Introduction

Hilsa shad (Tenualosa ilisha, Hamilton-Buchanan, 1822) is one of the popular fish species in Bay of Bengal (BoB) and Middle East countries due to its unique and delicious taste and high nutritional value. Nutritional studies have shown that it is rich in amino acids and essential fatty acids that can help reduce the risk of cardiovascular disease, lower cholesterol levels, and support brain development (Alam and Naser 2020). Hilsa shad has wide distribution in the marine, freshwater and estuarine environments. Its marine distribution ranges from the Persian Gulf to the west coast of India in the Arabian Sea and the Bay of Bengal (Pillay and Rosa 1963) (Fig. 1). This transboundary distribution of hilsa shad has significant economic, cultural and social value as it being one of the common food sources shared between nations (Ahmed et al. 2021).

Global hilsa shad production in 2019 was 558,356 tonnes and this production was mainly contributed by Bangladesh (95%) and India (4%). The rest is shared by the other countries such as Myanmar, Kuwait, Pakistan, Thailand, Iraq etc. (FAO 2020). In 2019, Bangladesh hilsa shad production increased by 3% compared to previous year with hilsa shad being the single species that made the highest contribution to the Bangladesh total fish production. The high importance of hilsa shad in Bangladesh has led it to being designated as the national fish in Bangladesh and also considered as the social and economic driving force of the country (Alam et al. 2021). Recently, Bangladesh hilsa shad has achieved geographical indication certificate that guarantee the quality of its origin. In India, most of the hilsa shad production comes from West Bengal region and marine hilsa shad fishery predominates over the other forms (i.e., estuarine and freshwater). Hilsa shad production in these countries also holds significant economic value, ranging from USD 867 million per year in the artisanal fisheries of Myanmar to USD 1.3 billion per year in Bangladesh (Burcham et al. 2020; Ahmed et al. 2021).

Hilsa shad fishery provides substantial livelihoods for the countries involved in it. In Bangladesh, hilsa shad fisheries provide livelihoods for 0.5 million fishermen and 2.5 million people in the value chain and distribution (Hossain et al. 2019c). In the West Bengal region of India, 0.46 million fishermen were actively engaged in hilsa shad fishery (Dutta et al. 2021). In Myanmar, 1.6 million fishermen engaged in hilsa shad fisheries in some of the country’s most impoverished areas (Bladon 2017). However, declining catches of hilsa shad have recently threaten the livelihoods and food security of the fishermen in all of these countries.

Hilsa shad is an anadromous fish, that migrates from seawater to freshwater for spawning. After spawning in freshwater reaches of rivers, adult fish may return to the sea and wait for the next reproductive cycle while larval fish grow up to juveniles (Wahab et al. 2019). Once the juveniles reach sexual maturity after 1–2 years, they join the breeding stock at sea and migrate to freshwaters. The major spawning grounds of hilsa shad include Meghna ecosystem in Bangladesh, Hooghly-Bhagirathi rivers in India and Ayeyarwady delta ecosystem in Myanmar (Hossain et al. 2019a). However, some studies suggest that a subset of the hilsa populations complete their lifecycle in freshwater without migrating to the sea (Hossain et al. 2019a). In recent decades, this anadromous behaviour of hilsa shad is greatly influenced by anthropogenic pressures such as flow regulations (i.e., damming), habitat degradation and overfishing that lead to a significant impact on hilsa shad production in Bangladesh and India (Hossain et al. 2019c). This alarming situation urged the sustainable management of hilsa shad fisheries to minimize the fishing pressure on natural fish stocks while fulfilling the growing demand for hilsa production (Hossain et al. 2019c). These management activities are largely supported by the existing scientific knowledge on hilsa shad fisheries in major producing countries.

