Potential use of green seaweed (Cladophora sp.) as a partial replacement for commercial feed in tilapia (Oreochromis sp.) cultured in a brackish water pond

Nguyen Thi Ngoc Anh, Pham Thi Tuyet Ngan, Ly Van Khanh and Tran Nguyen Hai Nam

College of Aquaculture and Fisheries, Can Tho University, 3/2 street, Ninh Kieu district, Can Tho city, Vietnam
ntnanh@ctu.edu.vn

Abstract

The study was carried out to evaluate the use of green weed (Cladophora sp.) as a partial substitute commercial feed in Tilapia (Oreochromis sp.) culture in the brackish water pond at Bac Lieu province, Vietnam. The trial comprised four feeding treatments that were randomly assigned in triplicate. (1) Fish were fed a single commercial feed (CF) every day as a control; in the other three treatments, fish were fed green seaweed Cladophora (CL) every day, of which (2) was a single CL; (3) and (4) were a combination of CF and CL, with CF being given every two days and every three days, respectively.

After 90 days of culture, feeding treatments had no effect (p>0.05) on tilapia survival, ranging from 96.67% to 98.89%. Fish performance in terms of final weight (144.58-206.35 g), specific growth rate (2.62-3.01%/day) and production (1.88-2.61 kg/m²) of which fish fed CL daily combined with CF every 2 days were comparable to the control group (p>0.05), with a lower feed conversion ratio and the cost of commercial feed could be reduced by 41.66%. Fish fillet analysis showed that fish fed commercial feed had the highest crude lipid content, while crude protein content was unaffected by green seaweed replacement in the feeding regime. These findings suggested that green seaweed Cladophhora sp. can be used as a feed to partially decrease commercial feed in the grow-out of tilapia.

Key words: growth rate, production, proximate composition

Introduction

Tilapia are the second most farmed fish worldwide, with global yearly production progressively increasing from 0,38 million tons in 1990 to 6.8 million tons in 2022 (Fitzsimmons 2023). Notably, tilapia Oreochromis sp. is considered an ideal fish species for rural aquaculture because of outstanding characteristics such as high tolerance of extreme environmental conditions, fast growth rates, disease resistance and the ability to feed on various food sources, high fecundity and high flesh quality (El-Sayed 2019). In Vietnam, tilapia is commonly cultured in monoculture and polyculture systems in both freshwater and brackish water regions, with the Mekong Delta being the main production area. According to the Vietnamese Ministry of Agriculture and Rural Development (2016), tilapia products will be developed with an effective value chain and sustainability up to 2030, with farming areas expected to reach 40,000 ha, cages 1.8 million m³, and yields of 400,000 tons.

Fish meal (FM) has been used as the main protein source in the aquafeed industry due to its high protein content and amino acid balance (Tacon et al. 2011). The rapidly expanding aquaculture industry has high fishmeal demand, but global fishmeal production is unlikely to increase further, resulting in increased prices (Olsen and Hasan 2012). Commercial feed used for tilapia farming in semi-intensive and intensive systems is the largest expense, accounting for up to 40–60% of total production costs and having a major influence on profitability (Manyala et al. 2015; Kuma et al. 2017). Therefore, numerous feeding regimes have been applied in tilapia culture, such as partial reduction of feeding rate or alternative feeding regimes, to reduce pellet feed supply and operating costs, improve economic efficiency, and minimize environmental impacts (Bolivar and Jimenez 2006; Siddik et al. 2014; Siddik and Anh 2015).

Green seaweed Cladophora, belonging to the Cladophoraceae family, is a cosmopolitan macroalga that can be found in huge biomass in various marine and freshwater bodies and functions as bioremediation, habitat and natural food for aquatic organisms (Michalak and Messyasz 2020). Moreover, Cladophora sp. is rich in biologically active compounds, amino acids, minerals, pigments and essential fatty acids (Khuantrairong and Traichaiyaporn 2012; Michalak and Messyasz 2020) and can be used as a feed ingredient (Anh et al. 2018) and direct feed for many herbivorous fishes (Tolentino-Pablico et al. 2008; Michalak and Messyasz 2020). Recent investigations found that Cladophora sp. are found throughout the year in improved-extensive shrimp farms and brackish waters in the Mekong Delta of Vietnam. This green seaweed typically overdevelops in shrimp farms, which causes detrimental effects on shrimp performance and is partially removed and discarded from farms by farmers (Anh et al., 2017; 2018). This indicates that large quantities of green seaweed, a discarded product from shrimp farms, are available in the regions. Tilapia are omnivorous, capable of ingesting natural aquatic foods such as detritus, plankton and macrophytes, as well as a variety of supplemental feeds (Assefa and Getahun 2015; El-Sayed 2019). Tilapia Oreochromis sp. is commonly found abundantly in brackish water ponds and in improved extensive shrimp ponds in the Mekong Delta of Vietnam (Hai et al. 2020) and it is considered as subsistence food for local farmers in the region (Quyen 2011).

