Effect of fermented aquatic macrophytes supplementation on growth performance, feed efficiency and digestibility of Nile Tilapia (Oreochromis niloticus) juveniles fed low fishmeal diets

Y Cruz Velásquez¹, C Kijora, S Wuertz^2,3 and C Schulz^2,4

Animal Breeding in the Tropics and Subtropics, Humboldt Universitaet zu Berlin. Philippstraße 13. D-10115, Berlin, Germany.
yorcelis.cruz@hu-berlin.de
¹ Instituto de Investigaciones Tropicales de la Universidad Del Magdalena (Intropic).
Carrera 32 No 22 – 08. A.P.:2-1-21630. Santa Marta, Colombia.
² GMA - Gesellschaft für Marine Aquakultur mbH, Hafentörn 3, 25761 Büsum, Germany
³ Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Dep Ecophysiology and Aquaculture, Müggelseedamm 310, 12587 Berlin, Germany
⁴ Institute for Animal Husbandry at the Christian-Albrechts-University of Kiel. Olshausenstraße 40 D-24098 Kiel, Germany

Abstract

An 8-week feeding experiment was conducted to evaluate the growth performance, feed efficiency, digestibility, carcass composition and, liver and intestine histology of juvenile Nile Tilapia (3.18 ± 0.02 g) fed on a control diet and four experimental diets (35% CP). The experiment aimed to assess the utilization of local macrophytes; namely fermented duckweeds at 15% (DW15) and 25% (DW25) as well as fermented water fern at 15% (WF15) and 25% (WF25).

The growth performance comprising of weight gain (WG), specific growth ratio (SGR) and protein efficiency ratio (PER) was assessed in triplicates and no significant differences between diets were observed. Feed conversion ratio (FCR) and ash content of the carcass were similar in all the treatments except for WF25 where significantly higher FCR and ash content were recorded compared to the control diet. However, lipid and protein content of the carcass was comparable between diets. Results showed that the inclusion of fermented duckweeds up to 25% and fermented water fern up to 15% in diets for Nile Tilapia was feasible without any negative effect compared to the control group.

Keywords: duckweed, histology, plant ingredients, water fern

Introduction

In contrast to Asia, where aquaculture industry is primarily dependent on farm-made feeds, Latin America’s industry is mainly dependent on manufactured feeds. Most operations of aquaculture industry in Latin America consist of semi-intensive and intensive production systems involving the subsequent use of commercial aquafeeds based on fish meal (Flores-Nava 2007). During the last years, domestic fish consumption and fish production in the region increased strongly (Flores-Nava 2007), consequently increasing the demand for aquafeeds. Thus, industrially manufactured feed has become more readily available in most Latin American countries, where it is produced by large-scale manufactures. In contrast, small-scale producers in rural areas do not have the capital for these costly commercial feeds and farm-made feeds are only occasionally produced in localized regions where locally grown agricultural by-products are utilized to replace or complement costly commercial diets (Hasan et al 2007). A basic knowledge of the use of alternative ingredients, especially of plant based sources, could therefore significantly contribute to the production of low cost, farm-made feeds for local fish production.

Imbalanced essential amino acid profile, the presence of antinutritional substances, low digestibility and poor palatability are the most common problems encountered when using plant sources as fish feed ingredients (Lovell 1989; Tacon 1994; Francis et al 2001). For a successful inclusion of plant based ingredients into diets, it is necessary to evaluate growth performance and feed conversion, and carefully monitor the potential adverse effects on the fish physiology. Aquatic macrophytes are known to be among the potential candidates for local production of aquafeeds (El-Sayed 2003; Bairagi et al 2002; El-Sayed 1999; Leng et al 1995). Moreover, they are widely used as feed for cultured omni-herbivorous fish in many Asian countries where aquaculture is primarily rural (Hasan and Chakrabarti 2009) and finances do not allow for expensive feed sources. Usually, they are processed either as sun dried material or after ensiling in order to improve their digestibility by fish (El-Sayed 2003; Bairagi et al 2002).

Due to its nutritional requirements, Tilapia can be grown on plant-based diets including algae, aquatic plants and a variety of feeds from diverse origins (Watanabe 2002) and thus it is relatively inexpensive. This makes it particularly beneficial in meeting the needs of small-scale farms in rural Colombia. A number of studies have been carried out on the use of different macrophytes in Tilapia feed (Mandal et al 2010; Abdelhamid et al 2010; Abdel-Tawwab 2008; Abdelhamid et al 2006; El-Sayed 2003; Fasakin et al 1999).