The scientific literature on hilsa shad dates back to the early 1900s. Since then, many studies have focused on various aspects of hilsa shad biology and its subdisciplines, such as physiology (Hossain et al. 2019c; Malick et al. 2018b), genetics (Behera et al. 2015; Hoolihan et al. 2004), nutritional profile (Daviglus et al. 2002; König et al. 2005), feeding ecology (Baker et al. 2014; Buckland et al. 2017) and reproductive biology (Almukhtar et al. 2016; Halim et al. 2020). In terms of fisheries management, numerous studies have focused on population dynamics, conservation and legal framework of hilsa shad (Hossain et al. 2019a; Rahman et al. 2020; Wahab and Mohammed 2013). However, quantification of these scientific efforts is limited. Quantification and periodical assessment of hilsa shad research may provide insight into understanding the knowledge gaps, current research needs and future research scopes. In this context, scientometric studies provide many tools to assess the scientific productivity, knowledge gaps and future directions of a scientific discipline (Bandara 2020) and proved to be succesful compared to conventional revieweing techniques (Sultan et al. 2022). Therefore, the objectives of the present study were (i) to determine the current status of the hilsa shad research in terms of temporal and geographical distribution, journal and author contributions and (ii) to understand key research areas in hilsa shad research emphasizing current knowledge, knowledge gaps and future research directions.

Fig. 1
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a) Distribution range of hilsa shad (Tenualosa ilisha) (the data is from https://www.fishbase.se) b) Hilsa shad harvest from Bangladesh (image credit: Jugum)

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Materials and Methods

Data Collection

A survey was conducted in Web of Science™ Core collection database (WOS) to retrieve the articles related to the hilsa shad. The Web of Science™ is the oldest scientific database published by the Institute for Scientific Information™ consisting of over 10000 journals (Chadegani et al. 2013) and has a wide reputation as the world’s premier citation search and analytical database (Li et al. 2018). In the WOS platform, keyword combination of “Tenualosa ilisha” OR “ilisha” OR “hilsa*” was employed to retrieve the articles related to the hilsa shad between 2000–2022. In keyword combination, both binomial nomenclature and common vernacular English names were used to maximize the search results. Keyword search has retrieved 323 articles (original articles-296, proceedings paper − 18, review-8, editorial material-1). However, for the current study, only the original research articles (~ 91% of total articles) written in the English language were selected. These articles were individually inspected for their title, abstract, and keywords to identify possible mismatches with the research on Tenualosa ilisha. Articles that were not related to the Tenualosa ilisha (i.e., Ilisha africana, Hilsa kelee and Ilisha megaloptera etc.) and studies that were not directly focused on hilsa shad (i.e., bycatch studies) were removed from the analysis. Finally, 204 articles were used for further analysis (Fig. 2).

Fig. 2
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Flow diagram of the document search for articles relating to Tenualosa ilisha during 2000–2022 on the WOS platform

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Data Analysis

Upon careful evaluation of each article, the metadata of the selected articles were exported in BibTeX (RIS) format. Bibliometric analyses were performed using the bibliometrix package (Aria and Cuccurullo 2017) and the R programing language version 3.4.2 (R Development Core Team 2017). Vosviewer (version 1.6.11) visualization tool was utilized to illustrate the bibliographic network matrices (van Eck and Waltman 2010).

Results and Discussion

Scholarly Production on Hilsa Shad

The total number of articles related to the hilsa shad research (204) was lower compared to similar, economically important species in the region (i.e., Indian carp) (Bandara 2020). This may be due to fewer articles on hilsa shad in WOS. The low number of articles in WOS may potentially hinder the reliable dissemination of the information related to hilsa shad studies. From 2000 to 2019, the number of articles on hilsa shad has increased. The highest number of articles (30) was recorded in 2019 while no articles were recorded in 2005 (Fig. 3a). From 2015 to 2019, there has been an increasing trend in the number of articles published. This increase in the number of articles published since 2015 may be due to the inclusion of the Emerging Sources Citation Index (ESCI) in the WOS (Aksnes and Browman 2015). The ESCI has more coverage in regional journals (Somoza-Fernández et al. 2018) that may be more interested in regionally important studies, such as hilsa shad research. From 2000 to 2020, there was a strong positive relationship between the world hilsa shad production and the number of articles on hilsa shad (Fig. 3b). This strong relationship was similar to the previously reported relationship between the number of publications in fisheries science and the world fish production, that indicate more economically important fish species are studied more often (Jarić et al. 2012).

