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Technical and economical evaluation of periphyton-based cage culture of tilapia (Oreochromis niloticus) in tropical freshwater cages
Aquaculture 218 (2003) 219 – 234 www.elsevier.com/locate/aqua-online
Technical and economical evaluation of periphyton-based cage culture of tilapia (Oreochromis niloticus) in tropical freshwater cages S.M.H. Huchette *, M.C.M. Beveridge1 Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, UK Received 14 November 2001; received in revised form 15 March 2002; accepted 2 August 2002
Abstract A 3-month study (March – June) was carried out in the Meghna – Gumti River (Bangladesh) to assess the potential for periphyton-based cage aquaculture. The growth of genetically improved farmed tilapia (GIFT) strain (Oreochromis niloticus) was monitored during a trial conducted in nine 2.5-m3 floating cages. The importance of periphyton in tilapia diets was assessed using three treatments. In the first treatment, the periphyton biomass was increased using additional substrates placed in the cage. In the second treatment, additional substrates were used and the diet was supplemented with a locally available compound feed. The third treatment had no additional substrate but received some supplementary feed. The effect of grazing on periphyton biomass was also assessed in a parallel experiment. Biomass was assessed by measuring ash, ash-free dry weight (AFDW), chlorophyll a, phaeopigments and respiration. Net production of fish ranged from 0.12 to 0.19 kg m 3 month 1 and specific growth rate (SGR) from 0.7% to 0.95% body weight day 1. Jaw abnormalities affected 33% of fish and reproduction was observed in fish as small as 35 g. Productivity of a 1 m2 of net located in the uppermost 50 cm of the water column, however, was estimated at 0.94 g fish day 1. The periphyton biomass dropped in just 2 days from 1.12 to 0.34 mg/ cm2 after the introduction of the fish in the cage. The fish fed on plankton, artificial supplementary feed and periphyton growing on substrate in the cage. Filter feeding and grazing on the cage net appeared to be the most important source of energy. The effect of the additional substrates placed in the cages was marginal. The periphyton-based cage aquaculture described here was not economically viable and the main economic constraints were the costs of cage material (net and bamboo poles) and
* Corresponding author. Present address: Department of Zoology, The University of Melbourne, 3052 Parkville, Melbourne, Victoria, Australia. E-mail address: [email protected] (S.M.H. Huchette). 1 Present address: FRS Fresh Fish. Laboratory, Faskally, Pitlochry, Perthshire PH16 5LB, Scotland, UK.
0044-8486/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 2 ) 0 0 4 1 4 - 3
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fry. Periphyton-based cage culture could be made more profitable by using local fish species, less expensive cage design, better substrate and by using adapted management techniques to improve periphyton productivity. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Tilapia; Biomass; Periphyton-based cage aquaculture
1. Introduction Different forms of fish culture have been developed in Bangladesh during the past 20 years. Among them, pond culture and rice – fish culture have been developed and made popular by various organisations and are now widespread. Rice –fish culture is now developing very fast in areas such as Parbatipur where the fingerling demand doubled between 1994 and 1995 (Morrice, 1997), and the supply of fingerlings is found to be a limiting factor (Ireland, personal communication). However, pond aquaculture or, at a lower scale, rice –fish culture has had a negative impact on the wild fisheries and is profitable only to the landowners. Cage aquaculture has been promoted in the past in Bangladesh; however, long-term adoption rate was very poor (Gregory and Kamp, 1995). With the development of this type of aquaculture, the landless people have seen their fishing grounds reduced and the quantities of small wild fish, which is the only fish affordable for them, have been declining as it is progressively replaced by the production of food fish (CNRS, 1995; Lewis et al., 1996). This situation is gradually getting worse as the demand for food fish in the urban markets is stimulating more commercially orientated productions. When promoted in a manner appropriate to local needs and resources, cage aquaculture can be successfully used to alleviate poverty among the rural poor. Since 1995, the Cage Aquaculture for Greater Economic Security (CAGES) development project of CAREBangladesh has successfully assisted more than 2500 families to successfully adopt cage farming (Hambrey et al., 2001; Mc Andrew et al., 2002). Nevertheless, one of the main factors limiting the access to aquaculture by the poorer sectors of society in the country remains to be the requirement for inputs of seed and feed (Hambrey et al., 2001). Dempster et al. (1993, 1995) demonstrated that Nile tilapia (Oreochromis niloticus L., 1757) could graze efficiently on periphyton, the community of microscopic plants and animals that attaches to the surface of stones and plants. In the Ivory Coast, the ‘acadja’ (Legendre et al., 1989), artificial reefs made of branches and bundles in African lagoons, and ‘acadja enclos’ (Hem and Avit, 1994), acadja permanently enclosed by nets in which fish are self-recruiting, and in Bangladesh, the Khata fisheries (MacGrory and Williams, 1996), aggregation of water hyacinth surrounded by bamboo poles planted in the river bed, are three traditional fish attracting device (FAD)-based fisheries systems. Studies of these systems recognised that fish production might be enhanced by grazing on the periphytic community that colonizes the substrates. Using bamboo or wooden sticks to enhance production of periphyton in aquaculture ponds (Wahab et al., 1999a,b) has recently been explored in order to reduce feeding inputs, which usually represents over 50% of operating costs (El-Sayed, 1999). The type of substrate (Keshavanath et al., 2001) and the
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fertilisation method (Azim et al., 2001b) used to have a significant effect on periphyton productivity and quality in ponds and subsequently on fish production. Periphyton-based aquaculture may improve the productivity for some herbivorous fish species such as tilapia (Hem and Avit, 1994), Labeo calbasu (Hamilton) (Wahab et al., 1999a), Tor khudree (Sykes) (Keshavanath et al., 2001) and Labeo rohita (Hamilton) but not others such as Labeo gonius (Linnaeus) (Azim et al., 2001a). Norberg (1999) has examined periphyton production in net cages and concluded that it contributed up to 1% of the energy requirement of caged tilapia. Huchette et al. (2000) showed that grazing significantly reduced periphyton biomass and reduced the biodiversity of the algal community by selectively removing larger species. Thus, grazing pressure has to be monitored carefully to maintain a higher productivity (Lamberti and Moore, 1984). This study sets out to assess the potential economical and technical feasibility of periphyton-based culture of Nile tilapia in small cages by evaluating the relative contributions to the diet of plankton, artificial feed and periphyton from the cage net and from additional substrate placed in the cage. This study also describes the change in composition over time of the periphyton biomass on the cage net. It concludes with a consideration on how to maximise available periphyton biomass.
2. Material and methods 2.1. General description The trial was conducted at the CARE-Bangladesh experimental farm at Baushia Ferry Ghat on the west bank of the Megnha – Goumti River, 40 km east of Dhaka. The 3-month experiment ran from 6 March until 7 June 1997. The experimental design comprised three treatments, carried out in triplicate (Table 1), using twelve 2.5-m3 (1.5 1.8 0.9 m) floating cages. Floating cages were made of polyethylene netting hanging from a floating bamboo raft. A description of the construction material used for the cage is given in Table 2. Nile tilapia fry of the genetically improved farmed tilapia (GIFT) strain, strain produced by the International Centre for Living Aquatic Resources Management (ICLARM) in the Philippines and introduced to Bangladesh, were obtained from the Fisheries Research Institute, Mymensingh and allowed to acclimatize to experimental conditions for 8 months prior to the beginning of the experiment. A mixed sex population was used, as is usual among local farmers.
Table 1 The three treatments used to assess the periphyton-based cage culture Treatment
Additional Supplemental Stocking Stocked Surface of substrate substrate (bottles) feeding weight (g) biomass (g) (net + bottles) (m2)
T1: fish, substrate and feed 77 T2: fish and substrate 77 T3: fish and feed – Data are means F S.D., n = 3.