Tilapia is consequently an ideal species for aquaculture and it is in high demand in the majority of the international and domestic markets. In the many countries of Latin America where Tilapia has been introduced, it plays an important role as high quality food source, especially in rural communities. In Colombia’s national aquaculture production, Tilapias occupy the first place (principally red Tilapia), followed by Cachama (Piaractus brachypomus) production (Espinal et al 2005; FAO 2012). Unfortunately, its production in rural regions is limited due to the high costs of fish feeds.

In an effort to minimize the use of expensive ingredients and reduce the production costs of Tilapia in small-scale aquaculture systems, the present study aimed to evaluate the utilization of fermented locally available aquatic macrophytes in a low fish meal content (3% of DM) diet.

Material and methods

Experimental diets

A total of five isonitrogenous (35% crude protein) and isoenergetic (17 kJ/g gross energy) diets were formulated. Soy cake and maizegluten were used as the main plant protein sources for the diets. Due to their comparable nutrient composition wheat bran was replaced by fermented duckweeds (Lemna minor and Spirodela polyrhiza) at 15% (DW15) and 25% (DW25) and by fermented water fern (Azolla filiculoides) at 15% (WF15) and 25% (WF25) inclusion levels. In each diet 3% fishmeal was included to assure palatability of the experimental diets. To maintain the level of protein in the low-fishmeal content diets, wheat bran as well as alphacellulose were removed as fermented duckweed or fern were added.

Aquatic macrophytes were fermented using a commercial silage LAB inoculant based on the bacterial strain Lactobacillus plantarum DSM 8862 and DSM 8866 (BIO-SIL^®, Dr. Pieper Technologie-und Produktentwicklung GmbH, Germany). For fermentation, freshly harvested plants were mixed with a part of dried aquatic macrophytes of the same sample until about 350 to 450 g/kg dry matter content was obtained. Molasses at 15% was added and the mixture was vacuum packed into plastic bags according to the method described by Johnson et al (2005). After four weeks plastic bags were opened and samples were dried in an oven at 45 °C for 48 h. Afterwards proximal composition of the fermented plant material was determined. Fermented macrophytes were then used to prepare the experimental diets. All ingredients of the diets were well ground, mixed and finally pelleted to pieces of 4 mm in diameter. Chromic oxide at 0.5% was added as an inert marker.

The proximate composition of the tested diets is shown in Table 1. The most prominent difference between diets was the increased ash and crude fibre content in the DW and WF diets compared to the control diet. Between experimental diets WF showed stronger increase in ash content (by 5% and 17% respectively) than DW at corresponding inclusion levels. In contrast, the fibre content depended only on the inclusion level of aquatic macrophytes and was found to be higher than in the control diet. Lipid content was relatively low but comparable in all diets.

Growth trial

The experiment was carried out at the aquaculture facilities of Humboldt University, Berlin. A total of 225 juveniles of Nile Tilapia ( niloticus) with an initial body mass of 3.18 ± 0.02 g (mean ± SD) were randomly distributed into fifteen 200L flow-through glass aquaria at a density of 15 fish per aquarium, providing three replicates for each experimental diet. Water parameters were monitored weekly and were within optimum ranges for normal growth and health of Tilapia juveniles according to Stickney (1979): 25.3 ± 1.07 °C, pH 8.43 ± 0.17, < 0.3 mgl/L NO _2, < 0.01 mgl/L NH_4,, 8.79 ±0.57 mgl/L dissolved oxygen respectively.

Fish were fed a commercial diet for two weeks of acclimatisation. Afterwards, the experimental diets were randomly assigned to the aquaria. Fish were handfed to apparent satiation for 56 days, twice a day at 09:00 and 17:00 h. Fish were weighed in bulk every two weeks and additionally, the total length and weight were measured individually at the end of the trial. During the experiment a photoperiod of 12 h light: 12 h darkness was maintained. At the end of the experiment three fish from each aquarium were sampled as bulk samples (n = 3 per treatment) for carcass analysis and stored at –20 °C.

Additionally, six individuals from each dietary group were dissected and samples of the liver and the intestine were weighed and taken for histological analysis.