Fig. 3
figure 3

(a) Article distribution on hilsa shad in Web of Science database from 2000–2022. (b) Relationship between the number of publications on hilsa shad and world hilsa shad production between 2000–2020 (Production data for hilsa shad were obtained from FAO (2020))

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Top 10 Journals with the Highest Article Count

Based on the number of publications on hilsa shad, the top 10 journals are listed in Table 1. These top 10 journals accounted for ~ 30% of the total number of articles on hilsa shad. Among the top 10 journals, the Indian Journal of Fisheries had the highest number of articles (13), while the lowest number of articles (4) was shared by the Journal of Applied Ichthyology and Journal of Fisheries. From 2000 to 2022, many studies on hilsa shad were published in regional journals such as the Indian Journal of Fisheries and Indian Journal of Geo-Marine Sciences. This high publication count in regional journals may be attributed to the regional importance of hilsa shad fisheries rather than its international significance. The Indian Journal of Fisheries and the Indian Journal of Geo-Marine Sciences were important journals in fisheries and aquaculture studies in South Asia (Bandara 2020; Jayashree and Arunachalam 2000). These Journals are published under the open-access scheme by the Indian Council of Agricultural Research (ICAR) and the National Institute of Science Communication and Information Resources of India (NISCAIR) respectively. Except for these regional journals, several articles were also published in the international journals that have high impact factors. Publication of research on hilsa shad in international journals that have high impact factors may indicate an improvement in the quality of hilsa shad research and increased global interest.

Table 1 Top 10 Journals with the highest number of articles (2000–2022)
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Authors, Affiliations and Country Level Collaborations

During the period from 2000 to 2022, 605 authors have contributed to hilsa shad research. Out of these 605, 6 authors have contributed to the single-authored documents while the other 599 contributed to the multi-authored documents. Based on the number of publications, the top 10 authors, the number of articles they have published each year (freq) and the number of citations received in each year (TC) have indicated in Fig. 4. In total, these top 10 authors have published 95 articles that received 451 citations from 2000 to 2022. Prior to 2015, the number of publications from these top 10 authors was scarce, but from 2015 on, a continuous number of articles was recorded from each author. Among the top 10 authors, Wahab MA (n = 14) and Suresh VR (n = 14) had the highest number of articles while DAS P received the highest number of citations (n = 52) for his scholarly work in 2016.

Fig. 4
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Top 10 authors (based on the total number of articles) and their productivity over time (TC = Total number of citations for the respective year, freq = number of articles published in the respective year)

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The distribution of articles by top 10 author affiliations has indicated in Fig. 5. From 2000 to 2022, global research effort on hilsa shad was distributed among 116 Universities and institutes. Among the top 10 universities/institutes, the highest percentage of articles (12%) were published by ICAR- Central Inland Fisheries Institute (CIFRI), India followed by University of Dhaka, Bangladesh (4%) and University of Chittagong, Bangladesh (4%). CIFRI is one of the important fisheries research institutes that focused on inland fisheries resources and conducts various research activities (ranching programs, nutrigenomic studies, socio-economic studies etc.) and extension programs related to hilsa fisheries (CIFRI 2018). Earlier studies have also highlighted the importance of Bangladesh research institutes in hilsa shad studies (Raja 1985). However, a recent study suggested that coordination among the Bangladesh institutes on fisheries research is lacking and hilsa shad research is sporadic (Islam et al. 2016).

Fig. 5
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Distribution of the articles on hilsa shad by the top 10 author affiliations between 2000–2022

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The distribution of articles by country indicated that 19 countries focused on hilsa shad studies (Fig. 6). Many of these countries are located around the BoB and Persian Gulf regions. Among them, India and Bangladesh had a higher publication output than the other countries. However, the number of articles from Myanmar was low. This may be due to the many of the publications on Myanmar’s hilsa fishery appear in the grey literature, that hinders proper access and scientific dissemination (Burcham et al. 2020; Merayo et al. 2020).

