CF) content was analyzed using standard methods described in AOAC (2000). Nitrogen-free extract (NFE) was calculated by subtracting the sum of crude protein, crude lipid/ether extract (EE), ash, and crude fiber from the corresponding dry matter value: (NFE (%) = DM-(CP +CL +CF +Ash). Gross energy (GE) was calculated as (CP x 23.6+CF x 39.5+NFE x 17.2)/100, expressed as MJ g^-1.

At the end of the experiment, four fish per tank were used for whole body analysis of crude protein, crude lipid, crude fiber, moisture and ash.

Water parameters such as pH, temperature (°C) and dissolved oxygen (mg L ^-1) were monitored twice daily (at 08.00 and 16.00 h) in each experimental tank, using a portable multi-parameter probe (Hanna HI 11310, Hanna Instruments Ltd., USA). Water temperature was kept at 27.0±0.1 °C using aquarium heaters (Aquazonic AZ-LED 100, Yi Hu Fish Farm Trading. Sungei Tengah, Singapore). Nitrite (mg L^-1) and ammonia (mg·L^-1) were monitored on a weekly basis using a Hach^® water analysis kit (DR/890 Colorimeter, Hach Company, Colorado, USA).

Growth performance and calculation of health indices

Growth performance and biological indices were calculated using the following equations:

Specific growth rate (SGR, %/day) = [(ln W_f−ln W_i)/T] ×100, where W_f is final weight and W_i is initial weight

Protein intake (g) = Feed intake (g) × Protein in the diet (%).

Total feed intake per fish (FI) = Total feed intake (g)/Number of fish

Survival rate (SR %) = (TF_f/TF_i) ×100, where TF _f is total number of fish at harvest and TF_i is total number of fish at start.

Feed conversion ratio (FCR) = Total feed intake (g)/Total wet weight gain (g)

PER = WG/PI, where WG is weight gain (g) and PI is protein intake (g)

Hepato-somatic index (HSI, %) = [100 × (Liver weight (g)/Body weight (g))].

Viscero-somatic index (VSI, %) = [100× (Viscera weight (g)/Body weight (g))].

Statistical analysis

Data on growth performance, feed utilization, and body composition were encoded into Microsoft Excel worksheets, and then imported into IBM SPSS STATISTIC (2011) program version 19 software for statistical analysis. One-way analysis of variance (ANOVA), followed by Duncan’s multiple range test, were used for comparisons of means (p<0.05 level of significance). Rearing tank was considered as experimental unit, and the same method was used for all parameter testing. All means were recorded, ± standard error of the mean (SEM).

Results

Proximate composition of feed ingredients and diets

The proximate composition of ingredients used in the experimental diets for Nile tilapia fingerlings is presented in Table 2. The CP content was highest in blood meal (701 g kg^-1 DM) and fish meal (547 g kg ^-1 DM), followed by soybean meal, SBY, cottonseed meal, SPLM, sunflower oil cake, and SBG (382 to 245 g kg^-1 DM). It was low in WM and maize middlings, rice bran and KBLM (178 to 67 g kg^-1 DM). The CL content varied by up to 10-fold between the ingredients (16-170 g kg^-1 DM), with the highest values in fishmeal and the lowest in blood meal. The CF content in the experimental ingredients showed different patterns, with a high content (>153 g kg^-1 DM) in soybean meal, cottonseed meal, rice bran, sunflower seedcake and SBG. The ash content ranged from 17 to 235 g kg^-1 DM, with the highest values in rice bran and SPLM (235 and 145 g kg^-1 DM, respectively) and the lowest in fishmeal (17 g kg^-1 DM). The NFE content varied between 141 and 618 g kg^-1 DM, with the lowest content in fishmeal and the highest in KBLM and WM.

The aim to produce iso-nitrogenous and iso-energetic diets was almost achieved, with CP content ranging between 282 and 300 g kg^-1 DM, and energy content between 16.5 and 17.3 MJ kg^-1 (Table 3).

Growth performance, feed utilization and somatic indices

Measurements of body weight changes over the rearing period showed a consistent trend of daily weight gain for all treatments (Figure 1). From day 14 until the end of the experiment, fish fed CD displayed higher growth than those in other treatments. The WM and KBLM diets consistently displayed lower growth performance.

