The increasing trend of fish feed cost poses a challenge in intensive culture systems (Haider et al., 2016; Iqbal et al., 2020a; Bjørndal et al., 2024), with dietary protein being identified as the costliest component in the production of manufactured fish food. Generally, the cost of feed constitutes around 56.45–58.49% of the total production expenses in aquaculture industry (Hossain et al., 2022). In specific cases, it may even escalate to 60–70% (Khan M. A. et al., 2018; Khan N. et al., 2018; Prodhan and Khan, 2018; Hossain et al., 2020a,b). This is primarily attributed to the heightened requirements for protein, contributing significantly to the overall costs. Fishmeal (FM) is widely employed as a protein source in the majority of formulated diets and is considered as the costliest component in fish diets (Moniruzzaman and Fatema, 2022). Additionally, it plays a crucial role in providing essential nutrients that promote fish growth and ensure their long-term well-being (Batool et al., 2018; Haider et al., 2018; Bjørndal et al., 2024). FM is well balanced with respect to essential amino acids, fatty acids and minerals, has a low carbohydrate content, and is free of anti-nutritional factors with high palatability and digestibility (Gatlin et al., 2007; Bhuyain et al., 2019). As a result, the rapid growth of aquaculture demands for higher FM (NRC (National Research Council), 2011). Small pelagic fishes such as anchovies, sardines, mackerel, capelin, and menhaden are known to contribute about 90% of produced FM worldwide (Tacon and Metian, 2009). However, FM resources are finite, continuous pressure on these fish species’ natural stock due to overfishing is likely to increase the scarcity and price of FM in the near future (Hardy, 2008; Savonitto et al., 2021). There is also acute scarcity in the supply of FM because of the equally high demand of this protein source from other animal husbandry practices and uncertainty in collecting wild trash fish, which is the primary source of FM (Naylor et al., 2000). Therefore, a higher cost and fluctuating FM supply necessitate replacing FM with cheaper, alternative protein sources with acceptable amino acid composition (Santigosa et al., 2011; Köprücü and Sertel, 2012; Al-Thobaitia et al., 2018). In this case, the more affordable and alternative FM replacement options could include plant protein ingredients (PPI), animal byproducts, and other novel protein feedstuffs (Kishawy et al., 2021).

The effectiveness of various PPI as a partial and complete replacement of FM in aquafeeds has been investigated by several researchers (Suprayudi et al., 2015; Aziza and El-Wahab, 2019) whereas, soybean, barley, corn, cottonseed, wheat, mustard oil cake, rice bran etc. can replace FM and are widely used in aquafeeds (El-Saidy and Gaber, 2002; Gatlin et al., 2007; Zamal et al., 2008, Koumi et al., 2009, Brinker and Friedrich, 2012; Khan et al., 2013; Ibrahem and Ibrahim, 2014). However, the inclusion of PPI (>50%) are sometimes reported to reduce the growth performance compared to that of fish fed FM-based diets (Collins et al., 2013; Yaghoubi et al., 2016; Turchini et al., 2019). Because PPI are possessing anti-nutritional factors and indigestible carbohydrates, protein digestion and absorption of amino acids are less efficient in fish (Lall and Anderson, 2005). On the other hand, using only FM to the diets sometimes results in the waste of excessive protein which increases the load of nitrogen and phosphorus in the water and deteriorate the water quality in fish pond (Hardy, 2010). Study shows that the partial replacement of FM by PPI can reduce Phosphorous (Ketola and Harland, 1993) and Nitrogen excretion (as ammonia) by reducing protein levels (Cheng et al., 2003). Therefore, additional research is needed to adequately determine the inclusion rate of PPI in the partial replacement of FM in fish diet which could be readily available, cheap and environmentally friendly (Hernández et al., 2016; Hossain et al., 2021). Studies indicate that diets allowing for the partial or complete substitution of FM can be feasible through a meticulous formulation process (Espe et al., 2006; Kousoulaki et al., 2012).