Apparent nutrient digestibility

During the last three weeks of the experimental period, faecal samples were collected twice from each aquarium between the feeding times at 1 h intervals. Samples were collected using a filtering net, rinsed in distilled water, then transferred to a filter paper, oven dried at 60 °C for 72 h and stored until laboratory analysis. Apparent digestibility coefficients (ADC) of dry matter, ash, protein and lipid were calculated using chromic oxide (Cr₂O₃) as inert marker following the formula described by Nose (1960):

ADC_diet = 100-100*(%Cr₂O_3diet/%Cr₂O_3faeces)*(%Nut_faeces/%Nut_diet)

Where:

ADC_diet = Apparent digestibility coefficient of the nutrients or energy in diets

%Cr₂O_3diet = % of chromium in diets

%Cr₂O_3faeces = % of chromium content in faeces

%Nut_diet = % of nutrient or energy in diets

%Nut_faeces = % of nutrient or energy in faeces

Chemical analyses

Proximate analysis of the diets, faeces and carcass was performed following AOAC (2005) procedures. Gross energy (GE) was determined by using an adiabatic bomb calorimeter (Parr 121 EA, USA). Chromic oxide in diets and faeces was determined spectrophotometrically by the method of Furukawa and Tsukahara (1966). All samples were analyzed in duplicate.

Histological analyses

For histological analysis, six fish per dietary group were dissected and the liver as well as the sac-like stomach was sampled. Macroscopically, no distinct regions of the stomach were sampled in order to standardize sampling with regard to controversial reports on distinct regions of the stomach in Nile Tilapia O. niloticus (Caceci et al 1997; Osman and Caceci, 1991; Al-Hussaini and Kholy 1953). Samples were fixed in 10% phosphate-buffered formalin, gradually dehydrated, cleared, embedded in paraffin, cut to 5 µm slices using a microtome (Leica RM 2135) and stained with haematoxylin and eosin (HE). Analysis was carried out with a light microscope (Leica DM 2500) equipped with a digital camera (Leica DFC 420). For the histological analysis any alterations and abnormalities were recorded. During histological analysis, samples of liver and stomach from fish O. niloticus fed standard commercial feed (standard group) were compared to the fish fed control and test diets.

Data calculation and statistical analyses

The criteria used to determinate growth performance, feed utilization and morphological measurements were:

- Specific growth rate (SGR) = [ln Wf (mean final weight) − ln Wi (mean initial weight)/56 days (d)] × 100

- Percent weight gain (WG) = 100(Final weight-Initial weight)/ Initial weight

- Feed conversion ratio (FCR) = total feed intake in dry basis (g) / wet weight gain (g)

- Protein efficiency ratio (PER) = total weight gain (g)/protein intake (g)

- Hepatosomatic Index (HSI) = [Liver mass (g) / body mass (g)] × 100

- Intestinal Somatic Index (ISI) = [Intestine mass (g) / body mass (g)] × 100

Data from each treatment were subjected to one-way analysis of variance (ANOVA) and are presented as mean ± standard deviation (SD) of triplicate groups (n=3). Data were analysed for normal distribution by Kolmogorov–Smirnov and equal variance by Levene Test (passed if p < 0.05) using SPSS 17.0 for Windows. For multiple comparisons, parametric Tukey’s multiple range test or non-parametric Dunnett`s T3 were used. Individuals were sampled from each replicate for histology analysis.

Results

Water quality parameters were optimal for Nile Tilapia and the percentage of survival ranged from 95.3% to 100% (Table 2). The parameters of growth, final weight (Wf), weight gain (WG) and specific growth rate (SGR) ranged from 15.5 to 16.6 g, 377-419% and from 2.8 to 2.9 g/day and were not different. DW15 and WF15 had a tendency to show higher values than control and the 25% supplementation group (Figure 3). In contrast, the feed efficiency parameters were significantly different displaying better values for control group and DW15. This is a result of higher feed consumption in DW25 and WF15 and WF25 (up to 140% in WF25).

Digestibility analysis (Table 2) revealed that dry matter, ash, lipid and protein digestibility were highest in the control at 88%, 84%, 98%, 96% followed by 15% inclusions. All nutrients showed significantly lower digestibility in the 25% groups compared to control group. In comparing the two aquatic plants, DW groups presented the tendency to have higher ADC values than WF groups at the same inclusion levels.

Figure 1. Mean body (mean ± SE, n=3) weight of Nile Tilapia fed experimental diets for 8 weeks. No significant differences between experimental groups were detected

The proximate carcass composition and morphological measurements of Nile Tilapia are presented in Table 4. There were no differences in the carcass protein (39.3 - 40.5%) and carcass lipid content (20.4 - 22.1%) between all treatments. The carcass ash content in group WF25 (7.1%) was significantly higher than in the control group (6.6%). No significant differences between the dietary groups were recorded in the ISI ranging from 4.97 (WF15) to 5.51 (WF25). Whereas, low HIS was observed at high macrophytes incorporations, in WF25 and DW25, 1.80 and 1.88, respectively, but a significant difference to the control was not detected.

