1. Introduction
In recent decades, aquaculture has been the world’s fastest-growing food-production sector, providing a protein-rich supplement [
1]. Bangladesh has been among the top five fish-producing countries [
2,
3], and production has increased by 53 percent since 2009 [
1]. By 2022, this country will be among the top four fish-producing countries [
4]. The country has a coastal area of 2.30 million hectares and a 714 km coastline along the Bay of Bengal and supports large numbers of artisanal and coastal fisheries. Moreover, post maritime boundary settlement with Myanmar and India, marine capture from the Bay of Bengal is likely to be increased manifold [
5,
6]. Seafood is the second most valuable export commodity from Bangladesh. It contributes almost 3.65% to gross domestic product (GDP), 25.30% of gross agriculture products, and 2.0% of total export earnings, worth
$526.45 million USD [
3,
7,
8,
9,
10]. This contribution would have been higher if the seafood waste had also been used effectively. Among the different types of seafood, frozen shrimp and fish are the main seafood export items [
7,
8,
9,
10]. Public and private organizations have installed more than 100 shore-based export-oriented fish processing plants at Chittagong, Khulna, Dhaka, and Sylhet divisions. Out of 100 processing plants, 76 are European Union (EU) compliant, and 30 plants are USFDA green ticketed [
11] with a total of 350,000 tons processing capacity [
8]. Bangladesh’s seafood processing industries have the vast potential for both vertical and horizontal expansion [
6,
7,
10,
12].
Seafood industries produce a considerable quantity of processing byproducts, which are mainly inedible and constitute approximately 40–60% of the wet weight [
13,
14,
15]. Seafood waste is mainly composed of heads, viscera, bones, and scales and is rich in lipids, proteins, and other bioactive compounds [
16]. The ever-increasing production of these byproducts without utilization has resulted in environmental pollution [
17]. Waste management activities include collection, transportation, processing, utilization, and disposal [
18]. Monitoring and controlling of waste management methods are also considered important [
19]. Inappropriate waste management (e.g., open dumping, indiscriminate littering) causes environmental pollution and forms breeding grounds for insects and vermin, posing significant public health risks [
20]. Consequently, waste management practice is coming under strict regulations due to environmental issues and has become an increased cost burden for the seafood industries [
21,
22]. The proper management and utilization of seafood processing waste and its conversion into value-added products will lead to better resource utilization and profit maximization [
23,
24]. This also results in significant environmental and economic improvement. In contrast, underutilization of byproducts leads to loss of potential revenues and additional disposal costs [
20,
23].
Seafood waste has many applications, among which the most important are animal feed, biodiesel, and biogas [
14,
25,
26]. Furthermore, seafood waste contains several bioactive compounds like chitin, collagens, biofilms, pigments, amino acids, and fatty acids [
27,
28,
29]. Efficient utilization of seafood waste can only be achieved when properly utilized and no wastage is allowed [
13,
30,
31,
32]. Value addition, product diversification, utilization, and efficient management of seafood waste will generate more profits [
33,
34,
35]. For efficient seafood waste management, information on the amount, types, and existing management status is essential. However, for many countries including Bangladesh, information on the amount of seafood waste produced and the present management status is still lacking [
36]. Due to biological instability, high moisture content, high enzymatic activities, and rapid auto-oxidation, utilization of seafood waste is difficult. Thus, waste management and disposal in the seafood processing industry pose problems in terms of environmental protection and sustainability [
31,
37]. In less technology-intensive countries, seafood waste is usually used to produce animal feed ingredients such as fishmeal and silage [
23,
38,
39,
40]. The seafood waste-based silage production process is relatively simple and is much less expensive than producing fishmeal [
41,
42,
43]. Moreover, seafood waste (fish and shrimp)-based silage production fits best at places where the supply chain is not well organized, and the amount and availability of waste are not sufficient to justify the operation of a fishmeal plant [
41].