Labeo rohita, locally known as rohu, is a culturally and economically significant fish species in Bangladesh, playing a vital role in aquaculture. Its vigorous biology and adaptability make it a preferred species for sustainable fish farming, contributing significantly to economic livelihoods, nutritional security, and the overall resilience of local communities (Jewel et al., 2020a; Pervin et al., 2020). The significance of developing aquaculture for L. rohita with low-cost feed in Bangladesh cannot be overstated as it is one of the sustainable practices. Because, by utilizing affordable feed options, aquaculture becomes more accessible to a broader spectrum of farmers, fostering widespread participation and contributing to poverty reduction. L. rohita, being a major cultivable species in the country, ensures that the production of this fish with low-cost feed provides an affordable protein source, positively impacting the nutritional well-being of the population. Additionally, the adoption of cost-effective feed formulations reduces the overall production costs, enhancing the competitiveness and profitability of aquaculture ventures in Bangladesh (Akter et al., 2018; Pervin et al., 2020; Jewel et al., 2023a).

In Bangladesh, high feed costs in aquaculture industry impose a significant economic burden on rural farmers, leading to reduced profitability and limiting the accessibility of aquaculture activities. Additionally, these elevated costs can contribute to food security concerns by potentially increasing fish prices and impacting the affordability of this essential protein source for local communities.

A primary factor contributing to this is the rising expense of fish feed ingredients, particularly the cost of FM in Bangladesh. Hence, replacing fish meal with plant-based protein sources has the potential to decrease costs. Considering the availability and crude protein content, a combination of Mustard oil cake (MOC), soybean meal (SBM) and rice-bran (RB) can be a suitable replacement option for FM. It is widely prevalent and extensively utilized as a component in aquafeed across the country. Because, mustard is one of the major oilseed crops occupying 78% of the cultivated area and contributing nearly 62% of the cultivated area of the total oilseed production in Bangladesh (Bangladesh Bureau of Statistics (BBS), 2003). MOC serves as a relatively good source of crude protein (Bhuyain et al., 2019). Additionally, it is more cost-effective compared to other oil cakes in Bangladesh. Moreover, SBM stands out as the most frequently employed plant ingredient, boasting high protein content (approximately 48% crude protein) and a relatively stable amino acid profile (Ye et al., 2019; Meng et al., 2020; Pervin et al., 2020). RB or polish, a by-product of rice, is abundantly available throughout the year in Bangladesh. Several studies also demonstrated that RB contain 13–15% protein and 11–12% lipid (Saunders, 1990; Alencar and Alvarenger, 1991; Nyirenda et al., 2000) which signifies its role as a suitable ingredient to be used for FM replacer. However, research on low-cost feed development using locally available ingredients specially using PPI for L. rohita culture in Bangladesh is very limited. Hence, this study aims to (i) assess the nutritional effectiveness of MOC, SBM, and RB as a replacement for FM in fish diets, (ii) investigate the economic impact of different FM replacement levels in a cage culture system (CSS), (iii) analyze the proximate composition of harvested fish for nutritional quality assessment, and (iv) conduct an economic analysis to determine the profitability of using PPI as a FM substitute in L. rohita culture. The insights gained from the development of low-cost aquafeeds can be extended to advance sustainable aquaculture practices for various species. This not only contributes to economic empowerment and food security but also emphasizes a commitment to responsible and inclusive aquaculture practices in Bangladesh and other developing nations.