Fig. 6
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Distribution of articles on hilsa shad by country during 2000–2022 (SCP-Single Country Publication: MCP-Multiple Country Publications)

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The co-authorship network of hilsa shad research at the country level provides information on the collaboration between different countries. The collaboration network consisted of 11 countries, each of which produced at least five articles (Fig. 7). In co-authorship network, the countries within the same colour belong to the same cluster and the size of the node for each cluster is proportional to the number of articles produced by that country. Among all the clusters, India had the largest node with 82 articles, followed by Bangladesh (54 articles). The thickness of the line between two nodes (link strength or ls) indicates the size of the quantitative collaboration between two different countries (i.e., higher the ls, higher the collaboration). Among all the clusters, Bangladesh had the strongest collaboration with other countries. The collaboration between Bangladesh and Malaysia was the highest as indicated by the strongest link strength (ls=10) followed by Bangladesh-Japan (ls=7), Bangladesh-Canada (ls=6) and Bangladesh-India (ls=6). Although India had the highest number of publications, the weak collaboration with other countries indicated that most of the publications from India were single-country publications (Fig. 6). India’s highest collaborations were with Bangladesh (ls=2) and England (ls=2). In terms of publication output, the collaboration between major hilsa producing countries (India, Bangladesh and Myanmar) was not strong. This weak relationship among countries has been illustrated in the studies dealing with the management of hilsa shad populations as collaborative initiatives to manage the transboundary hilsa shad stock have generally been unsuccessful (FAO 2016). This collaboration network also indicated that the collaboration strength between the major hilsa shad producers and the western countries is also scarce. However, weak collaboration can be alleviated through knowledge and expertise sharing among the countries. For years, Bangladesh is known for extensive research in hilsa shad and Bangladesh Fisheries Research Institute (BFRI) is the leading institute that conducts research on various disciplines of hilsa shad. In India, Central Inland Fisheries Resource Institute (CIFRI), Barrackpore, West Bengal has initiated a number of projects on hilsa shad conservation. In Myanmar, sustainable management of hilsa shad is promoted through Ayeyarwady integrated river basin management project (Bladon 2017). Currently, a number of organizations are working to enhance these collaborations (i.e., WorldFish; an international nonprofit organization that aimed to improve sustainable fisheries in developing countries, Asian Fisheries Society: A organization dedicated to sustainable fisheries and aquaculture in south Asia). This collaboration can be further strengthened by involving additional organizations (i.e., South Asian Cooperative Environment Program (SACEDP) and the International Union for Conservation of Nature (IUCN) etc.). Moreover, knowledge sharing platforms (i.e., webinars, blogs and social media), research symposia, collaborative research projects and multi stakeholder platforms that involve researchers, policymakers and practitioners could also be employed to facilitate knowledge sharing and cooperation.

Fig. 7
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The co-authorship network of hilsa shad research at the country level

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Sources of Funding for Hilsa Shad Research

Among the 204 articles, 142 articles provided information regarding the funding sources for hilsa shad research. Figure 8 indicates the top 10 funding agencies with the highest publication count. In terms of publication count, Ministry of Science and Technology, Bangladesh had the highest number of funding for hilsa shad research. We also observed that several publications were also funded by the United States Agency for International Development (USAID) under the ECOFISH (Ecosystems Improved for Sustainable Fisheries) project in Bangladesh. ECOFISH was one of the largest projects implemented by the government of Bangladesh with the collaboration of WorldFish that aimed to “improve household and community resilience by backing up the livelihoods and coping capacities of the marginalized poor, including women, and reducing their risk to ecosystem threats, including impacts of climate change sustainable management of hilsa shad fisheries in Bangladesh” (ECOFISH Annual Report 2018). The project has successfully achieved its multiple objectives in sustainable management of hilsa fisheries and provided a number of funds for multiple research areas (Abdul et al. 2018). Indian council for Agricultural Research (ICAR) was the main funding agency for hilsa shad research in India. Among the international donors, Ministry of Higher Education, Malaysia and Department of International Development, UK had also contributed to funding. Other possible funding sources for hilsa shad research may also include International Foundation for Science (for early career researchers) and Defra’s Darwin initiative, UK.