Fish silage is a protein-rich liquid produced from enzymatic hydrolysis of fish byproducts and bycatch [
32,
43,
44] that is rich in a mixture of hydrolyzed proteins, lipids, and minerals. It is easily digestible and absorbed by terrestrial and aquatic animals [
35,
45,
46]. Silage is prepared by combining ground processing waste material with inorganic acids to attain a lower pH (<4.0) where bacterial growth is inhibited, and spoilage is prevented. Thus, silage can be stored for years and used when needed [
47]. Details of the fish and shrimp silage production techniques are available in the literature [
48,
49,
50]. Fish and shrimp silage has the high potential for use in aquaculture due to the similarity in raw materials, protein source, and low operation cost compared to fishmeal [
45,
51,
52,
53]. Fish and shrimp silage can act not only as a useful feed ingredient but also as a feed additive [
32]. Fish silage-based diets fed to
Litopenaeus vannamei [
51,
54,
55]; Nile tilapia,
Oreochromis niloticus [
22,
56,
57,
58]; pacu,
Piaractus mesopotamicus [
46,
59]; broiler chicken [
39,
40,
60]; quails,
Coturnix japonica [
61]; and rohu,
Labeo rohita [
53] animals showed improved growth, digestibility, and physiological fitness. Therefore, the application of silage-based diets for aquaculture and livestock production is promising. However, for Bangladesh, there are insufficient data available on the amount of seafood waste generated and the present status of waste management and utilization. Information on the potential for seafood waste-based silage production is lacking. Thus, a better understanding of the amount of seafood waste produced and its management status is essential to prepare an effective seafood waste management plan for silage production. In this study, quantitative and qualitative estimation was conducted to identify the amount of seafood waste generated and to understand existing management practices. Furthermore, animal feed producers were also surveyed to assess the potential for fish silage production and utilization.
4. Discussion
Shrimp and fish are the second-largest export items in Bangladesh after readymade garments [
5]. Every year a considerable quantity of processing byproducts are produced [
83] in the seafood processing industries [
84,
85]. However, information on the amount of seafood waste generated in Bangladesh was not available, which made it difficult to compare the findings of this present study. To the best of our knowledge, this is the first baseline study on the amount of fish waste produced and present waste management status in Bangladesh. In Chittagong and Khulna regions, the highest amount of shrimp and fish waste is produced. Seafood industries processed both captured (freshwater, marine water) and cultured shrimp and finfish [
8]. Fish landed at Chittagong and Khulna through 76 and 12 landing centers, respectively [
86], from farms, wild stocks, and marine catch. These fish are exported as gutted, beheaded, and scaled products, which generates a substantial amount of waste. The current survey indicated that significant quantities and varieties of finfish and shellfish wastes are being produced. An interesting corollary of increased demand for exports of shrimp shell and fish scale to other Asian countries has been contributing slightly to a reduced volume of wastes. The survey also identified opportunities where a significant amount of waste can be used to produce sustainable aquaculture and livestock feeds.
An essential aspect of the present study was the quantitative evaluation of seafood waste for further processing. Such evaluation was difficult due to poor recordkeeping. Thus, to cross-check and validate the present study findings, the estimated seafood amount was compared with seafood export data. In 2017, the total exported amount was 68,935 tons [
11], of which 65% was shrimp, 27% was fish, and the remaining 8% was fins and air bladders [
8]. Another approach was followed to quantify the volume and value of fish and shrimp waste from the export data of the Bangladesh Frozen Food Export Association [
8]. The estimated volume and value of the present findings agree with the 2016-2017 export data (
Table 11). Fresh shrimp and fish heads were found to be consumed by local people and used to feed farm fish. However, the fish feeding process was found wasteful and inefficient (
Supplementary File 3). Direct application of seafood waste as fish feed acts as a threat to sustainable aquaculture. The used raw seafood waste was produced in different environments, thus increasing the chance to introduce potentially harmful pathogens (algae, amoebae-protozoans, and viruses) to which farmed fish and shrimp can be susceptible [
31]. Excessive bio-deposition of applied waste as feed puts aquaculture at risk by deteriorating water quality [
9,
87]. A portion of the fish air bladders, scales, fins, and shrimp heads and appendages (both spoiled and fresh) are dried and exported to China, Thailand, and Vietnam. However, sometimes, specifically during abundant supply, fish entrails, scales, gills are carried away and discarded as landfill. However, the estimation of the waste export volume was not possible for lack of recordkeeping. Seafood wastes are an important source of environmental pollution [
37]. Environmental impacts from seafood waste appear at all stages of the product lifecycle, from the collection of resources, processing, and disposal [
23]. Fish waste affects not only the surrounding area but is also directly affected by the effluent discharge. However, it can also alter wider areas at different ecosystem levels and the biomass, density, and diversity of the benthos, plankton, and nekton in the receiving water body [
34].