yes no yes
27.0 F 3.5 1456 F 24 27.1 F 3.5 1462 F 9 26.7 F 3.6 1440 F 33
9.6 + 5.5 = 15.1 9.6 + 5.5 = 15.1 9.6 + 0 = 9.6
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Table 2 Details of the construction cost of one floating cage in Bangladeshi takas depreciated for one production cycle (56 days in the present study) Raw material
Quantity
Unit price
Durability (year)
Cost per crop
6- and 8-mm plastic line (kg) Polyethylene net (m2) Feeding tray (bamboo) Bamboo poles Anchor rope (kg) Iron sinker Tree branch Anchor Construction labour (man day 1) Plastic bottle Total
0.55 11.5 1 5 0.5 22 5 1 1.5 77
76 40 3 100 180 0.82 2.85 385 60 0.9
1 2.5 1 1.5 2.5 2.5 1 10 1 5
4.8 21.2 0.4 38.4 4.1 0.8 1.6 4.4 10.4 1.6 87.7
Inexpensive, locally available plastic bottles (n = 77; surface area per cage = 5.5 m2) were halved along their vertical and suspended in the cages to provide additional substrate for enhancement of periphyton biomass in two treatments (T1 and T2). All cages were stocked with the same biomass of fish (600 g m 3) at 22 fish m 3 with fish of approximately 27.0 F 3.5 g. To assess the contribution of artificial feeding, supplementary feed was administered in two treatments (T1 and T3) (Table 1). All fish were weighed ( F 0.1 g) and total lengths ( F 1 mm) determined at the beginning and end of the experiment. Samples of 20 fish were measured fortnightly to monitor growth and to adjust feeding rate. Fish from all treatments were harvested after 56 days. As the fish began to reproduce after 4 weeks, the sex of each fish was also recorded at harvest. The numbers of fish with unusually prominent lower jaws were also recorded at harvest. Prior to the beginning of the experiment, fry had been fed twice daily on a diet composed of 50% local rice bran and 50% mustard oil cake at a rate of 10% body weight day 1. Throughout the experiment, fish from treatments T1 and T3 received supplementary feed comprising 25% mustard oil cake and 75% rice bran. During the first 2 weeks, fish were fed at a rate of 6% body weight day 1; thereafter, the feeding rate was reduced to 2% body weight day 1. Food was administered twice daily at 9:00 am and 4:00 pm. Food conversion ratio (FCR) was used to evaluate the quality of the feed. As the ingestion of natural food by fish may influence FCR values, a modified FCR (FCRm) value was derived using data from treatment T2, which did not receive supplementary feed, by subtracting the average fish biomass production of T2 from the biomass values in treatments T1 and T3. Although the interactions between natural food and artificial feed could not be quantified precisely because both plankton and periphyton growing on the cage nets were always present, the additional substrates in treatments T1 and T2 gave an indication of the contribution of periphyton to the diet. Variables measured included net productivity, change in condition factor (CF) (initial CF (weight in g/(total length in cm)3) final CF), weight gained, specific growth rate (SGR), sex ratio and abnormality ratio (abnormal fish/total harvested fish). Samples of feed ingredients and periphyton were taken and analysed at the fish nutrition laboratory, Department of Aquaculture, Faculty of Fisheries, Bangladesh Agri-
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culture University, Mymensingh (Table 3). The protein content of the feed was approximately 18% and the price was Tk 6 kg 1 (£0.08 kg 1). The results of the growth experiment were used to carry out an economical analysis of the periphyton-based cage aquaculture. For the purpose of the analysis, the following assumptions were made. The farm gate price (Tk 40 kg 1) is independent of the size of the fish sold. It was the price prevailing in the local market at the time of the study. This is often the case in Bangladesh as fish of all sizes are often found in the market. However, in general, bigger fish (>100 g) would reach a better market price, up to Tk 70 in the best case. The price of the seed fish generally varies according to the size of the seeds. For convenience in the analysis, the price was fixed at Tk 40 kg 1 as this was very close to the market price. Prices may vary according to location and time of purchase. The price of the floating cages was calculated from the construction cost of the experimental cages (Table 2). The cost of the cage was depreciated on a daily basis based on an estimation of the longevity of each material. 2.2. Nutritional contribution of the periphyton on the cage net In order to collect complementary information to Huchette et al. (2000) and assess the contribution of the periphyton growing on the cage net to the diet of the fish, the following short-term experiment was carried out. Twenty four pieces (10 10 cm) of 14-mm black polyethylene net secured to a wooden frame were hung at four different depths (0, 20, 40 and 100 cm) in an empty floating cage (Fig. 1). The nets were introduced to the cages 3 weeks prior to stocking with tilapia for the periphyton to develop fully. Tilapias were allowed to graze on both sides of the nets, 1 m2 of net is represented by a piece of 50 100 cm. Each of these pieces of net was sampled just before stocking and 1, 2, 3, 6 and 8 days after stocking. In order to estimate the ash-free dry weight (AFDW), chlorophyll a and phaeopigment content of the periphyton, samples of net with known surface area were carefully removed and processed as described in Huchette et al. (2000). The net production of the grazed periphyton community of cage netting in the uppermost 50 cm of the water column was estimated by Huchette et al. (2000) to be 0.12 g C m 2 h 1 and was assumed to apply to periphyton production in the present trials. Periphyton dry matter content was estimated using the equation: 1 g C = 2 g dry Table 3 Results from the nutritional analysis in percentage ( F S.D., n = 3) of the feed ingredients used during the experiment and nongrazed periphyton collected within the experimental cages
Moisture Protein Lipid Ash Crude fibre Nitrogen free extract
Mustard oil cake
Automated mill rice bran
Periphyton (wet basis)
Periphyton (dry basis)
9.14 F 0.03 31.58 F 0.19 10.18 F 0.45 7.69 F 0.06 11.40 F 0.27 30.01 F 0.62
10.81 F 0.26 14.27 F 0.41 13.82 F 0.39 7.90 F 0.24 17.64 F 0.24 35.56 F 0.2
89.85 F 0.17 1.27 F 0.03 1.70 F 0.09 6.56 F 0.03 0.58 F 0.06
12.6 16.9 65.3 5.77
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Fig. 1. Drawing of the experimental design showing the position of the pieces of net in the water.
matter (DM) (Vymazal, 1995), and the energy content calculated from the results of the nutritional analysis conducted in triplicate on ungrazed periphyton collected within the cages and using conversion factors of 23.6, 39.5 and 17.2 kJ g 1 for protein, lipid and carbohydrates, respectively (Ross, 2000). An energy budget was calculated using the equations established for O. niloticus by Ross (2000) and the assumptions of Dempster et al. (1995). The weight of the tilapia at the beginning of the experiment (30 g) was used in the calculation. Fish moisture content was assumed to be 80%. The digestibility of the periphyton, mainly diatoms, was estimated at 60%, an average of values found by Getachew (1993) for organic matter and Moriarty and Moriarty (1973) for Nitzchia sp. The resulting energy budget was summarized in the following equation: 100C ¼ 16P þ 44R þ 40F þ 2U where C = consumption, P = production, R = respiration, F = faeces and U = nonfaecal losses. Light penetration was estimated between 12:00 am and 1:00 pm using a Secchi disc while water temperature was measured in the morning and in the afternoon by an electronic thermometer ( F 0.1 jC). 2.3. Statistical analysis Homogeneity of variance was tested and an ANOVA used to compare treatments. Treatments T1 –T3 were compared after 56 days. Where significant differences were found, treatment means were compared using Tukey’s test. Student’s t-tests were used to compare average weights of abnormal fish with those of normal fish and average weights of mature fish (male and female) with those of immature fish (Fowler and Cohen, 1993). For each treatment, net fish production was calculated on the basis of mean fish weight gain and numbers recovered at harvesting. A multiple regression analysis of the average weight gained was carried out against treatment characteristics (i.e. feeding rate and surface of substrates). An ANOVA was performed to test if periphyton characteristics changed with depth.
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3. Results 3.1. Cage fish species composition The results of the growth trial are shown in Table 4. Wild fish of various genera (i.e. Puntius, Danio, Salmostella, Bakela, Chanda, and Tetrada), the biggest and most abundant being Puntius species (Cyprinideae), were found in the cages containing additional periphyton substrate. They were not stocked in the cage but had entered when still small and subsequently became too big (0.5 – 9 g) to be able to leave. Wild fish were more abundant in cages with substrates and without tilapia. The distribution of abnormal fish (33%) within the cages was not found to be significantly different among the treatments (Table 4). The mean weight of abnormal fish (38.9 F 7.27 g, n = 146) was significantly lower (z = 22.5, p < 0.001) than that of normal fish (45.0 F 7.4 g, n = 313). Although the fish used for the experiment were relatively small (Table 4), they began to reproduce within the first 4 weeks after stocking. This had a major impact on fish growth. Growth of mature tilapia was reduced in comparison with immature fish (Legendre, 1986; Legendre et al., 1989). The average size of females (42.0 F 7.8 g, n = 221) is significantly less than the average size of males (44.7 F 7.8 g, n = 215) at harvest (z = 10.3, p < 0.01). Differences were not significant (Table 4). Mature fish (43.3 F 7.9 g, n = 436) had a significantly higher weight than immature ones (38.0 F 5.9 g, n = 23) at harvest (z = 10.2, p < 0.01). The change in each of the variances describing growth is shown in Fig. 2. The average weights of fish at harvest were found to be significantly different among
Table 4 Results from the periphyton-based growth trial (standard deviation, n = 3) Variables
ANOVA result
T1: fish, substrate and feed
T2: fish and substrate
T3: fish and feed
Stocking weight (g) Biomass at stocking (g) Harvest weight (g) Biomass at harvest (g) Net production (g) Average weight gained Fish recovered (%) Sex ratio Abnormality ratio (%) Specific growth rate (% day 1) Change in CF Feed conversion ratio (FCR) FCRm
p > 0.05 p > 0.05 p < 0.05 p < 0.01 p < 0.01 p < 0.05 p > 0.05 p > 0.05 p > 0.05 p < 0.01 p < 0.01 p > 0.05 p > 0.05
27.0 F 3.5 1455.9 F 3.8 45.4 F 7.3 2240.3a F 38.7 784.4a F 17.8 18.4a F 0.9 91.4 1.21 F 0.43 32.7 F 0.1 0.931a F 0.030 0.090b F 0.017 2.94 F 0.15 10.28 F 1.98
27.1 F 3.5 1461.6 F 9.3 40.2 F 7.2 2077.7b F 55.5 616.1b F 48.9 13.2b F 1.2 95.7 0.69 F 0.28 27.9 F 0.1 0.708b F 0.051 0.130a F 0.012
26.7 F 3.6 1440.5 F 32.9 43.7 F 8.2 2273.1a F 41.6 832.7a F 48.8 17.0a F 0.9 96.3 1.35 F 0.43 34.0 F 0.1 0.882ab F 0.048 0.102b F 0.003 3.18 F 0.17 14.02 F 3.68
An ANOVA was carried out on each variable to compare the treatments. Tukey’s test results are specified by superscript letters. FCRm: FCR modified for the contribution of natural food.
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Fig. 2. Change in average weight, average total biomass and SGR during the experiment for each treatment.
treatments. Fish biomass at harvest in treatments T1 and T3 were higher than in treatment T2, suggesting a significant contribution of the supplementary feed to the resulting fish biomass. The recovery rate did not vary significantly between cages (Table 4). Mortalities always occurred after stocking or sampling sessions and must be attributed to handling. A few fish also escaped during sampling. Fish from treatment T1 had a higher SGR than fish in treatment T2, emphasising the positive effect of the supplementary feed. Differences in growth rate between treatments T1 and T3 demonstrated that the additional substrate had no significant effect on net fish production.