Therefore, this study was to assess the possibility of partially replacing commercial feed with green seaweed Cladophora sp. for the culture of tilapia Oreochromis sp. in a brackish water pond. These findings could motivate farmers to use available green seaweed as a food source for fish to reduce feed costs and improve their profits, as well as generate the added value of discarded green seaweed from their farms.

Materials and methods

Study site and culture system

The study was conducted in Vinh Hau commune (9°13′25″ N, 105°40′42″ E), Hoa Binh district, Bac Lieu province (southeast coast of the Vietnamese Mekong Delta), where brackish water is available all year. Twelve hapa nets were placed within the brackish water pond (Figure 1) to ensure similar water quality and other factors among the different hapa nets during the culture duration. The pond was connected to the supply canal by inlet and outlet sluices. Each hapa net was 2.25 m² in size (5 m x 5 m) with a height of 1.4 m and was made of green plastic net with a 7 mm mesh size.

Photo 1. Experimental system and some sampling activities

Source of tilapia, green seaweed and commercial feed

Tilapia (Oreochromis sp.) juveniles were purchased from a local farmer near the experimental site in Bac Lieu province and acclimated similarly to the salinity (22 ppt) of the culture pond. Green seaweed Cladophora sp. available in the experimental pond was used during feeding trial. Commercial feed (Tilapia feed UP) was produced by Uni-President company, Vietnam. The proximate compositions of Cladophora and commercial feed were analyzed by the method of AOAC (2000), as shown in Table 1.

Experimental design, feeding and management

The trial included four feeding treatments that were assigned to triplicate hapa nets at random. (1) Fish were fed a single commercial feed (CF) every day as a control treatment; in the other three treatments, fish were fed green seaweed Cladophora (CL) every day, of which (2) was a single CL; (3) and (4) were a combination of CF and CL, with CF being given every two days and every three days, respectively. The treatments were as follows:

- Treatment 1: Single commercial feed every day (CF, control)

- Treatment 2: Single Cladophora every day (CL)

- Treatment 3: CF every 2 days + CL every day (CF2+CL)

- Treatment 4: CF every 3 days + CL every day (CF3+CL)

Thirty uniform-sized tilapia with a mean initial weight of 13.37±1.18 g were reared in each 2.25 m² hapa net, which was equivalent to a stocking density of 13 fish/m². For commercial feed, fish was fed to satiation twice a day at 8:00 am and 4:00 pm according to the feeding treatments assigned. For green seaweed, 500 g of fresh seaweed was offered at each feeding time. This seaweed was bunched, hung inside a hapa net and monitored daily. When the seaweed was gone, a new one was added to ensure the continuous availability of seaweed as a feed for tilapia during the culture period. The total amount of seaweed added to the hapa net was recorded to calculate the feed conversion ratio.

The water in the culture pond was exchanged twice a month at each spring time, following the tidal regime. Each time, about 40% of the pond volume was exchanged. The hapa nets were cleaned once a week to remove periphyton attachments and maintain a continuous water exchange between the hapa nets and the pond. The trial lasted for 90 days.

Data collection

The water quality in the culture ponds, such as the temperature, pH, salinity, transparency and water level, was determined every five days. Temperature and pH were measured at 7:00 and 14:00 h with a mercury thermometer and pH meter, respectively. Salinity was measured by a refractometer, transparency by a Secchi disk and water level by a depth gauge. The concentrations of dissolved oxygen (DO), total ammonia nitrogen (TAN) and NO₂^-were determined every 15 days using test kit (Sera, Germany).