This study attempted to investigate replacing FM with PPI for L. rohita reared in CCS. Significantly higher finishing weight of fish was achieved in Diet 2 group and lower in Diet 3. Weight gain and SGR alluded to substantially higher performance in Diet 2 that declined in, Diet 3. The replacement of animal protein sources (FM) up to a certain level with plant protein sources was not detrimental, as was evident in the findings of Furuya et al. (2004) and Lin and Luo (2011). Even a 50% replacement of FM in diets has been reported to be favorable for the overall fish growth performance (Viola et al., 1982; Jahan et al., 2012). However, replacing 75% animal-source protein reduced fish growth in the present experiment in Diet 3. Incorporating PPI higher than the sub-optimal level might negatively affect fish growth. Although, we have not measured the level of anti-nutritional factors (ANFs) present in PPI in our study. However, we presume that the greater PPI might increase toxic components (ANFs) and imbalance the amino acid profile responsible for intestinal irritation and reduced growth (Olvera-Novoa et al., 2002). This understanding corroborates the earlier outcomes by Hua and Bureau (2012), who opposed the total replacement of FM protein with plant protein as it could be detrimental to the cultured organisms. Complete FM replacement is also reported to decrease protease activities in the intestine and hepatopancreas in Juvenile Tilapia (Lin et al., 2010). Jalili et al. (2013) also found reduced digestive enzyme activities and subsequent lower growth in rainbow trout fed the diet with 75 and 100% FM replacement. In the present study, 75% replacement of FM by PPI in Diet 3 may be the reason for the reduced growth performance of L. rohita to the other treatments.

The proportion of protein and non-protein energy sources is necessary while preparing a balanced diet. An excess protein in the diet causes higher ammonia, affecting fish growth performance (Kaushik and Medale, 1994). When adequate non-protein energy sources are available in the diet, it could minimize the use of protein as an energy source and enhance fish’s growth performance (Iqbal et al., 2020b). Carps are the most efficient exploiters of carbohydrates (Kumar et al., 2005). The intrusion of carbohydrates in the form of PPI in Diet 2 could impart a protein-sparing effect that may enhance the feed utilization by fish. The protein-sparing effect of suboptimal levels of carbohydrates was also reported in silver barb by Mohanta et al. (2007). An appropriate level of carbohydrate in the diet can reduce protein degradation and amino acid oxidation which results in improved growth (Frick et al., 2008). However, several studies also reported that dietary carbohydrate beyond the optimal level can cause lower growth and feed utilization in fish (Tan et al., 2009; Gao et al., 2010; Yu et al., 2022). Therefore, lower growth and feed utilization in Diet 3 group was lower despite of the higher carbohydrate level. Therefore, replacing FM in the diet did not affect fish survival. Jahan et al. (2007) found no significant difference between the treatments regarding survival rate in a partial replacement experiment of FM with SBM for the fry of Cirrhinus cirrhosus. Replacement of FM in fish diets can significantly affect the total fish yield, whereas the higher production was recorded in Diet 2 group with a 50% replacement of FM. However, the higher inclusion rate of PPI in Diet 3 caused a significant reduction in the total yield.

The FCR decreased in the fish of Diet 2 compared to the Diet 1 fish and increased significantly in Diet 3. The earlier investigations have argued that low FCR indicates higher feed utilization efficiency, balancing bioavailability and partitioning dietary nutrients toward growth (Angelidis et al., 2005). Zamal et al. (2009) reported a high-energy diet produced a lower FCR and higher nutrient retention in the fish body. Therefore, 50% FM replacement in feed increased the feed efficiency, but it decreased in growing proportion for PPI in the feed (Diet 3). The present findings are supported by the conclusions of Devi et al. (1999), who reported relatively better (lower) FCR in L. rohita fingerlings fed on a diet including 20% SBM.

Similarly, they found a higher FCR while increasing the proportion of SBM up to 40 and 60%. PER is related to the dietary protein intake and its conversion into fish weight gain (Koumi et al., 2009). The fish fed on varying levels of a FM replacement diet showed a significantly higher PER in Diet 2 and lower in Diet 3. The lower PER obtained in Diet 3 might be due to the imbalance of amino acid profile as affected by the higher inclusion rate of PPI, which was also supported by Espe et al. (2008). Plant protein sources generally have lower biological value and palatability properties (Estevez et al., 2011), which may be responsible for lower feed utilization in Diet 3 group. However, a balanced proportion of FM and PPI in Diet 2 might provide essential amino acids and increase fish feed utilization. The study by Espe et al. (2006, 2010) and Estevez et al. (2011) also showed that the palatability of a plant protein-based diet could be enhanced by adding essential amino acids and other animal proteins protein sources.