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The FCR values obtained for the overall experiment were around 3 (Table 4). When corrected for the contribution of the natural food, FCRm became two to three times higher (Table 4). Therefore, the use of supplementary diet may not be an economically efficient way of improving yield in the periphyton-based cage system for the stocking densities used in the present study. 3.2. Multiple regression analysis of net production Regression equation obtained was: Weight gained (g) = 5.40 (t = 6.9, p < 0.001) feeding rate (%) + 1.42 (t = 1.8, p = 0.122) substrate surface area (m 2 ) + 11.64 (t = 12.1, p < 0.001), with R2 = 0.89 and p < 0.01. Only the coefficient for ‘‘feeding’’ provided a good estimate for the average weight gained in the cage. While the coefficient for substrate surface area was not significant, it is still positively correlated with the average weight gained. It should be noted that the high constant value suggests the contribution to the growth of other factors equally present in all treatments. The economic analysis of the results obtained during this experiment is shown in Table 5 (US$1 = Tk 50). The obvious extra cost of floating cage, which required more bamboo poles and anchoring system, leads us to extrapolate the economical results using fixed cage design. Fixed cages are made of polyethylene netting attached to half
Table 5 Economic analysis of the periphyton-based cage aquaculture for each treatment Economic variables
T1: fish, feed and substrate in floating cages
T2: fish and substrate in floating cages
T3: fish and feed in floating cages
Extrapolation of T2 for fixed cage
Depreciated cage cost Stocking density (fish m 3) Survival (%) Stocking weight (g) Harvest weight (g) Price of seed (Tk) Feed cost (Tk/production cycle) FCR Net production (kg) Total income (Tk) Profit (Tk)
88 22 90 27 45.4 60 15 2.94 0.77 90 71
87 22 90 27 40.4 60 – – 0.52 80 67
86 22 92 27 43.8 60 13 3.18 0.73 89 71
40 22 90 27 40.4 60 – – 0.52 80 20
16 66 31 2
– 74 35 3
15 67 31 0
– 74 35 3
Important parameters Feed cost as % income Seed cost as % income Netting cost as % income Substrate cost as % income
Treatment T2 was extrapolated for fixed cages (all prices are expressed in Bangladeshi takas). £1 = 69 Tk at the time of study. Seed and farm gate fish prices are assumed to be fixed at Tk 40 kg 1. Depreciated costs were calculated according to the period of production and durability of construction material. A period of production of 56 days was used for the analysis.
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Fig. 3. Change in chlorophyll a content, ash free dry weight (AFDW), ash and phaeopigment of the periphyton growing on the cage net over 1 week of grazing. Fish were introduced in the cage on the 6th of May.
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Table 6 Chlorophyll a content and biomass of the grazed periphyton growing on black polyethylene net at four different depth 1 day before and 1 day after stocking ( F S.D., n = 2)
2 – 8 days after stocking ( F S.D., n = 4)
Depth (cm)
Chlorophyll a (Ag/cm2)
AFDW (mg/cm2)
Chlorophyll a (Ag/cm2)
AFDW (mg/cm2)
0 – 10 20 – 30 40 – 50 100 – 110
7.5 F 1.5 8.6 F 5.8 6.1 F 0.3 0.7 F 0.0
2.4 F 2.1 1.4 F 0.9 0.5 F 0.2 0.2 F 0.0
2.2 F 1.5a 1.8 F 1.4a 2.1 F 1.4a 0.6 F 0.8b
0.38 F 0.6a,b 0.43 F 0.6a 0.26 F 0.5b 0.31 F 0.6b
Superscript letter indicates the result of the Tukey’s test between depths.
bamboo poles planted vertically in the sediment and do not require floating device or anchoring. 3.3. Nutritional contribution of the periphyton fouling on the cage net During the period of sampling, light penetration in the water averaged 128 F 6 cm (n = 4) while the temperature was 29.5 F 0.3 jC (n = 8). The composition of the periphyton biomass are shown in Fig. 3. The average nutritional values of the grazed periphyton at each depth in the cage are given in Table 6. The ash proportion in the dry matter was much higher in the periphyton (>60%) than in the plankton (26%) while the phaeopigment content was higher in the phytoplankton (26%) than in the grazed periphyton ( < 20%). The greatest biomass of periphyton was found in the uppermost 30 cm of the water column close to the compensation depth. However, the chlorophyll a content appeared to be less sensitive to light penetration than the ash-free dry weight (AFDW). These results are similar to those found by Konan-Brou and Guiral (1994). Highest periphyton biomass levels occurred just before and immediately after stocking. After stocking, the biomass decreased considerably in the uppermost three sampling depths although the periphyton biomass below 100 cm showed no change over time. The chlorophyll a content and AFDW of periphyton were generally lower following the onset of grazing, the greatest difference occurring in the uppermost 30 cm. Grazing did not appear to affect periphyton ash content although it affected phaeopigment content, phaeopigments decreasing below 20%, a sign of increasing community health (Wetzel and Likens, 1991). The nutrition value of the periphyton, summarised in Table 3, was found to be similar to that of diatoms (Brown and Jeffrey, 1995). The total energy production of 1 m2 of net located in the uppermost 50 cm of the water column was estimated as 1.4 g C day 1 = 2.8 g DM day 1 = 29.68 kJ day 1, sufficient to support an estimated fish productivity of 0.94 g wet weight m 2 net day 1.
4. Discussion The similar quantities among the cages of fish with abnormally large lower jaws clearly suggested that the abnormalities had a genetic basis or were due to injuries
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received before the experiment began. The former is more likely because smaller fish in the farm of the same origin also presented the same abnormalities. Thus, these fish were not ‘‘genetically improved’’, and they may not have been able to feed efficiently. Oedemas were frequently observed on the lower jaw suggesting that they may have had difficulty grazing on the periphyton. Their stunted state may be due to undernutrition. However, the difference between the average weight of the normal and the abnormal fish did not vary significantly among treatments, suggesting that the supplementary feed did not improve their growth. Abnormal fish may well be unable to compete with the other fish. The cages appeared to act like attraction devices in other fisheries such as Katha fisheries and ‘acadja enclos’ (Legendre et al., 1989; MacGrory and Williams, 1996). The small wild fish entering the cages stocked with tilapias may have been chased by them, which may be the reason why their number remained low in the stocked cages. The attraction of wild fish is an interesting alternative to the supply of quality seeds for periphyton-based cage aquaculture in Bangladesh. It may also however be a source of conflict with local fishermen. Problems of reproduction in cages are usually limited as the eggs are easily lost through the bottom of the cage (Coche, 1982; Beveridge, 1996). But an inflection in the growth of the tilapia was observed after the 4th week of experiment in all treatments (Fig. 2), most probably because of reproduction. Although they were small, it was subsequently discovered that the fish were approximately 10 months old at the beginning of the experiment. This stunted population found a lot of natural food in the periphyton-based cage at first and may have enjoyed good conditions during the first month when they showed excellent specific growth rates (SGR) of 0.6 – 0.8% body weight per day. The environment was therefore good enough to prepare them for reproduction. It seems also very possible that starvation of juvenile fish may cause them to begin gonad development early, as they do not ‘‘expect’’ to reach a large size or to live long. In other words the onset of reproduction may be phenotypically plastic as suggested by other studies (El-Sayed et al., in press). As the reproduction process took place, the fish allocated less energy for growth. In treatment T3, even though fish length kept increasing slightly, their weight declined between weeks 4 and 6 suggesting the beginning of the spawning period (Fig. 2). Yields obtained in extensive cage aquaculture in the Philippines range from 0.05 to 1.25 kg m 3 month 1 without input (Beveridge, 1996), which are in agreement with this study (0.12 – 0.24 kg m 3 month 1). Considering the problem of reproduction, the yield in this study could probably be improved with seed of better quality. In a parallel experiment, smaller fish were stocked at the same biomass as the one used in the present study (initial biomass = 1.475 F 0.024 g, n = 3) with 55 fish (mean wt. = 10.7 F 1.7 g) m 3 in floating cages with the same surface area of added substrate. The productivity (g m 3 month 1) was higher after only 6 weeks (final biomass = 2380.4 F 61.9 g, n = 3). The better result obtained with smaller fish showed that the potential for extensive cage aquaculture in Bangladesh remains unexplored. This may be for two reasons: the existence of natural food available proportionally to the number of fish (i.e. plankton), or the existence of energy spent proportionally by the fish (i.e. reproduction).