The final weight and length of fish were individually measured at the end of culture and counted to determine growth rate and survival, respectively. Growth performance of fish, such as weight gain (WG), daily weight gain (DWG), specific growth rate in weight (SGR_W) and length (SGRL), survival and feed conversion ratio (FCR), were calculated as follows:

Weight gain (g) = Final weight - Initial weight

DWG (g/day) = (final weight – initial weight)/days of culture

SGR _W (%/day) = [(ln final weight) – (ln initial weight)]/day of culture x 100

SGR _L (%/day) = [(ln final length) – (ln initial length)]/day of culture x 100

Survival (%) = Final number of prawn/Initial number of prawn x 100

FCR = feed provided (dry weight)/weight gain (wet weight)

Production (kg/m²) = Biomass of fish harvested (kg)/area of hapa net (m²)

The proximate composition of fish was evaluated at the end of the trial by randomly selecting 4 fish from each happa net. The moisture, crude protein, crude lipid and ash content in fish fillet samples were analyzed following the standard method of AOAC (2000).

Statistical analysis

All percentage values were arcsine transformed and variance homogeneity was tested using Levene's test. A one-way ANOVA to determine the overall effect of the treatment. Turkey test was used to identify significant differences between the mean values at a significance level of p<0.05 (SPSS, version 20.0).

Results and discussion

Environmental parameters in the culture pond

The water temperature and pH in the pond during the culture period ranged from 26.7 to 31.8^oC and from 7.8 to 8.3, respectively. Water depth and transparency were in the range of 70-95 cm and 28-45 cm, respectively (Table 2). Salinity was highest at the start of the experiment (22 ppt), then gradually decreased over the course of rearing, reaching its lowest value (7 ppt) by the end of the culture due to the rainy season (Figure 1). Tilapia species have been found to be highly adaptable to a variety of environmental conditions and euryhaline (Makori et al. 2017; El-Sayed 2019). Environmental factors in the culture pond greatly fluctuated during this trial, but these factors were still within the recommended ranges for tilapia growth.

Figure 1. Variations in the contents of TAN, NO2 - and salinity during culture duration

The concentrations of TAN and NO₂^- in the culture ponds were very low, varying in the ranges of 0.12-0.19 mg/L and 0.18-0.26, respectively. This suggested that the water quality in the rearing pond was optimal for tilapia culture ((Makori et al., 2017; El-Sayed, 2019).

Growth and survival of tilapia after 90 days of culture

Figure 2. Growth curve in weight of tilapia during culture duration

Figure 2 showed that the individual weight of tilapia (60.9-85.3 g) was affected by feeding treatments from the first 30 days, with the largest observed in the control group (fish fed single CF every day), followed by the combined feeding regime with CF every 2 days (CF2+CL) and CF every 3 days (CF3+CL) and the smallest in the CL group (fish fed single green seaweed). A similar trend was found from day 60 onward.

After 90 days of rearing, the final length and specific growth rate of fish (SGR_L) were 19.50–21.30 cm and 0.62-0.72%/day, respectively, with the highest in the CF group and the lowest in the CL group. There were significant differences (p< 0.05) between these two treatments, but they were insignificant (p>0.05) with the two remaining groups Table 3).

The final weight and WG of fish varied from 144.58 to 206.35 g and 130.86 to 192.62 g, respectively, corresponding to DWG and specific growth rates (SGR_W) of 1.45-2.14 g/day and 2.62-3.01 %/day, respectively. The growth rate in the CF2+CL was comparable to that in the control treatment (p> 0.05), while both were superior (p <0.05) to the CF3+CL and CL treatments (Table 3).

The survival of fish ranged from 96.67% to 98.89%, was not influenced by the reduction of commercial feed supply (p>0.05). The average fish production ranged from 1.88 to 2.66 kg/m², with the CF2+CL and control groups producing nearly equal fish quantities and significantly (p < 0.05) higher than the CL group (Table 3).

The average FCRs of CL were 0.74–1.59, with the highest value in the single-feeding CL and the lowest in the CF2+CL treatments. The FCR of CF was highest in the control groups (1.13) and lowest in the CF3+CL group (0.52).