Fig. 8
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Funding information on hilsa shad research (2000–2022)

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Thematic Areas of Hilsa Shad Research

To identify the thematic areas described by the articles mentioned above, a network analysis of the keywords in the articles was performed. On the keyword co-occurrence map, keywords with similar colour belong to the same cluster (Fig. 9; Table 2). The size of each node/vertex is proportional to the weights of the keywords (Liao et al. 2018) and the thickness of the line between the two nodes (link strength or ls) is proportional to the quantitative relationship between two keywords (i.e., the higher the thickness, the higher the relationship) (Gu et al. 2017). Among all keywords, Tenualosa ilisha had the highest frequency resulting the largest node size. A strong relationship between the keywords Tenualosa ilisha and Bangladesh was also reported (ls=14). The keyword co-occurrence map identified several clusters that represent different thematic areas in hilsa shad studies (Fig. 9; Table 2). Cluster 1 consisted of the keywords such as “heavy metals”, “bioaccumulation”, “PAH (Poly-aromatic-hydrocarbons)” and “safety” etc. that can be translated into studies dealing with “Aquatic pollutants and hilsa shad”. Approximately, 11% of the total article count was represented under this theme. Many of the studies in this theme have focused on the bioaccumulation of heavy metals (Pb, Cd, Cr, and As etc.) and PAH in hilsa shad from various geographical regions (Table S1). Increased urbanization and industrialization that discharge a large volume of wastewater containing heavy metals into rivers and estuaries (i.e., Bangladesh: Karnaphuli River, India: Hugali estuary) can lead to the accumulation of these heavy metals in fish. Among the number of studies conducted on the heavy metal accumulation of hilsa shads, a large number of studies has focused on Pb, Cd, Cr and As. In contrast, fewer studies have focused on the other heavy metals such as Ni, Cu, Co etc. The reported concentrations of Pb, As and Ni in many studies were above the maximum recommended daily intake of these respective heavy metals (USFDA 1993; WHO 1993). Other pollutants (i.e., organochlorine pesticides) recorded under this theme, could be accumulated in hilsa shad and mobilized into fish eggs (Das and Das 2004).

Fig. 9
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Keyword co-occurrence network in hilsa shad studies (2000–2020)

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Table 2 Key thematic areas as depicted by keyword co-occurrence map
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Cluster 02 consisted of keywords such as “conservation”, “migration”, “livelihood”, “fishing ban” and “spawning” etc. that may focus on the studies dealing with the “conservation of hilsa shad”. Barriers in the migratory pathways, pollution, overfishing, siltation and the capture of broodstock and juveniles have attributed to a severe decline in hilsa shad production in Bangladesh in the early 2000s (Wahab and Mohammed 2013). Similar to Bangladesh, declining hilsa shad production was also recorded in other countries such as India and Myanmar (Chacraverti 2020). In response to this decline, Bangladesh implemented the Hilsa Fish Management and Action Plan (HFMAP) in 2003 to conserve the hilsa shad resources and increase the hilsa shad production (Rahman et al. 2020). HFMAP included several conservation measures such as establishing sanctuaries, implementing banning periods, payments for ecosystem services (economic incentives) and providing alternative occupations for fishermen etc. However, the effectiveness of most of these measures is depended on the socio-economic conditions of the fishermen. Poor socio-economic conditions of fishermen often lead to non-compliance with the existing regulations of hilsa shad management. As a result of that, there is a significant threat to collapse the hilsa fisheries management system in Bangladesh (Mozumder et al. 2018). More recently, USAID funded ECOFISH project have also focused on the conservation of hilsa shad (Hossain et al. 2019c). Compared to other hilsa shad producing countries, this significant focus on the conservation aspect of hilsa fisheries in Bangladesh may be attributed to high number of conservation-related articles (65%).

Keyword combinations of cluster 03 (“Bay of Bengal”, “co-management”, “Eco-system-based Management” etc.) may depict the holistic thematic area of “Management of hilsa fisheries- the Bay of Bengal (BoB) and surrounding countries”. The BoB is rich in biodiversity and is one of the important spawning grounds for hilsa shad (Hossain et al. 2019b). Moreover, the deltaic ecosystem around the Ganges, Brahmaputra and Meghna rivers in the BoB region provides substantial hilsa shad production and these stocks are shared by the countries in this region (India, Bangladesh and Myanmar). However, there is no formal transboundary agreement to protect this common resource and this may jeopardise the hilsa shad stocks (Rahman et al. 2012). However, Bay of Bengal Large Marine Ecosystem (BOBLME) project provides comprehensive guidelines for sustainable utilization of fishery resources in the region (Hossain et al. 2019a). Under this, the BOBLME is focusing more on ecosystem-based fisheries management and the conservation of the hilsa shad fishery within this region. In addition to this regional approach, country-specific hilsa shad fisheries management plans were also recorded in few publications. The concept of social resilience has also been coined in many articles under this theme, indicating the importance of improving the livelihoods of local fishermen especially in Bangladesh.