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Tilapias have been primarily described as filter feeders on phytoplankton (Moriarty and Moriarty, 1973; Bocci, 1999; Turker et al., in press). However, it has been shown in recent works that the Nile tilapia is also a periphyton grazer (Beveridge and Baird, 2000). Periphyton is mainly composed of diatoms, an important component of wild Nile tilapia diet (Getachew, 1993). The stomach content showed that the tilapias grown in cages in the Meghna River were both filter feeding and grazing (Huchette et al., 2000). The proportions of each feeding mode were difficult to determine, but a visual estimate of gut contents suggested a higher contribution from periphyton. Comparing treatments T1 and T3 with T2, the supplementary feed allowed approximately 29% more growth yielding a high corrected FCRm between 10 and 14 (Table 4). Thus, the supplementary feed would be an expensive input for farmers for a low efficiency. A better formulation of the diet should be possible considering recent work looking at alternatives for fish meals in tilapia diets (reviewed by ElSayed, 1999). However, the formulation of the diet used in this study was constrained by the local availability and the cost of raw material. The contribution of the additional substrates was not significant suggesting that the periphyton productivity of these substrates was low or that the range of substrate area in the experiment was too small to alter the food supply markedly. Thus, the fish were either mostly feeding on plankton or feeding preferentially on the periphyton growing on the cage net. The change in condition factors suggested that supplementary feed substantially maintained the fish in a better physical condition. Even if affected by the reproduction process, the specific growth rates (SGR) obtained in the parallel study were higher for smaller (SGR = 1.447 F 0.079% day 1, n = 3) than for bigger fish (Table 4), suggesting that the smaller fish are able to collect proportionally more food than the larger ones. Ross and Ross (1983) showed that the quantity of water passing through the gills of smaller fish was proportionally higher in response to higher oxygen requirements in small fish. By getting more water through the gills, the smaller fish could filter proportionally more phytoplankton and gather more metabolisable energy for growth than the larger ones. To improve the supply of natural food, various studies (Dempster et al., 1993, 1995; Konan-Brou and Guiral, 1994; Guiral et al., 1993) suggested increasing the amount of periphyton available to the fish in the rearing system, as grazing on periphyton allows a more efficient collection of energy than filter feeding on plankton. In extensive rearing systems such as ‘acadja enclos’ in the Ivory Coast or bamboo ponds in Bangladesh, the stocking density is very low (approximately 1 fish m 2), thus the low grazing pressure allows better development of the periphyton communities. In cages such as the ones used in the present studies, the stocked biomass has to be higher for economic reasons. Thus, the periphyton community is put under greater pressure. The grazing pressure maintained the standing crop of periphyton at the lowest level on the additional substrate, which did not appear to be sufficiently productive to contribute significantly to the diet of the fish (Huchette et al., 2000). However, the fish could only access the periphyton growing on the inside of the cage nets. This may maintain a higher periphyton biomass on the net, making it more productive than the plastic bottles. As floating cages are expensive and represent a considerable investment for the poorest farmers, the results for economic feasibility were extrapolated for the best
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treatment (T2) in fixed cages (Table 5). Fixed cages need less bamboo sticks and represent a lower investment, having a better profitability. In the fixed cages, a bagshaped net is maintained open in the water using four bamboo sticks planted vertically in the bottom of the river. None of the treatments yielded economically viable results. The net productivity was insufficient in the cages even in the best case (T2). The farm gate price was too low and the cost of seed represented 66 –74% of total income. The price of tilapia is not very high in Bangladesh because local fish, like the Indian carps, are abundant and preferred because of higher fat content. This may change in the future as fisheries production is steadily declining and as the Bangladeshis become accustomed to eating tilapias. The prices of fish are controlled by highly organised fish traders (Lewis et al., 1996). The cost of the substrate appeared to be only marginal, ranging between 2% and 3% of the total income in all treatments. Periphyton-based aquaculture was in recent years perceived to have real potential in extensive culture system (Wahab et al., 1999a,b). In cages, where the stocking densities must be high, it is especially difficult to control the production of natural food. Cages usually have a very limited surface area and thus a limited periphyton productivity, which cannot be enhanced by fertilizers like in enclosed systems (Azim et al., 2001b). Recently, Norberg (1999) tried to quantify the energetic contribution of fouling on the net of tropical cages in intensive tilapia culture. Periphyton biomasses on the black polyethylene net pieces were significantly reduced following tilapia stocking, showing that the fish grazed efficiently on the nets. Periphyton under 100-cm depth did not seem to contribute to the diet of the fish. The most important proportion of the biomass was produced above 30-cm depth. Periphyton appeared to trap and concentrate the mineral matter suspended in the water. This effect may be accentuated as nets are filtering the water. The lipid level of the periphyton was higher than the protein content, which is unusual for the microorganism community. This showed the presence of diatoms in the periphyton and a lot of cell wall material. The price of 1 m2 of polyethylene net is equivalent to 1 kg of fish (Tk 40 m 2), which requires a little less than 3 years to recover the investment. Therefore, a periphyton-based culture of tilapia using net as substrate would not be profitable. However, periphyton biomass and production could be improved using various management techniques. Substrates could be alternatively grazed and protected from grazing, allowing the rapid development of a more diverse community. Substrates protected from grazing could be set horizontally at a depth close to the compensation depth. Other systems using enclosed and fertilised water bodies could also be used to produce more periphyton and to reduce siltation problems. By increasing the amount of nutrients available, the production of periphyton could be up 10-fold (Fairchild et al., 1985). Other inexpensive substrates could be used instead of net, but they should not affect water volume and should not decompose. A supplementary feed could also be given and this would contribute to improve the periphyton growth by increasing the nutrients available in the water through the excretion of faecal and nonfaecal materials by the fish. If the production of periphyton could be multiplied by four, this would allow the system to be profitable within a few months.
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Acknowledgements We thank the DFID Aquaculture Research Programme for making available the funds for the present study. We also thank the staff of CARE-Bangladesh CAGES project, and DFID, Bangladesh, for their cooperation and technical assistance throughout the experiment. Special thanks are due to Narunnabi, Mark Ireland and the technical staff of the experimental farm. We are grateful to the staff at the Faculty of Fisheries, BAU, for providing access to their laboratory facilities. Discussions with Marc Verdegem, University of Wageningen, Jon Norberg, University of Stockholm, Rob Day, University of Melbourne and our colleague, Trevor Telfer proved useful. We are grateful to two anonymous reviewers for improving the manuscript.
References Azim, M.E., Wahab, M.A., Van Dam, A.A., Beveridge, M.C.M., Verdegem, M.C.J., 2001a. The potential of periphyton-based culture of two Indian major carps, rohu Labeo rohita (Hamilton) and gonia Labeo gonius (Linnaeus). Aquac. Res. 32, 209 – 216. Azim, M.E., Wahab, M.A., Van Dam, A.A., Beveridge, M.C.M., Mitstein, A., Verdegem, M.C.J., 2001b. Optimization of fertilization rate for maximizing periphyton production on artificial substrates and the implication for periphyton-based aquaculture. Aquac. Res. 32, 749 – 760. Beveridge, M.C.M., 1996. Cage Aquaculture, 2nd ed. Fishing News, Oxford, p. 346. Beveridge, M.C.M., Baird, D.J., 2000. Diet, feeding and digestive physiology. In: Beveridge, M.C.M., McAndrew, B.J. (Eds.), Tilapias: Biology and Exploitation. Kluwer Academic Publishing, Dordrecht, Netherlands, pp. 59 – 87. Bocci, M., 1999. Modelling the growth of Nile tilapia (Oreochromis niloticus) feeding on natural resources in enclosures in Laguna de Bay (Philippines). Ecol. Model. 119, 135 – 148. Brown, M.R., Jeffrey, S.W., 1995. The amino acid and gross composition of marine diatoms potentially useful for mariculture. J. Appl. Phycol. 7 (6), 521 – 527. CNRS, 1995. Community-Based Fisheries Management and Habitat Restoration Project: Annual Report Centre for Natural Resource Studies, Dhaka 30 pp. Coche, A.G., 1982. Cage culture of tilapias. In: Pullin, R.S.V., Lowe-McConnell, R.H. (Eds.), The Biology and Culture of Tilapias. ICLARM Conference Proceedings, vol. 7. International Centre for Living Aquatic Resources Management, Manila, Philippines, pp. 205 – 246, 432 pp. Dempster, P.W., Beveridge, M.C.M., Baird, D.J., 1993. Herbivory in the tilapia Oreochromis niloticus: a comparison of feeding rates on phytoplankton and periphyton. J. Fish Biol. 43, 385 – 392. Dempster, P.W., Baird, D.J., Beveridge, M.C.M., 1995. Can fish survive by filter feeding on microparticles? Energy balance in tilapia grazing on algal suspensions. J. Fish Biol. 47, 7 – 17. El-Sayed, A.-F.M., 1999. Alternative dietary protein sources for farmed tilapia, Oreochromis spp. Aquaculture 179, 149 – 168. El-Sayed, A.-F.M., Mansour, C.R., Ezzar, A.A., 2002. Effects of dietary protein level on spawning performance of Nile tilapia (Oreochromis niloticus) broodstock reared at different water salinities. Aquaculture (in press). Fairchild, G.W., Lowe, R.L., Richardson, W.B., 1985. Algal periphyton growth on nutrient-diffusing substrates: an in situ bioassay. Ecology 66, 465 – 472. Fowler, J., Cohen, L., 1993. Practical Statistics for Field Biology. Wiley, Chichester, p. 223. Getachew, T., 1993. The composition and nutritional status of the diet of Oreochromis niloticus in Lake Chamo, Ethiopia. J. Fish Biol., 865 – 874. Guiral, D., Arfi, R., Da, K.P., Konan-Brou, A.A., 1993. Communaute´s, biomasses et productions algales au sein d’un re´cif artificiel (acadja) en milieu lagunaire tropical. Rev. Hydrobiol. Trop. 26, 219 – 228. Gregory, R., Kamp, K., 1995. Aquaculture extension in Bangladesh: experiences from the Northwest fisheries extension project 1989 – 1992. BAFRU, Dhaka, 150 pp.