It is well known that green seaweed Cladophora has excellent nutrition compositions, including essential amino acids and fatty acids, minerals, pigments and bioactive compounds (Khuantrairong and Traichaiyaporn 2012; Michalak and Messyasz 2020). This seaweed has been used as a feed ingredient (Anh et al. 2018) or as direct feed for herbivorous fish species (Tolentino-Pablico et al. 2008; Michalak and Messyasz 2020). Therefore, tilapia in the present study fed green seaweed combined with commercial feed every 2 days could achieve nutrient balance for their growth.

Our findings are in accordance with the study of Siddik et al (2014), who found that the growth performance of Nile tilapia Oreochromis niloticus in the alternating feeding treatments of 1 day of commercial feed (CF) followed by 1 day of fresh or dried gut weed was not significantly different from that of the control group receiving only CF, but fish fed the fresh or dried gut weed as a sole diet showed the poorest growth. Analogous results were detected for herbivorous fish species including red tilapia Oreochromis sp., spotted scat Scatophagus argus and giant gourami Osphronemus goramy (Siddik and Anh 2015). These authors also stated that feeding treatments with solely CF or gut weed, as well as combinations of both had no adverse impacts on the survival of these species. An and Anh (2020) discovered that Nile tilapia co-cultured with red seaweed (Gracilaria tenustipitata) without receiving CF had survival comparable to those in monoculture fed CF to satiation.

Notably, tilapia in this study fed CF every three days along with green seaweed Cladophora (CL) or only CL as a feed, resulting in their growth rates being noticeably worse than the fish fed CF every day. This could be due to the differing proximate compositions of CF and green seaweed, as CF comprises 29.88% crude protein and 6.04% lipid, while Cladophora has 14.59% crude protein and 1.04% lipid (Table 1). This indicated higher protein and lipid in CF than in green seaweed. According to Winfree and Stickney (1981), the protein requirement of tilapia decreased with age and size, with higher dietary crude protein levels required for fry (30–56%) and juveniles (30–40%) but lower protein levels (28–30%) for larger fish. Lipid levels of 5–12% are optimal in tilapia diets (Lim et al. 2009). For these reasons, fish fed CF every 3 days or only CL as feed may not satisfy their nutritional requirements, resulting in the substantial reduction in growth rate found in the present study.

Results of this study indicated that FCR and CF cost decreased with increasing feeding frequency of CF, which is consistent with the findings of Siddik et al. (2014), who discovered that FCR and feed cost in tilapia fed alternate feeding regimes of CF and gut weed were considerably lower than those fed single CF. Similar results were seen in spotted scat, giant gourami and red tilapia (Siddik and Anh 2015) or tilapia co-cultured with red seaweed with a reduction in feeding rate (An and Anh 2020).

The current findings illustrated that using the green seaweed Cladophora sp. available in the culture pond as a feed for tilapia can partially diminish the need for commercial feed, helping to lower feed costs and enhance economic efficiency as well as control overgrowth of green seaweed in the improved-extensive shrimp ponds.

Proximate composition of fish fillet

The contents of moisture (water content of fresh fish fillet) and crude protein in fish fillet showed no significant differences (p<0.05) across feeding treatments, varying in the ranges of 74.98–78.02% and 64.39–68.75%, respectively. The highest lipid level (10.47%) was observed in fish-fed single CF and this level decreased with a decrease in CF supply, with the lowest value (6.24%) found in the single CL-fed group. Ash contents were from 7.11% to 8.19%, where the control group was significantly lower (p<0.05) than in other feeding treatments (Table 5).

Previous research has found that the biochemical composition of tilapia is influenced by numerous factors, including culture conditions, food type, size, sex and species (Olopade et al. 2016; Jim et al. 2017). Our results are supported by the study of Siddik et al (2014). A carcass analysis of Nile tilapia showed the highest crude lipid content in fish fed single commercial feed and the lowest in fish fed single gut weed, while protein content was not influenced by the replacement of commercial feed with gut weed. However, An and Anh (2020) revealed that in a co-culture of Nile tilapia and red seaweed, feeding rates significantly affected the proximate composition (moisture, protein, lipid and ash) of fish fillet.

Conclusion

Green seaweed Cladophora sp. can be used as a feed for tilapia Oreochromis sp. combined with feeding pellet feed every 2 days without affecting growth and production of fish.

Acknowledgements

This study was funded by Can Tho University for scientific activities. The authors would like to thank Nguyen Thi Thuy An, Nguyen Thi Mai and Nguyen Hoang Tam for their assistance during the experiment.

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