Cluster 04 focused on the “genetics and physiology of hilsa shad”. While there is more information on the stock structure of hilsa shad populations in Bangladesh, fewer studies have been conducted on hilsa shad genetic structure in other countries (i.e., India and Myanmar etc.). Many studies in this category have focused on the analysis of hilsa shad population structure using the differences in mitochondrial DNA. The use of mitochondrial DNA in these studies may be linked to its simple mode of transmission, maternal inheritance and high mutation rate (Hoolihan et al. 2004). Restriction Fragment Length Polymorphism (RFLP) and Random Amplified Polymorphic DNA (RAPD) were the most common techniques used to differentiate hilsa shad populations and these techniques have yielded various outcomes. Lal et al. (2004) used RFLP to study the population structure of riverine hilsa shad in various rivers (i.e., Brahmaputra, Padma, Ganga, Hooghly, and Feeder canal) in India and found a panmictic population. However, Brahmane et al. (2006) identified two populations of hilsa shad in india using the RAPD markers. Using the Cytochrome b gene, Behera et al. (2015) found distinct hilsa shad populations from the Bay of Bengal and Arabian sea regions. Similarly, Starch gel electrophoresis results from marine, freshwater and estuarine hilsa shad populations from Bangladesh indicated the presence of more than one gene pools (Rahman and Naevdal 2000). Mazumder and Alam (2009) also found at least two populations of hilsa shad in Bangladesh. More recently, use of these multiple markers through next generation sequencing method has identified three separate hilsa shad ecotypes (marine/brackish, muddy freshwater and clear/freshwater) in Bangladesh (Asaduzzaman et al. 2019, 2020). Besides analysing the population structure, other articles in this category have focused on the physiological aspects of hilsa shad migration. These studies have investigated the mechanisms of osmoregulation and associated genes of hilsa shad during its upstream migration. A few studies, have also focused on the basic sensory processes such as olfaction, gustation and chemical sensing, that aid hilsa shad migration (Malick et al. 2018a). However, there were relatively few physiological studies on hilsa shad migration compared to other commercially important anadromous fish species.

Cluster 05 consists of keywords such as “simulation”, “population dynamics”, “Tenualosa ilisha” etc. that can be translated into the “studies focusing on stock assessment/population dynamics of hilsa shads”. This category represents a higher percentage of publications (~ 20%) that highlights the focus on global hilsa shad research. These articles include studies on the assessment of basic length-weight parameters and studies on stock assessments of hilsa shad. The assessment of the basic growth parameters such as asymptotic length (Linf), growth constant (K) and length at first capture (Lc) has been used to evaluate the health of hilsa stocks since early 1960s (Rajayalakshmi 1973). However, many of these growth parameters were estimated using traditional stock assessment tools (i.e., FiSAT) that have limitations.

Studies that focused on the “nutritional profile of hilsa shad” has been represented in cluster 06 as indicated by the major keyword combination “muscle”, “EPA”, “DHA”, “protein” etc. Fish is often considered one of the important nutrient sources rich in essential fatty acids, amino acids and other nutrients. Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are essential fatty acids that help to enhance cognitive development, improve cardiovascular function, and reduce cholesterol levels (Daviglus et al. 2002; König et al. 2005). Under the publications in this category, several studies have focused on quantifying the EPA and DHA profiles of hilsa shad in different seasons (Majumdar and Basu 2009), various ecosystems (Hossain et al. 2014) and in different size/maturity levels (Nath and Banerjee 2012). A few articles have focused on the nutrigenomic studies focused on the fatty acid metabolism of hilsa shad. Except for the fish muscle, fatty acid composition of hilsa shad eggs were also analysed in few articles. Studies focused on the proximate composition/biochemical composition of hilsa have also been recorded. However, the percentage article count in this category was lower as ~ 7% compared to other categories.