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Hambrey, J., Beveridge, M.C.M., McAndrew, K., Bulcock, P., 2001. Cage aquaculture and poverty alleviation. Part I: Bangladesh. World Aquac. 32(1), 50 – 58. Hem, S., Avit, J.L.B., 1994. First results on ‘‘acadja-enclos’’ (bamboo reefs) used as an extensive aquaculture system Cote d’Ivoire (West Africa). Bull. Mar. Sci. 55, 1038 – 1049. Huchette, S.M.H., Beveridge, M.C.M., Baird, D.J., Ireland, M., 2000. The impacts of grazing by tilapias (Oreochroomis niloticus) on periphyton communities growing on artificial substrate in cages. Aquaculture 186, 45 – 60. Keshavanath, P., Gangadhar, B., Ramesh, T.J., Van Rooij, T.M., Beveridge, M.C.M., Baird, D.J., Verdegem, V.C.J., Van Dam, A.A., 2001. The potential of artificial reefs to enhance production of herbivorous fish in Indian freshwater ponds. Aquac. Res. 32, 189 – 197. Konan-Brou, A.A., Guiral, D., 1994. Available algal biomass in tropical brackish water artificial habitats. Aquaculture 119, 175 – 190. Lamberti, G.A., Moore, J.W., 1984. Aquatic insects as primary consumers. In: Resh, V.H., Rosenberg, D.M. (Eds.), The Ecology of Aquatic Insects. Praeger, New York, pp. 164 – 195. Legendre, M., 1986. Influence de la densite´, de l’elevage monosexe et de l’alimentation sur la croissance de Tilapia guineensis et de Sarotherodon melanotheron e´leve´s en cage-enclos en lagune Ebrie´ (Coˆte d’Ivoire). Rev. Hydrobiol. Trop. 19 (1), 19 – 29. Legendre, M., Hem, S., Cisse, A., 1989. Suitability of brackish water tilapia species from Ivory Coast for lagoon aquaculture. II: Growth and rearing methods. Aquat. Living Resour. 2, 81 – 89. Lewis, D.J., Wood, G.D., Gregory, Y.R., 1996. Trading the Silver Seed: Local Knowledge and Market Moralities in Aquacultural Development. University Press Ltd., Dhaka, Bangladesh, 199 pp. Macgrory, J., Williams, D., 1996. Katha Fishing: Economics, Access Patterns and Potential Conflicts with Fish Cage Culture. CARE, Bangladesh 74 pp. Mc Andrew, K., Brugere, C., Beveridge, M.C.M., Ireland, M.J., Tapash, R., Yesmin, K., 2002. Improved management of small-scale tropical cage culture systems in Bangladesh: potential benefits of an alliance between an NGO and a western research institute. In: Edwards, P., Little, D.C., Demain, H. (Eds.), Rural Aquaculture. CAB International, Wallingford, Oxon, pp. 129 – 141. Moriarty, C.M., Moriarty, D.J., 1973. Quantitative estimation of the daily ingested of phytoplankton by Tilapia nilotica and Haplochromis nigripinnis in Lake George, Uganda. J. Zool. (Lond.) 171, 15 – 23. Morrice, C.P., 1997. Annual Report on the 1995/96 Season of the Extension and Training Section of the Northwest Fisheries Extension Research NFEP, Parbatipur, Dinajpur 203 pp. Norberg, J., 1999. Periphyton fouling as a marginal energy source in tropical tilapia cage farming. Aquac. Res. 30, 427 – 430. Ross, L.G., 2000. Environmental physiology and energetics. In: Beveridge, M.C.M., McAndrew, B.J. (Eds.), Tilapias: Biology and Exploitation. Kluwer Academic Publishing, Netherlands, pp. 89 – 128. Ross, B., Ross, L.G., 1983. The oxygen requirements of Oreochromis niloticus under adverse conditions. In: Fishelson, L., Yaron, Z. (Eds.), Proceedings of the 1st International Symposium on Tilapia in Aquaculture, Tel Aviv Univ. Press, Israel, 8 – 13 May, pp. 134 – 143. Turker, H., Eversole, A.G., Brune, D.E., 2002. Filtration of green algae and cyanobacteria by Nile tilapia, Oreochromis niloticus, in the partitioned aquaculture system. Aquaculture (in press). Vymazal, J., 1995. Algae and Element Cycling in Wetlands. Lewis Publishers, Boca Raton, FL, p. 689. Wahab, M.A., Azim, M.E., Ali, M.H., Beveridge, M.C.M., Khan, S., 1999a. The potential of periphyton-based culture of a native major carp calbaush, Labeo calbasu Hamilton. Aquac. Res. 30, 409 – 420. Wahab, M.A., Mannan, M.A., Huda, M.A., Azim, M.E., Tollervey, A., Beveridge, M.C.M., 1999b. Effects of periphyton grown on bamboo substrates on growth and production of Indian major carp rohu (Labeo rohita Ham.). Bangladesh J. Fish. Res. 3, 1 – 10. Wetzel, R.G., Likens, G.E., 1991. Limnological Analysis, 2nd ed. Verlag, New York.
Technical and economical evaluation of periphyton-based cage culture of tilapia (Oreochromis niloticus) in tropical freshwater cages S.M.H. Huchette *, M.C.M. Beveridge1 Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, UK Received 14 November 2001; received in revised form 15 March 2002; accepted 2 August 2002
Abstract A 3-month study (March – June) was carried out in the Meghna – Gumti River (Bangladesh) to assess the potential for periphyton-based cage aquaculture. The growth of genetically improved farmed tilapia (GIFT) strain (Oreochromis niloticus) was monitored during a trial conducted in nine 2.5-m3 floating cages. The importance of periphyton in tilapia diets was assessed using three treatments. In the first treatment, the periphyton biomass was increased using additional substrates placed in the cage. In the second treatment, additional substrates were used and the diet was supplemented with a locally available compound feed. The third treatment had no additional substrate but received some supplementary feed. The effect of grazing on periphyton biomass was also assessed in a parallel experiment. Biomass was assessed by measuring ash, ash-free dry weight (AFDW), chlorophyll a, phaeopigments and respiration. Net production of fish ranged from 0.12 to 0.19 kg m 3 month 1 and specific growth rate (SGR) from 0.7% to 0.95% body weight day 1. Jaw abnormalities affected 33% of fish and reproduction was observed in fish as small as 35 g. Productivity of a 1 m2 of net located in the uppermost 50 cm of the water column, however, was estimated at 0.94 g fish day 1. The periphyton biomass dropped in just 2 days from 1.12 to 0.34 mg/ cm2 after the introduction of the fish in the cage. The fish fed on plankton, artificial supplementary feed and periphyton growing on substrate in the cage. Filter feeding and grazing on the cage net appeared to be the most important source of energy. The effect of the additional substrates placed in the cages was marginal. The periphyton-based cage aquaculture described here was not economically viable and the main economic constraints were the costs of cage material (net and bamboo poles) and
* Corresponding author. Present address: Department of Zoology, The University of Melbourne, 3052 Parkville, Melbourne, Victoria, Australia. E-mail address: [email protected] (S.M.H. Huchette). 1 Present address: FRS Fresh Fish. Laboratory, Faskally, Pitlochry, Perthshire PH16 5LB, Scotland, UK.
0044-8486/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 4 - 8 4 8 6 ( 0 2 ) 0 0 4 1 4 - 3
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fry. Periphyton-based cage culture could be made more profitable by using local fish species, less expensive cage design, better substrate and by using adapted management techniques to improve periphyton productivity. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Tilapia; Biomass; Periphyton-based cage aquaculture
1. Introduction Different forms of fish culture have been developed in Bangladesh during the past 20 years. Among them, pond culture and rice – fish culture have been developed and made popular by various organisations and are now widespread. Rice –fish culture is now developing very fast in areas such as Parbatipur where the fingerling demand doubled between 1994 and 1995 (Morrice, 1997), and the supply of fingerlings is found to be a limiting factor (Ireland, personal communication). However, pond aquaculture or, at a lower scale, rice –fish culture has had a negative impact on the wild fisheries and is profitable only to the landowners. Cage aquaculture has been promoted in the past in Bangladesh; however, long-term adoption rate was very poor (Gregory and Kamp, 1995). With the development of this type of aquaculture, the landless people have seen their fishing grounds reduced and the quantities of small wild fish, which is the only fish affordable for them, have been declining as it is progressively replaced by the production of food fish (CNRS, 1995; Lewis et al., 1996). This situation is gradually getting worse as the demand for food fish in the urban markets is stimulating more commercially orientated productions. When promoted in a manner appropriate to local needs and resources, cage aquaculture can be successfully used to alleviate poverty among the rural poor. Since 1995, the Cage Aquaculture for Greater Economic Security (CAGES) development project of CAREBangladesh has successfully assisted more than 2500 families to successfully adopt cage farming (Hambrey et al., 2001; Mc Andrew et al., 2002). Nevertheless, one of the main factors limiting the access to aquaculture by the poorer sectors of society in the country remains to be the requirement for inputs of seed and feed (Hambrey et al., 2001). Dempster et al. (1993, 1995) demonstrated that Nile tilapia (Oreochromis niloticus L., 1757) could graze efficiently on periphyton, the community of microscopic plants and animals that attaches to the surface of stones and plants. In the Ivory Coast, the ‘acadja’ (Legendre et al., 1989), artificial reefs made of branches and bundles in African lagoons, and ‘acadja enclos’ (Hem and Avit, 1994), acadja permanently enclosed by nets in which fish are self-recruiting, and in Bangladesh, the Khata fisheries (MacGrory and Williams, 1996), aggregation of water hyacinth surrounded by bamboo poles planted in the river bed, are three traditional fish attracting device (FAD)-based fisheries systems. Studies of these systems recognised that fish production might be enhanced by grazing on the periphytic community that colonizes the substrates. Using bamboo or wooden sticks to enhance production of periphyton in aquaculture ponds (Wahab et al., 1999a,b) has recently been explored in order to reduce feeding inputs, which usually represents over 50% of operating costs (El-Sayed, 1999). The type of substrate (Keshavanath et al., 2001) and the
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fertilisation method (Azim et al., 2001b) used to have a significant effect on periphyton productivity and quality in ponds and subsequently on fish production. Periphyton-based aquaculture may improve the productivity for some herbivorous fish species such as tilapia (Hem and Avit, 1994), Labeo calbasu (Hamilton) (Wahab et al., 1999a), Tor khudree (Sykes) (Keshavanath et al., 2001) and Labeo rohita (Hamilton) but not others such as Labeo gonius (Linnaeus) (Azim et al., 2001a). Norberg (1999) has examined periphyton production in net cages and concluded that it contributed up to 1% of the energy requirement of caged tilapia. Huchette et al. (2000) showed that grazing significantly reduced periphyton biomass and reduced the biodiversity of the algal community by selectively removing larger species. Thus, grazing pressure has to be monitored carefully to maintain a higher productivity (Lamberti and Moore, 1984). This study sets out to assess the potential economical and technical feasibility of periphyton-based culture of Nile tilapia in small cages by evaluating the relative contributions to the diet of plankton, artificial feed and periphyton from the cage net and from additional substrate placed in the cage. This study also describes the change in composition over time of the periphyton biomass on the cage net. It concludes with a consideration on how to maximise available periphyton biomass.