No striking differences in fat accumulation between the intestinal coils were observed upon dissection. Histology of the sac-like stomach is illustrated in Figure 1. The digestive tract exhibited the typical four-layered structure comprising a folded mucosa, the submucosa, the muscularis and the serosa, which is typically found in vertebrates. The strongly folded mucosa consisted of a single-layered columnar epithelium with interspersed mucus-secreting goblet cells facing the lumen of the stomach. A muscularis mucosa separating the lamina propria from the submucosa was not observed here, forming a connective tissue core with blood vessels. The thin submucosa extended into the folds, thereby supporting the mucosa. Two prominent layers of striated muscle (inner longitudinal. outer circular) formed the muscularis, which was succeeded by the serosa. Fish fed experimental diets had a similar appearance when compared to the control. Consequently, the diets did not affect the morphology of the stomach.

Figure 2. Histology of the sac-like stomach of Tilapia revealing no differences in fish fed control and experimental diets. Serosa (Se). muscularis (mus), longitudinal muscle (lm), circular muscle (cm), submucosa (su), mucosa (muc). Scale bars: 150µm. (H&E).

Macroscopically, the hepatopancreas in all fish were light brown (Figure 3) not indicating severe fattening as suggested by pale coloration in other studies, irrespective of diet fed. Histopathology of the liver did not reveal major abnormalities, neither in the control diets nor in the fish fed experimental diets. Hepatocytes were arranged in a typical parenchyma and pancratic tissue was evenly scattered within the liver tissue in close proximity to the blood vessels. Sinoids were irregularly distributed between the polygonal hepatocytes without any abnormalities such as congestion of sinoids. Congruently, yellow ceroid pigments, indicating nutritional stress, were rarely observed. Again, no difference between dietary groups was observed.

Figure 3. Histological sections of the hepatopancreas of Nile Tilapia in control fish group (a). and fish fed experimental diets DW15 (b). WF15 (c) and WF25 (d). Exogenous pancreas (P) embedded in liver tissue (L). Scale bars: 100µm. (HE-stained).

Discussion

The digestive system of Tilapias is relatively simple, comprising of a short oesophagus, a small sack-like stomach, and a very long coiled intestine, which can reach 7 to 13 times bigger in length compared to the total fish length (Beveridge and Baird 2000). This long digestive system indicates the herbivorous feeding behaviour of Tilapias. Therefore, this study focused on the utilization of plant material as feed sources. El-Sayed (1999) has reviewed this topic and has reported the potential of terrestrial plants (oilseed plants, grain legumes and plant concentrate proteins. and aquatic plants) as dietary protein sources for Tilapias. Although plant material results in a low biological performance compared to standard fish diets, he has recommended its use as fish feed due to its economical efficiency.

In Colombia aquatic plants have a high potential as fish feed since they are not used as food and they are greatly available in rural regions throughout the whole year. The nutritive value and potential of dried aquatic macrophytes as plant protein for fish feed has been examined in numerous studies (Fasakin et al 1999; Kalita et al 2007 and Abdel-Tawwab 2008). It was reported that an inclusion level up to 25% supported fish growth, when fish meal content ranged between 7.5 to 22% in the diet. More recently, Cruz et al (2011) evaluated the nutritional potential of sundried and fermented aquatic plants found in rural areas of Colombia. They recommended the used of fermented plant material as fermentation reduced the content of antinutrients and fibre.

Studies on evaluating aquatic macrophytes as feed ingredient for Nile Tilapia reported similar or even lower SGR than those reported in this study. El-Sayed (2003) reported SGR of 2.9 and 2.8 (%/day) for fish fed on diets (35% CP and 38% FM inclusion level) containing 20% of molasses-fermented and yeast-fermented water hyacinth (Eichornia crassipes), respectively. In our study with comparable CP, but lower FM inclusion (3% FM), a SGR of 2.8 and 2.9 (%/day) was achieved at 25% and 15% fermented macrophytes inclusion. Abdel-Tawwab (2008) reported a lowest SGR of 0.8 (%/day) for Nile Tilapia fed on diets supplemented with sundried Azolla pinnata at 25%. In his study diets contained 20% CP and 10% FM inclusion level. In other study, Nhi and Preston (2011) used a biodigester effluent as fertilizer for the natural food in ponds and for the production of duckweeds as supplement feed for fish. They reported a SGR of 2.67 and 2.39 for Nile Tilapia growing in an integrated culture with Common carp (Cyprinus carpio) at a density of 3 and 5 fish/ m², respectively. In a later study, Nhi and Preston (2012), they reported a highest SGR of 3.35 for Nile Tilapia in monoculture by using natural feed from the effluent-fertilized ponds and duckweeds as supplement feed. The growth rate in this study was supported by the natural food chain in the ponds.