Seafood waste is continuously gaining ground as a waste management field. Research has been carried out to convert these wastes into useful products [
88,
89,
90,
91]. Among the most prominent current uses of seafood waste are collagen and antioxidant isolation for cosmetics, biogas, biodiesel, fertilizers, chitosan, packaging materials (e.g., gelatin, chitosan), enzymes (e.g., proteases), feed ingredients (e.g., fish sauce, fishmeal, and fish silage) [
30,
31,
92]. Most of the technologies known for the utilization of seafood waste are not economically attractive due to the high initial investment [
92]. Among these options, some are technology-intensive and require higher investment costs. However, fish waste can be profitably used as fish feed through fishmeal and fish silage production [
20], since seafood waste represents half of the raw material and could be an excellent source of low-cost nutrients [
30]. In some countries, fishmeal is being traditionally produced from seafood waste, but this requires high investment, a high degree of coordination, a well-organized supply channel (with cold chain facilities), and an adequate volume of steady supply [
16,
23]. Seafood waste transportation to fishmeal plants is not always sustainable practice for the seafood processing industry [
34,
65,
93]. In Bangladesh, these facilities are not available. Thus, it is essential to find suitable alternatives to manage waste within the industries or nearby, emphasizing energy savings, environmental concerns, and sustainability. Fish silage production can be a viable alternative, as it is an easy-to-make product that is less technology-intensive that requires low investment and little space [
42,
67,
94]. Moreover, the advantages of silage production over fishmeal are: The process is virtually independent of the supply scale (e.g., amount, volume, and raw material quality); simple technology; little initial investment for large-scale production; reduced effluent and odor problems; and the product can be used locally [
42,
95]. In addition, fish and shrimp silage has good nutritional values and can therefore be used as an attractive alternative to fishmeal [
46,
57,
96].
Seafood byproducts and waste-based silage production procedures are safe, cost-effective, and environmentally friendly [
97]. The product has the excellent nutritional quality and can be sufficient for animal feeding [
22,
45,
98,
99]. To understand the potential for the production and utilization of fish and shrimp silage from seafood waste, we surveyed the feed producers and fish farmers. The majority of seafood processors were unaware of fish silage and its potential as a profitable waste management method. The incorporation of seafood waste-based silage will be a new development in the production of animal feeds. Moreover, the use of fish silage in Bangladesh has not been studied yet. In the present study, most seafood industries and animal feed millers have shown their interest in fish and shrimp silage because both aquaculture and livestock farming are substantially dependent on high-cost feed inputs and exploring options for cheaper costs and improved feed conversion ratios [
100,
101,
102]. However, for seafood processors, hands-on training is required to start fish and shrimp-based silage production. Seafood processors and feed millers are reluctant to produce and use silage for the following reasons: (i) a lack of fish and shrimp silage technology, (ii) reliability of fish waste supply for silage production and fish silage supply to the feed millers, and (iii) transport cost to carry fish waste and fish silage. Higher transport costs for fish silage than dry components (e.g., fishmeal) are cited as a disincentive [
20,
45]. While this proposition is not true on a per ton of protein basis, it discounts the fishmeal transport costs from abroad [
103].