Figure 1. Growth performance the 70-day of Nile tilapia (Oreochromis niloticus) fed the control diet (CD) and test diets based on spent brewer’s yeast (SBY), wheat middlings (WM), kidney bean leaf meal (KBLM), sweet potato leaf meal (SPLM), and spent brewer’s grain (SBG)

During the 70-day experimental period, fish grew from an initial average weight of 28.9±1.88 g/fish to a high final weight 60.2±2.81g/fish, and there was no difference in fish initial body weight between the treatments (Table 4). Final body weight (FBW) and weight gain (WG) were significantly highest for fish fed CD, followed by fish fed diets SPLM, SBY, SBG, WM, and KBLM, in that order. Specific growth rate (SGR) was significantly highest in CD and SPLM fish, followed by SBY and SBG fish, and lowest in fish fed diets WM and KBLM. Feed conversion ratio (FCR) was highest in KBLM fish and lowest in CD and SPLM fish. Other feed utilization indices, including feed intake (FI) and protein efficiency ratio (PER), were not significantly affected by dietary treatment and showed only small numerical differences across treatment groups. In addition, survival rate (SR) was not significantly different between treatments (range 75.0-87%) and no differences were detected in HIS and VSI between fish on CD and the test diets.

Water quality

Water quality parameters recorded during the experiment remained stable and showed no differences between treatments (p>0.05). Average temperature (°C) was 27.3±0.66, pH was 7.40±0.20, and dissolved oxygen content was 5.50±0.70 mg L^-1. The concentration of total ammonia-N and nitrite-N was 0.30±0.03 mg L^-1 and 0.10±0.02 mg L^-1, respectively.

Discussion

Tilapia fish fed SPLM (up to 32% replacement) achieved as good growth performance as fish fed the fishmeal-based control (CD), indicating high suitability of SPLM as a feed ingredient for tilapia fingerlings. This agrees with previous findings that tilapia, which is typically an omnivorous species, is capable of using nutrients from animal and plant feedstuffs, including leaves of vegetable crops such as sweet potato (Felix et al 2020; Adewolu 2008; El-Sayed 1999). High nutritive value of SPLM has been reported previously (Ishida 2000; Woolfe 1992). Furthermore, sweet potato leaves contain various bioactive compounds (Nguyen et al 2021) and several essential minerals (Fe, Ca, Mg) and essential trace elements (Cr, Co, Ni, Cu, Zn) (Taira et al 2013).

The fish in the present study fed brewer’s by-products (SBG, SBY) generally showed adequate growth performance relative to fish fed CD, indicating that tilapia can utilize high amounts of agro-industrial-by products in their diet, which agrees with findings by Felix et al (2020) and El-Sayed (1999). Up to 50% fishmeal replacement with SBY gave high growth performance, as evidenced by high SGR and FCR, and also WG and FWG similar to that in CD fish (Table 4). High growth of tilapia fed SBY is in accordance with findings in earlier studies on tilapia cultured in different systems (Islam et al 2021; Abdel-Tawwab et al 2020; Nhi et al 2018). Nhi et al (2018) reported good growth and protein efficiency for tilapia fed up to 30% SBY, but for fish cultured in a biofloc environment instead of a clear water recirculating system. The good performance observed in our study for SBY can be related to its good dietary qualities, making it suitable for use as an alternative to fishmeal protein in fish feed (Agboola et al 2021). For instance, it has a high protein content and favorable amino acid profile , in addition to containing important bioactive compounds such as β-glucan, nucleic acids, mannan oligosaccharides, etc. that can substantially improve fish growth and health (Vidakovic et al 2020; Øvrum Hansen et al 2019; Ferreira et al 2010). Thus, according to our results and those in other studies, SBY can partly replace fishmeal in commercial diets for Nile tilapia.