Although unconventional plant protein sources frequently limit fish growth performance (In the literature, many factors are enumerated to explain this effect: the reduced protein content, the unbalanced amino acid profile, the high fibre and ash content, and the presence of antinutritional substances). The study of Nhi and Preston (2012) showed that the use of unconventional feed resources, which are locally available can be better used into alternative systems of fish production based on recycle of organic wastes. So fish production would not depend on costly feed and economic profit would favour the farmers.

The tested aquatic macrophytes showed moderate protein contents (241 - 264 g/kg CP) compared to standard commercial plant ingredients. However, the amino acid profile of DW and WF seems to fulfil the requirements for lysine and methionine of common cultured tropical fish (Cruz et al 2011). To avoid adverse effects of antinutritional substances the tested aquatic macrophytes were fermented. As Cruz et al (2011) showed, trypsin inhibitor, phytates, soluble and condensed tannins, and oxalates were tremendously diminished.

Since fibre content in DW25 (134 g/kg) and WF25 (137 g/kg) diets was notably higher than in the control diet (68.2 g/kg) overall digestibility might have been reduced in comparison to the control diet. In the past, increased fibre content of diets containing plant ingredients have shown to negatively affect weight gain, growth response and protein utilization of Nile Tilapia (Omoregie and Ogbemudia 1993; Fagbenro et al 2004). Also, Anderson et al (1984) reported that a fibre level above 100 g/kg reduced feed efficiency and nutrient digestibility of Nile Tilapia causing poor fish growth. Here a high fibre content (over 13%) at 25% DW and WF revealed significantly reduced dry matter.

In fact, the significantly higher FCR (2.5) in WF25 diets when compared to FCR (1.8) in control diets can be attributed to the high dietary fibre and ash content of the WF25 diets. An even higher FCR (4.2) was reported by Abdel-Tawwab (2008) for Nile Tilapia fed on diets supplemented with sundried WF ( Azolla pinnata) at 25% inclusion level. In contrast, El-Sayed (2003) reported lower FCR (from 1.6 to 1.8) for fish fed on diets containing 20% of molasses-fermented and yeast-fermented water hyacinth, suggesting a better FCR of fermented ingredients. Tick and Preston (2012) reported a much better FCR from 0.76 to 1.08 for Nile Tilapia fed on duckweeds and growing in effluent-fertilized ponds with natural feed supply.

Increased dry matter, ash content as well as protein digestibility revealed that feed utilization in DW15. DW25 and WF15 was comparable to the control, but was substantially reduced in WF25. Interestingly, ADC of lipids was significantly lower in diets containing higher levels of both aquatic macrophytes and particularly in those containing WF. This could possibly be explained by the low lipid content (31.0 g/kg) of aquatic macrophytes which could have also affected growth and feed utilisation as a result of the reduction of available dietary energy and deficiency of essential fatty acids. Otherwise, lipid body composition of Nile Tilapia fed the test diets was not significantly affected by dietary treatments. This coincides with El-Sayed (2003) findings evaluating fermented water hyacinth for Nile Tilapia. Even so, the level of whole body ash was significantly higher in fish fed on WF25 diets compared to the control group, which may be attributed to the particularly high ash content in WF.

Differences between the ISI values were not found between the diets. However, the HSI was significantly lower in fish fed on DW25 and WF25. This could be attributed to the significantly lower deposits of fat in these groups directly affecting the size of the liver and could be furthermore explained by the lowest ADC of lipids found in the diets containing higher levels of macrophytes. Tusche et al (2011) also referred to this observation and reported it as a sign of the effects of short-term starvation on fish hepatocytes.

In conclusion, fermented DW and WF up to 15% can be utilised in low-fish meal diets to reduce feeding costs without an impact on growth performance, feed conversion and animal welfare.

Acknowledgements

The authors gratefully acknowledge the German Academic Exchange Services (DAAD) for the PhD scholarship award for Ms. Cruz as well as the Instituto Colombiano para el desarrollo de la ciencia y la tecnología (Colciencias) and the Universidad del Magdalena for the financial support. The authors would also like to thank Prof. Dr. Werner Kloas and Prof. Dr. Frank Kirschbaum for providing the laboratory facilities to conduct the histological analyses. This work was financially supported by Project Colciencias Cod. 1117-452-21305

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Received 20 August 2014; Accepted 20 June 2015; Published 1 September 2015

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