2. Material and methods 2.1. General description The trial was conducted at the CARE-Bangladesh experimental farm at Baushia Ferry Ghat on the west bank of the Megnha – Goumti River, 40 km east of Dhaka. The 3-month experiment ran from 6 March until 7 June 1997. The experimental design comprised three treatments, carried out in triplicate (Table 1), using twelve 2.5-m3 (1.5 1.8 0.9 m) floating cages. Floating cages were made of polyethylene netting hanging from a floating bamboo raft. A description of the construction material used for the cage is given in Table 2. Nile tilapia fry of the genetically improved farmed tilapia (GIFT) strain, strain produced by the International Centre for Living Aquatic Resources Management (ICLARM) in the Philippines and introduced to Bangladesh, were obtained from the Fisheries Research Institute, Mymensingh and allowed to acclimatize to experimental conditions for 8 months prior to the beginning of the experiment. A mixed sex population was used, as is usual among local farmers.
Table 1 The three treatments used to assess the periphyton-based cage culture Treatment
Additional Supplemental Stocking Stocked Surface of substrate substrate (bottles) feeding weight (g) biomass (g) (net + bottles) (m2)
T1: fish, substrate and feed 77 T2: fish and substrate 77 T3: fish and feed – Data are means F S.D., n = 3.
yes no yes
27.0 F 3.5 1456 F 24 27.1 F 3.5 1462 F 9 26.7 F 3.6 1440 F 33
9.6 + 5.5 = 15.1 9.6 + 5.5 = 15.1 9.6 + 0 = 9.6
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Table 2 Details of the construction cost of one floating cage in Bangladeshi takas depreciated for one production cycle (56 days in the present study) Raw material
Quantity
Unit price
Durability (year)
Cost per crop
6- and 8-mm plastic line (kg) Polyethylene net (m2) Feeding tray (bamboo) Bamboo poles Anchor rope (kg) Iron sinker Tree branch Anchor Construction labour (man day 1) Plastic bottle Total
0.55 11.5 1 5 0.5 22 5 1 1.5 77
76 40 3 100 180 0.82 2.85 385 60 0.9
1 2.5 1 1.5 2.5 2.5 1 10 1 5
4.8 21.2 0.4 38.4 4.1 0.8 1.6 4.4 10.4 1.6 87.7
Inexpensive, locally available plastic bottles (n = 77; surface area per cage = 5.5 m2) were halved along their vertical and suspended in the cages to provide additional substrate for enhancement of periphyton biomass in two treatments (T1 and T2). All cages were stocked with the same biomass of fish (600 g m 3) at 22 fish m 3 with fish of approximately 27.0 F 3.5 g. To assess the contribution of artificial feeding, supplementary feed was administered in two treatments (T1 and T3) (Table 1). All fish were weighed ( F 0.1 g) and total lengths ( F 1 mm) determined at the beginning and end of the experiment. Samples of 20 fish were measured fortnightly to monitor growth and to adjust feeding rate. Fish from all treatments were harvested after 56 days. As the fish began to reproduce after 4 weeks, the sex of each fish was also recorded at harvest. The numbers of fish with unusually prominent lower jaws were also recorded at harvest. Prior to the beginning of the experiment, fry had been fed twice daily on a diet composed of 50% local rice bran and 50% mustard oil cake at a rate of 10% body weight day 1. Throughout the experiment, fish from treatments T1 and T3 received supplementary feed comprising 25% mustard oil cake and 75% rice bran. During the first 2 weeks, fish were fed at a rate of 6% body weight day 1; thereafter, the feeding rate was reduced to 2% body weight day 1. Food was administered twice daily at 9:00 am and 4:00 pm. Food conversion ratio (FCR) was used to evaluate the quality of the feed. As the ingestion of natural food by fish may influence FCR values, a modified FCR (FCRm) value was derived using data from treatment T2, which did not receive supplementary feed, by subtracting the average fish biomass production of T2 from the biomass values in treatments T1 and T3. Although the interactions between natural food and artificial feed could not be quantified precisely because both plankton and periphyton growing on the cage nets were always present, the additional substrates in treatments T1 and T2 gave an indication of the contribution of periphyton to the diet. Variables measured included net productivity, change in condition factor (CF) (initial CF (weight in g/(total length in cm)3) final CF), weight gained, specific growth rate (SGR), sex ratio and abnormality ratio (abnormal fish/total harvested fish). Samples of feed ingredients and periphyton were taken and analysed at the fish nutrition laboratory, Department of Aquaculture, Faculty of Fisheries, Bangladesh Agri-
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culture University, Mymensingh (Table 3). The protein content of the feed was approximately 18% and the price was Tk 6 kg 1 (£0.08 kg 1). The results of the growth experiment were used to carry out an economical analysis of the periphyton-based cage aquaculture. For the purpose of the analysis, the following assumptions were made. The farm gate price (Tk 40 kg 1) is independent of the size of the fish sold. It was the price prevailing in the local market at the time of the study. This is often the case in Bangladesh as fish of all sizes are often found in the market. However, in general, bigger fish (>100 g) would reach a better market price, up to Tk 70 in the best case. The price of the seed fish generally varies according to the size of the seeds. For convenience in the analysis, the price was fixed at Tk 40 kg 1 as this was very close to the market price. Prices may vary according to location and time of purchase. The price of the floating cages was calculated from the construction cost of the experimental cages (Table 2). The cost of the cage was depreciated on a daily basis based on an estimation of the longevity of each material. 2.2. Nutritional contribution of the periphyton on the cage net In order to collect complementary information to Huchette et al. (2000) and assess the contribution of the periphyton growing on the cage net to the diet of the fish, the following short-term experiment was carried out. Twenty four pieces (10 10 cm) of 14-mm black polyethylene net secured to a wooden frame were hung at four different depths (0, 20, 40 and 100 cm) in an empty floating cage (Fig. 1). The nets were introduced to the cages 3 weeks prior to stocking with tilapia for the periphyton to develop fully. Tilapias were allowed to graze on both sides of the nets, 1 m2 of net is represented by a piece of 50 100 cm. Each of these pieces of net was sampled just before stocking and 1, 2, 3, 6 and 8 days after stocking. In order to estimate the ash-free dry weight (AFDW), chlorophyll a and phaeopigment content of the periphyton, samples of net with known surface area were carefully removed and processed as described in Huchette et al. (2000). The net production of the grazed periphyton community of cage netting in the uppermost 50 cm of the water column was estimated by Huchette et al. (2000) to be 0.12 g C m 2 h 1 and was assumed to apply to periphyton production in the present trials. Periphyton dry matter content was estimated using the equation: 1 g C = 2 g dry Table 3 Results from the nutritional analysis in percentage ( F S.D., n = 3) of the feed ingredients used during the experiment and nongrazed periphyton collected within the experimental cages
Moisture Protein Lipid Ash Crude fibre Nitrogen free extract
Mustard oil cake
Automated mill rice bran
Periphyton (wet basis)
Periphyton (dry basis)
9.14 F 0.03 31.58 F 0.19 10.18 F 0.45 7.69 F 0.06 11.40 F 0.27 30.01 F 0.62
10.81 F 0.26 14.27 F 0.41 13.82 F 0.39 7.90 F 0.24 17.64 F 0.24 35.56 F 0.2
89.85 F 0.17 1.27 F 0.03 1.70 F 0.09 6.56 F 0.03 0.58 F 0.06
12.6 16.9 65.3 5.77
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Fig. 1. Drawing of the experimental design showing the position of the pieces of net in the water.
matter (DM) (Vymazal, 1995), and the energy content calculated from the results of the nutritional analysis conducted in triplicate on ungrazed periphyton collected within the cages and using conversion factors of 23.6, 39.5 and 17.2 kJ g 1 for protein, lipid and carbohydrates, respectively (Ross, 2000). An energy budget was calculated using the equations established for O. niloticus by Ross (2000) and the assumptions of Dempster et al. (1995). The weight of the tilapia at the beginning of the experiment (30 g) was used in the calculation. Fish moisture content was assumed to be 80%. The digestibility of the periphyton, mainly diatoms, was estimated at 60%, an average of values found by Getachew (1993) for organic matter and Moriarty and Moriarty (1973) for Nitzchia sp. The resulting energy budget was summarized in the following equation: 100C ¼ 16P þ 44R þ 40F þ 2U where C = consumption, P = production, R = respiration, F = faeces and U = nonfaecal losses. Light penetration was estimated between 12:00 am and 1:00 pm using a Secchi disc while water temperature was measured in the morning and in the afternoon by an electronic thermometer ( F 0.1 jC). 2.3. Statistical analysis Homogeneity of variance was tested and an ANOVA used to compare treatments. Treatments T1 –T3 were compared after 56 days. Where significant differences were found, treatment means were compared using Tukey’s test. Student’s t-tests were used to compare average weights of abnormal fish with those of normal fish and average weights of mature fish (male and female) with those of immature fish (Fowler and Cohen, 1993). For each treatment, net fish production was calculated on the basis of mean fish weight gain and numbers recovered at harvesting. A multiple regression analysis of the average weight gained was carried out against treatment characteristics (i.e. feeding rate and surface of substrates). An ANOVA was performed to test if periphyton characteristics changed with depth.