The diets with SBG (32% replacement) and SPLM (32% replacement) gave only minor differences in weight gain compared with the control. SBG was moderately rich in CP (23.4 ± 0.2), CL 9.4, and cellulose 51 ± 0.7 and also contains other nutrients (Yu et al 2020). The CL content in our study was higher than the 66 g kg^-1 DM reported by Nhi et al (2018), and much higher than the 39 g kg^-1 DM reported for oven-dried SBG by Santos et al. (2003). Elevated dietary lipid content above the minimum required level can support higher growth rates, partly due to protein-saving effects (NRC 2011). Our values were within the optimum range (5-12% lipid) reported for tilapia diets (Lim et al 2011). However, higher dietary lipid content may increase flesh lipid levels in freshwater fish (Guo et al 2019).

In general, growth performance indices may vary due to differences in nutritional quality or properties between feed ingredients used, size and age of fish, and culture systems, in addition to environmental conditions, feeding duration, and other unknown factors (Nhi et al 2018; Liti et al 2006). For ingredients such as brewer’s by-products, nutritional quality or properties may also vary with factors including the type of barley, malting and mashing conditions, and additives used during beer processing (Robertson et al 2010). Our results indicated that the body indices evaluated (HSI, VSI) did not differ across treatments, i.e., there were no significant effects of the dietary treatments on the physiological condition of the fish, fat accumulation and adaptation to the environment, and thus on fish welfare (Robb 2008).

From our results, it can be concluded that SPLM, SBY, and SBG are nutritionally adequate as sources of protein, fiber, carbohydrate, and energy and can be beneficial for tilapia fish growth performance, so their use in commercial tilapia diets can be recommended. The experimental diets containing those ingredients also did not affect fish somatic indices, so there were no obvious negative health effects in any treatment. In contrast, the results showed that weight gain for fish fed the KBLM and WM diets only reached about 40% and 45% of that in the control (CD) group. Fish fed KBLM and WM showed consistently decreasing growth (FW, WG, SGR), even though the replacement rate of fishmeal was only 5-7 g/kg in those diets, indicating that KBLM and WM should not be included as feed ingredients or should be kept at low levels in diets so as not to affect growth of tilapia. Tilapia has the capability to efficiently utilize high levels (30-70%) of dietary carbohydrates as a primary energy source, giving protein-saving effects for growth (FAO 2018; Kamalam et al 2017). Fish fed WM and KBLM displayed the worst growth performance of all treatments, possibly due to the presence of anti-nutritional factors such as phytate, trypsin inhibitor, and polyphenols commonly found in cereals and legumes, which reduce the bioavailability of nutrients and minerals (Ram et al 2020). Cereal grain has low overall phytic acid content (1-2% by weight), but in wheat this compound is concentrated in the external cover of the pericarp and the aleurone layer (Brouns et al 2012), which make up the wheat middlings fraction. Previous studies have concluded that use of plant-derived materials, including legume seeds, leaf meals, and root tuber meals, as fish feed ingredients is limited by the presence of different anti-nutritional substances, particularly protease inhibitors, phytates, glucosinolates, saponins, tannins, lectins, oligosaccharides and non-starch polysaccharides, gossypols, cyanogens, mimosine, and antivitamins (Francis et al 2001;Vasconcelos and Oliveira 2004). Tannin, oxalate, and phytate have been detected in bean leaves (Alalade et al 2016). Based on results in the present study, WM and KBLM should only be used in limited amounts in the diet of tilapia.

Conclusions

Inclusion of SPLM in the diet of tilapia resulted in no or minor differences in WG, FWG, SGR and FCR compared with a control diet based on fishmeal, while inclusion of SBY and SBG gave only minor differences. Therefore these alternative protein sources can replace fishmeal in the diet of Nile tilapia without adverse effects on growth, feed utilization, or body indices, and can be valuable for future sustainable tilapia production in Rwanda and other countries in Africa. However, inclusion of KBLM and WM gave consistently poor growth in fish, indicating that these ingredients should be excluded or kept at low levels in diets so as not to affect growth of tilapia. These findings have practical implications for optimized inclusion of local ingredients, allowing aquaculture nutritionists to tailor practical diet formulations.

Acknowledgements

The authors would like to express sincere thanks to Professor Torbjörn Lundh, SLU, for his valuable contribution to this study. We thank hatchery and laboratory technicians in UR-FFRS at Rwasave and Busogo campus in the College of Agriculture, Animal Sciences and Veterinary Medicine/ University of Rwanda, for their valuable help during sample handling and proximate analysis.

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