Fish and shrimp silage is a liquid product produced from the whole fish and shrimp or parts, to which acids, enzymes, or lactic-acid-producing bacteria are added, with the liquefaction of the mass provoked by the action of enzymes from the fish [
45,
104]. Silage can be stored for years and used when needed [
47]. The preparation of silage usually depends on locally available raw materials and conditions [
105]. Organic acid, most preferably formic acid, is the best choice for silage preparation. Formic-acid-made silage is not excessively acidic and does not require neutralization before use and thus can be used directly [
45,
106]. The increasing demand and progressive scarcity of fishmeal in the international market boosted its price and launched attempts to reduce fishmeal in fish diets and the consequent search for alternative, acceptable, and digestible protein sources [
106,
107]. Fish and shrimp silage can be an excellent fishmeal alternative. This will be an incentive to utilize seafood and fish waste as a protein source for aquaculture and livestock [
108] production in Bangladesh. Fish and shrimp silage is considered a valuable feed ingredient that has been shown to improve animal feed quality [
32,
44]. With fish and shrimp silage, 20–75% of fishmeal can be replaced during feed preparation [
47]. The use of fish silage as a feed ingredient could make differences in some ways, e.g., by (i) reducing levels of waste: environmental impact; (ii) providing nutrients and bioactive components: animal health; (iii) economic gains: as an alternative to replacing expensive fishmeal [
32,
47]. Seafood waste-based silage production draws potential attention and has increased importance compared to fishmeal due to the simple and easy production technology and lower investment cost [
52]. Feeding cost represents 50% of the operational cost in the aquaculture industry to ensure expensive dietary protein sources [
109,
110]. There is a shortage in the world production of fishmeal, which is the main protein source [
46,
100]. Currently, the increased demand for fishmeal for aquaculture is considered a possible deterrent for aquaculture growth [
34]. Fish and shrimp silage can substitute for fishmeal without impacting animal growth. Research has shown fish silage as a good feedstuff for aquafeeds in terms of nutritional benefits and economic feasibility [
30]. Inclusion of fish silage in feed increases the appetite and growth of farm animals [
47]. For example, fish and shrimp silage-based diets used to feed Nile tilapia,
Oreochromis niloticus, squilla,
Oratosquilla nepa, and chicken had good growth performance equivalent to a fishmeal diet [
45,
98,
108,
111,
112]. Likewise, fish silage-based diets supplied to Nile tilapia
O. niloticus [
113] and African catfish,
Clarias garipinus provided 25% to 50% fishmeal replacement [
97]. Digestibility and growth studies have shown that fish silage is highly digestible and can be used to replace up to 75% of fishmeal in aqua-feeds [
52]. For shrimp,
Litopenaeus vannamei fed with 25% to 50% silage-based diets showed higher growth performance [
51,
114] and improved gut health [
54]. Fish silage not only acts as a feed ingredient but also an essential feed additive [
32]. Moreover, 100% fishmeal replacement is also possible if fish silage is used along with other protein-rich plant ingredients, for example, soybean meal [
51] and rice bran [
113]. The free amino acids and peptides in the silage are pre-digested proteins, and the presence in the feed results in improved growth. Moreover, the organic acids in fish and shrimp silage have useful antimicrobial properties, enabling livestock to perform better against diseases and in terms of mortality. Eventually, this could contribute to eliminating the use of non-therapeutic antibiotics in livestock feed [
32,
46,
47].
In Bangladesh, there is a considerable gap between production origin and use of fish, shrimp, and poultry feed ingredients [
70,
115]. The incorporation of seafood byproduct-based silage could contribute to reducing the gap. Given the relatively wide geographical area (Khulna, Chittagong, Dhaka, Sylhet) covered by Bangladesh’s seafood industry and the variability in the amount and composition of species involved, fish and shrimp silage can be one of the most suitable options to utilize seafood waste. With this goal in mind, a partnership is required between seafood and animal feed industries. In this study, it was acknowledged that if the utilization of seafood waste were to be successful on a broad scale, it would require a considerable level of coordination and cooperation among seafood companies and animal feed producers. Thus, a more suitable alliance and a firm agreement among seafood industries with future fish and shrimp silage producers are required to secure raw material (seafood waste) supply.