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3. Results 3.1. Cage fish species composition The results of the growth trial are shown in Table 4. Wild fish of various genera (i.e. Puntius, Danio, Salmostella, Bakela, Chanda, and Tetrada), the biggest and most abundant being Puntius species (Cyprinideae), were found in the cages containing additional periphyton substrate. They were not stocked in the cage but had entered when still small and subsequently became too big (0.5 – 9 g) to be able to leave. Wild fish were more abundant in cages with substrates and without tilapia. The distribution of abnormal fish (33%) within the cages was not found to be significantly different among the treatments (Table 4). The mean weight of abnormal fish (38.9 F 7.27 g, n = 146) was significantly lower (z = 22.5, p < 0.001) than that of normal fish (45.0 F 7.4 g, n = 313). Although the fish used for the experiment were relatively small (Table 4), they began to reproduce within the first 4 weeks after stocking. This had a major impact on fish growth. Growth of mature tilapia was reduced in comparison with immature fish (Legendre, 1986; Legendre et al., 1989). The average size of females (42.0 F 7.8 g, n = 221) is significantly less than the average size of males (44.7 F 7.8 g, n = 215) at harvest (z = 10.3, p < 0.01). Differences were not significant (Table 4). Mature fish (43.3 F 7.9 g, n = 436) had a significantly higher weight than immature ones (38.0 F 5.9 g, n = 23) at harvest (z = 10.2, p < 0.01). The change in each of the variances describing growth is shown in Fig. 2. The average weights of fish at harvest were found to be significantly different among
Table 4 Results from the periphyton-based growth trial (standard deviation, n = 3) Variables
ANOVA result
T1: fish, substrate and feed
T2: fish and substrate
T3: fish and feed
Stocking weight (g) Biomass at stocking (g) Harvest weight (g) Biomass at harvest (g) Net production (g) Average weight gained Fish recovered (%) Sex ratio Abnormality ratio (%) Specific growth rate (% day 1) Change in CF Feed conversion ratio (FCR) FCRm
p > 0.05 p > 0.05 p < 0.05 p < 0.01 p < 0.01 p < 0.05 p > 0.05 p > 0.05 p > 0.05 p < 0.01 p < 0.01 p > 0.05 p > 0.05
27.0 F 3.5 1455.9 F 3.8 45.4 F 7.3 2240.3a F 38.7 784.4a F 17.8 18.4a F 0.9 91.4 1.21 F 0.43 32.7 F 0.1 0.931a F 0.030 0.090b F 0.017 2.94 F 0.15 10.28 F 1.98
27.1 F 3.5 1461.6 F 9.3 40.2 F 7.2 2077.7b F 55.5 616.1b F 48.9 13.2b F 1.2 95.7 0.69 F 0.28 27.9 F 0.1 0.708b F 0.051 0.130a F 0.012
26.7 F 3.6 1440.5 F 32.9 43.7 F 8.2 2273.1a F 41.6 832.7a F 48.8 17.0a F 0.9 96.3 1.35 F 0.43 34.0 F 0.1 0.882ab F 0.048 0.102b F 0.003 3.18 F 0.17 14.02 F 3.68
An ANOVA was carried out on each variable to compare the treatments. Tukey’s test results are specified by superscript letters. FCRm: FCR modified for the contribution of natural food.
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Fig. 2. Change in average weight, average total biomass and SGR during the experiment for each treatment.
treatments. Fish biomass at harvest in treatments T1 and T3 were higher than in treatment T2, suggesting a significant contribution of the supplementary feed to the resulting fish biomass. The recovery rate did not vary significantly between cages (Table 4). Mortalities always occurred after stocking or sampling sessions and must be attributed to handling. A few fish also escaped during sampling. Fish from treatment T1 had a higher SGR than fish in treatment T2, emphasising the positive effect of the supplementary feed. Differences in growth rate between treatments T1 and T3 demonstrated that the additional substrate had no significant effect on net fish production.
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The FCR values obtained for the overall experiment were around 3 (Table 4). When corrected for the contribution of the natural food, FCRm became two to three times higher (Table 4). Therefore, the use of supplementary diet may not be an economically efficient way of improving yield in the periphyton-based cage system for the stocking densities used in the present study. 3.2. Multiple regression analysis of net production Regression equation obtained was: Weight gained (g) = 5.40 (t = 6.9, p < 0.001) feeding rate (%) + 1.42 (t = 1.8, p = 0.122) substrate surface area (m 2 ) + 11.64 (t = 12.1, p < 0.001), with R2 = 0.89 and p < 0.01. Only the coefficient for ‘‘feeding’’ provided a good estimate for the average weight gained in the cage. While the coefficient for substrate surface area was not significant, it is still positively correlated with the average weight gained. It should be noted that the high constant value suggests the contribution to the growth of other factors equally present in all treatments. The economic analysis of the results obtained during this experiment is shown in Table 5 (US$1 = Tk 50). The obvious extra cost of floating cage, which required more bamboo poles and anchoring system, leads us to extrapolate the economical results using fixed cage design. Fixed cages are made of polyethylene netting attached to half
Table 5 Economic analysis of the periphyton-based cage aquaculture for each treatment Economic variables
T1: fish, feed and substrate in floating cages
T2: fish and substrate in floating cages
T3: fish and feed in floating cages
Extrapolation of T2 for fixed cage
Depreciated cage cost Stocking density (fish m 3) Survival (%) Stocking weight (g) Harvest weight (g) Price of seed (Tk) Feed cost (Tk/production cycle) FCR Net production (kg) Total income (Tk) Profit (Tk)
88 22 90 27 45.4 60 15 2.94 0.77 90 71
87 22 90 27 40.4 60 – – 0.52 80 67
86 22 92 27 43.8 60 13 3.18 0.73 89 71
40 22 90 27 40.4 60 – – 0.52 80 20
16 66 31 2
– 74 35 3
15 67 31 0
– 74 35 3
Important parameters Feed cost as % income Seed cost as % income Netting cost as % income Substrate cost as % income
Treatment T2 was extrapolated for fixed cages (all prices are expressed in Bangladeshi takas). £1 = 69 Tk at the time of study. Seed and farm gate fish prices are assumed to be fixed at Tk 40 kg 1. Depreciated costs were calculated according to the period of production and durability of construction material. A period of production of 56 days was used for the analysis.
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Fig. 3. Change in chlorophyll a content, ash free dry weight (AFDW), ash and phaeopigment of the periphyton growing on the cage net over 1 week of grazing. Fish were introduced in the cage on the 6th of May.
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Table 6 Chlorophyll a content and biomass of the grazed periphyton growing on black polyethylene net at four different depth 1 day before and 1 day after stocking ( F S.D., n = 2)
2 – 8 days after stocking ( F S.D., n = 4)
Depth (cm)
Chlorophyll a (Ag/cm2)
AFDW (mg/cm2)
Chlorophyll a (Ag/cm2)
AFDW (mg/cm2)
0 – 10 20 – 30 40 – 50 100 – 110
7.5 F 1.5 8.6 F 5.8 6.1 F 0.3 0.7 F 0.0
2.4 F 2.1 1.4 F 0.9 0.5 F 0.2 0.2 F 0.0
2.2 F 1.5a 1.8 F 1.4a 2.1 F 1.4a 0.6 F 0.8b
0.38 F 0.6a,b 0.43 F 0.6a 0.26 F 0.5b 0.31 F 0.6b
Superscript letter indicates the result of the Tukey’s test between depths.
bamboo poles planted vertically in the sediment and do not require floating device or anchoring. 3.3. Nutritional contribution of the periphyton fouling on the cage net During the period of sampling, light penetration in the water averaged 128 F 6 cm (n = 4) while the temperature was 29.5 F 0.3 jC (n = 8). The composition of the periphyton biomass are shown in Fig. 3. The average nutritional values of the grazed periphyton at each depth in the cage are given in Table 6. The ash proportion in the dry matter was much higher in the periphyton (>60%) than in the plankton (26%) while the phaeopigment content was higher in the phytoplankton (26%) than in the grazed periphyton ( < 20%). The greatest biomass of periphyton was found in the uppermost 30 cm of the water column close to the compensation depth. However, the chlorophyll a content appeared to be less sensitive to light penetration than the ash-free dry weight (AFDW). These results are similar to those found by Konan-Brou and Guiral (1994). Highest periphyton biomass levels occurred just before and immediately after stocking. After stocking, the biomass decreased considerably in the uppermost three sampling depths although the periphyton biomass below 100 cm showed no change over time. The chlorophyll a content and AFDW of periphyton were generally lower following the onset of grazing, the greatest difference occurring in the uppermost 30 cm. Grazing did not appear to affect periphyton ash content although it affected phaeopigment content, phaeopigments decreasing below 20%, a sign of increasing community health (Wetzel and Likens, 1991). The nutrition value of the periphyton, summarised in Table 3, was found to be similar to that of diatoms (Brown and Jeffrey, 1995). The total energy production of 1 m2 of net located in the uppermost 50 cm of the water column was estimated as 1.4 g C day 1 = 2.8 g DM day 1 = 29.68 kJ day 1, sufficient to support an estimated fish productivity of 0.94 g wet weight m 2 net day 1.