Replacement of 50 and 75% FM in Diet 2 and Diet 3 resulted in a 20.62 and 32.76% reduction in total feed cost, while 15.61% higher net economic return was obtained from Diet 2 compared to the Diet 1 group. A study conducted by Khan et al. (2013) reported that 24% of feed formulation costs could be reduced by the replacement of FM with rice polish (up to 20%) and MOC (up to 22%) without changing the nutritional quality in an experiment on Oreochromis niloticus. However, a higher inclusion rate of PPI in Diet 3 reduced the fish growth and subsequent reduction of 41.61% net economic return compared to Diet 1 in the present study. Finally, the replacement of 50% FM provided a higher BCR (1.72) compared to the other treatments. Apart from fish feed types, various environmental factors such as water temperature, turbidity, pH and ammonia play a crucial role in the CSS that must be considered (Ara et al., 2020, 2023; Jewel et al., 2020b, 2023b; Bashak et al., 2021).

Replacement of fish meal with FM at different levels significantly impacted the muscle protein and lipid content of L. rohita. Fish in Diet 2 retained a higher protein level compared to other treatments. However, lipid content was reduced considerably in Diet 3 compared to the Diet 1 group, potentially due to a higher inclusion rate of PPI in the diet. A similar observation by Devi et al. (1999) has reported higher protein and lower lipid levels in the muscle tissues of L. rohita fingerlings fed with a plant protein-based diet (SBM-based rations) compared to the Diet 1 group (0% SBM). Contradictory results were observed by Khan M. A. et al. (2018) and Khan N. et al. (2018). They reported that incorporating plant protein by replacing FM did not significantly affect the whole-body composition of Indian major carps, Catla catla, L. rohita and C. cirrhosus. A significant effect of FM replacement on the proximate composition of fish during the present study may be linked to anti-nutritional factors in PPI, although several studies reported the influence of FM replacement with plant protein sources on the proximate composition of fish (Olli and Krogdahl, 1995; Elangovan and Shim, 2000).

The present study affirmed that 50% replacement of FM by PPI could be economically efficient in reducing the feed formulation cost by approximately 20.45% without changing the proximate composition (nutritional quality) of L. rohita reared in the CSS. A replacement of 50% FM with PPI also increased the total net return by 15.61%, which was the most economical among all the experimental treatments for the cage culture of L. rohita in ponds. This study can also be extended to other leading aquaculture fish species to test the potential of using PPI as an alternative protein source of FM to reduce the increasing pressure on FM demand. This approach aligns with sustainable practices by promoting environmentally friendly alternatives and encouraging knowledge-sharing among practitioners. Therefore, the development of aquaculture for L. rohita using low-cost feed will not only supports economic empowerment and food security but also embodies a commitment to responsible and inclusive aquaculture practices in Bangladesh or other developing nations. However, in the current investigation, ANFs of PPI were not examined, which may restrict the acceptability of the designed diet. As a result, the current study recommends that the PPI’s ANFs be examined before to their usage in feed composition.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

The animal study was approved by University of Rajshahi, Bangladesh. The study was conducted in accordance with the local legislation and institutional requirements.

SA: Data curation, Writing – original draft. AH: Data curation, Writing – original draft. A-AS: Supervision, Writing – review & editing. UA: Software, Writing – review & editing. SI: Software, Writing – review & editing. PS: Writing – review & editing. BP: Writing – review & editing. TA: Writing – review & editing. MH: Resources, Software, Writing – review & editing.