4. Discussion The similar quantities among the cages of fish with abnormally large lower jaws clearly suggested that the abnormalities had a genetic basis or were due to injuries
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received before the experiment began. The former is more likely because smaller fish in the farm of the same origin also presented the same abnormalities. Thus, these fish were not ‘‘genetically improved’’, and they may not have been able to feed efficiently. Oedemas were frequently observed on the lower jaw suggesting that they may have had difficulty grazing on the periphyton. Their stunted state may be due to undernutrition. However, the difference between the average weight of the normal and the abnormal fish did not vary significantly among treatments, suggesting that the supplementary feed did not improve their growth. Abnormal fish may well be unable to compete with the other fish. The cages appeared to act like attraction devices in other fisheries such as Katha fisheries and ‘acadja enclos’ (Legendre et al., 1989; MacGrory and Williams, 1996). The small wild fish entering the cages stocked with tilapias may have been chased by them, which may be the reason why their number remained low in the stocked cages. The attraction of wild fish is an interesting alternative to the supply of quality seeds for periphyton-based cage aquaculture in Bangladesh. It may also however be a source of conflict with local fishermen. Problems of reproduction in cages are usually limited as the eggs are easily lost through the bottom of the cage (Coche, 1982; Beveridge, 1996). But an inflection in the growth of the tilapia was observed after the 4th week of experiment in all treatments (Fig. 2), most probably because of reproduction. Although they were small, it was subsequently discovered that the fish were approximately 10 months old at the beginning of the experiment. This stunted population found a lot of natural food in the periphyton-based cage at first and may have enjoyed good conditions during the first month when they showed excellent specific growth rates (SGR) of 0.6 – 0.8% body weight per day. The environment was therefore good enough to prepare them for reproduction. It seems also very possible that starvation of juvenile fish may cause them to begin gonad development early, as they do not ‘‘expect’’ to reach a large size or to live long. In other words the onset of reproduction may be phenotypically plastic as suggested by other studies (El-Sayed et al., in press). As the reproduction process took place, the fish allocated less energy for growth. In treatment T3, even though fish length kept increasing slightly, their weight declined between weeks 4 and 6 suggesting the beginning of the spawning period (Fig. 2). Yields obtained in extensive cage aquaculture in the Philippines range from 0.05 to 1.25 kg m 3 month 1 without input (Beveridge, 1996), which are in agreement with this study (0.12 – 0.24 kg m 3 month 1). Considering the problem of reproduction, the yield in this study could probably be improved with seed of better quality. In a parallel experiment, smaller fish were stocked at the same biomass as the one used in the present study (initial biomass = 1.475 F 0.024 g, n = 3) with 55 fish (mean wt. = 10.7 F 1.7 g) m 3 in floating cages with the same surface area of added substrate. The productivity (g m 3 month 1) was higher after only 6 weeks (final biomass = 2380.4 F 61.9 g, n = 3). The better result obtained with smaller fish showed that the potential for extensive cage aquaculture in Bangladesh remains unexplored. This may be for two reasons: the existence of natural food available proportionally to the number of fish (i.e. plankton), or the existence of energy spent proportionally by the fish (i.e. reproduction).
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Tilapias have been primarily described as filter feeders on phytoplankton (Moriarty and Moriarty, 1973; Bocci, 1999; Turker et al., in press). However, it has been shown in recent works that the Nile tilapia is also a periphyton grazer (Beveridge and Baird, 2000). Periphyton is mainly composed of diatoms, an important component of wild Nile tilapia diet (Getachew, 1993). The stomach content showed that the tilapias grown in cages in the Meghna River were both filter feeding and grazing (Huchette et al., 2000). The proportions of each feeding mode were difficult to determine, but a visual estimate of gut contents suggested a higher contribution from periphyton. Comparing treatments T1 and T3 with T2, the supplementary feed allowed approximately 29% more growth yielding a high corrected FCRm between 10 and 14 (Table 4). Thus, the supplementary feed would be an expensive input for farmers for a low efficiency. A better formulation of the diet should be possible considering recent work looking at alternatives for fish meals in tilapia diets (reviewed by ElSayed, 1999). However, the formulation of the diet used in this study was constrained by the local availability and the cost of raw material. The contribution of the additional substrates was not significant suggesting that the periphyton productivity of these substrates was low or that the range of substrate area in the experiment was too small to alter the food supply markedly. Thus, the fish were either mostly feeding on plankton or feeding preferentially on the periphyton growing on the cage net. The change in condition factors suggested that supplementary feed substantially maintained the fish in a better physical condition. Even if affected by the reproduction process, the specific growth rates (SGR) obtained in the parallel study were higher for smaller (SGR = 1.447 F 0.079% day 1, n = 3) than for bigger fish (Table 4), suggesting that the smaller fish are able to collect proportionally more food than the larger ones. Ross and Ross (1983) showed that the quantity of water passing through the gills of smaller fish was proportionally higher in response to higher oxygen requirements in small fish. By getting more water through the gills, the smaller fish could filter proportionally more phytoplankton and gather more metabolisable energy for growth than the larger ones. To improve the supply of natural food, various studies (Dempster et al., 1993, 1995; Konan-Brou and Guiral, 1994; Guiral et al., 1993) suggested increasing the amount of periphyton available to the fish in the rearing system, as grazing on periphyton allows a more efficient collection of energy than filter feeding on plankton. In extensive rearing systems such as ‘acadja enclos’ in the Ivory Coast or bamboo ponds in Bangladesh, the stocking density is very low (approximately 1 fish m 2), thus the low grazing pressure allows better development of the periphyton communities. In cages such as the ones used in the present studies, the stocked biomass has to be higher for economic reasons. Thus, the periphyton community is put under greater pressure. The grazing pressure maintained the standing crop of periphyton at the lowest level on the additional substrate, which did not appear to be sufficiently productive to contribute significantly to the diet of the fish (Huchette et al., 2000). However, the fish could only access the periphyton growing on the inside of the cage nets. This may maintain a higher periphyton biomass on the net, making it more productive than the plastic bottles. As floating cages are expensive and represent a considerable investment for the poorest farmers, the results for economic feasibility were extrapolated for the best
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treatment (T2) in fixed cages (Table 5). Fixed cages need less bamboo sticks and represent a lower investment, having a better profitability. In the fixed cages, a bagshaped net is maintained open in the water using four bamboo sticks planted vertically in the bottom of the river. None of the treatments yielded economically viable results. The net productivity was insufficient in the cages even in the best case (T2). The farm gate price was too low and the cost of seed represented 66 –74% of total income. The price of tilapia is not very high in Bangladesh because local fish, like the Indian carps, are abundant and preferred because of higher fat content. This may change in the future as fisheries production is steadily declining and as the Bangladeshis become accustomed to eating tilapias. The prices of fish are controlled by highly organised fish traders (Lewis et al., 1996). The cost of the substrate appeared to be only marginal, ranging between 2% and 3% of the total income in all treatments. Periphyton-based aquaculture was in recent years perceived to have real potential in extensive culture system (Wahab et al., 1999a,b). In cages, where the stocking densities must be high, it is especially difficult to control the production of natural food. Cages usually have a very limited surface area and thus a limited periphyton productivity, which cannot be enhanced by fertilizers like in enclosed systems (Azim et al., 2001b). Recently, Norberg (1999) tried to quantify the energetic contribution of fouling on the net of tropical cages in intensive tilapia culture. Periphyton biomasses on the black polyethylene net pieces were significantly reduced following tilapia stocking, showing that the fish grazed efficiently on the nets. Periphyton under 100-cm depth did not seem to contribute to the diet of the fish. The most important proportion of the biomass was produced above 30-cm depth. Periphyton appeared to trap and concentrate the mineral matter suspended in the water. This effect may be accentuated as nets are filtering the water. The lipid level of the periphyton was higher than the protein content, which is unusual for the microorganism community. This showed the presence of diatoms in the periphyton and a lot of cell wall material. The price of 1 m2 of polyethylene net is equivalent to 1 kg of fish (Tk 40 m 2), which requires a little less than 3 years to recover the investment. Therefore, a periphyton-based culture of tilapia using net as substrate would not be profitable. However, periphyton biomass and production could be improved using various management techniques. Substrates could be alternatively grazed and protected from grazing, allowing the rapid development of a more diverse community. Substrates protected from grazing could be set horizontally at a depth close to the compensation depth. Other systems using enclosed and fertilised water bodies could also be used to produce more periphyton and to reduce siltation problems. By increasing the amount of nutrients available, the production of periphyton could be up 10-fold (Fairchild et al., 1985). Other inexpensive substrates could be used instead of net, but they should not affect water volume and should not decompose. A supplementary feed could also be given and this would contribute to improve the periphyton growth by increasing the nutrients available in the water through the excretion of faecal and nonfaecal materials by the fish. If the production of periphyton could be multiplied by four, this would allow the system to be profitable within a few months.
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Acknowledgements We thank the DFID Aquaculture Research Programme for making available the funds for the present study. We also thank the staff of CARE-Bangladesh CAGES project, and DFID, Bangladesh, for their cooperation and technical assistance throughout the experiment. Special thanks are due to Narunnabi, Mark Ireland and the technical staff of the experimental farm. We are grateful to the staff at the Faculty of Fisheries, BAU, for providing access to their laboratory facilities. Discussions with Marc Verdegem, University of Wageningen, Jon Norberg, University of Stockholm, Rob Day, University of Melbourne and our colleague, Trevor Telfer proved useful. We are grateful to two anonymous reviewers for improving the manuscript.
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