Cluster 7 consists of the major keywords as ‘TheIndian ocean, ‘stomach content’, ‘gut content’ that may depict “studies on the feeding ecology of hilsa shad”. The number of publications in this category was the lowest (~ 2%). These studies have focused on the gut content analysis of hilsa shads to understand the predator-prey relationship. Fish gut content analysis is a less expensive and convenient method for understanding the trophic relationships. However, traditional gut content analyses usually overestimate the slowly digested and intact prey (Baker et al. 2014) that could lead to false interpreation of trophic relationships (Buckland et al. 2017).

Cluster 08 primarily addressed the studies dealing with hilsa shad conservation and the legal framework surrounding it. The legal framework for hilsa shad conservation includes both country-specific rules and regulations as well as regulations to protect transboundary stocks. Several studies have recommended various mechanisms for hilsa shad conservation such as maintaining closed seasons, restrictions of fishing gears and fishing periods, and regulation of fishing vessels etc. (Islam et al. 2016; Wahab et al. 2019). Islam et al. (2016) provided extensive detail on the institutional and legal framework for hilsa shad conservation in Bangladesh and recommended amendments for existing legal framework. Wahab et al. (2019) noted that the open nature of hilsa shad fisheries in India hindered conservation efforts there. Conservation measures for Indian hilsa shad populations include enforcing legal mesh size (> 100 mm), strict banning of capturing juveniles and declaring breeding sanctuaries for hilsa shad fisheries (Wahab et al. 2019). However, few publications reported that current regulations for hilsa shad fishery management in India are not sufficient and not exclusively focused on the hilsa shads. The comprehensive management of Indian hilsa shad is also limited by the less resources and expertise, as these management measures are taken by state government of west Bengal than the central government (Dutta et al. 2021).

Studies focusing on the “post-harvesting and processing of hilsa shad” products appeared in cluster 9. In Bangladesh, 75% of hilsa shad captured in the coastal regions are consumed outside of those regions and, significant post-harvest losses of hilsa shad were recorded at every stage of the supply chain (Ahmed 2007). There are various processing and post-harvesting methods exist for hilsa shad including, sun drying, freezing, salt-fermentation, smoking, canning etc. Many articles from this cluster focused on biochemical and microbiological analyses of traditional post-harvesting products.

Cluster 10 focused on reproductive biology (i.e., oocyte maturation and gonadotrophins) of hilsa shad. Within this category, several studies have focused on the oocyte and embryonic development of hilsa shad as well as hormonal regulation of oocyte development. A few studies have also focused on the broodstock development, captive management and artificial propagation of hilsa shad. Although artificial breeding of hilsa shad has been successful, many studies has reported that growing out of hilsa shad juveniles were difficult and standardized methods for artificial propagation of hilsa shad were scare.

The evolution of key-thematic areas in hilsa shad research (Fig. 10) indicated that studies with ‘Feeding ecology of hilsa shad (cluster 7)’ and ‘Management of hilsa shad fisheries in the BoB and surrounding countries (cluster 3)’ have been of recent interest among the scientific community. In contrast, studies on ‘post-harvesting of hilsa fishery products’ were more conventional. However, keywords within the same cluster with varying evolution may also depict the dynamism of the respective research theme.

Fig. 10
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Evolution of the key research themes in hilsa shad research (2000–2022)

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Knowledge Gaps and Future Research Directions

Our study focused on investigating the current status of the hilsa shad research, including its temporal and geographical distribution as well as contributions from different journals and authors. Additionally, we aimed to identify key research areas, knowledge gaps and future research directions using a scientometric approach. As a part of our analysis, we identified 10 thematic areas in the hilsa shad research and propose several actions to mitigate those gaps.

Under the thematic area of “Aquatic pollutants and hilsa shads (theme 01)”, we found that many articles focused on common heavy metals such as Pb, Cd, Cr and As. However, a few studies have explored the presence of trace elements (Ni, Cu and Co) in hilsa shad, that may cause health risks to humans if present in large quantities (Mertz 1993). We found that several studies reported a high level of Ni and Cu (Islam and Habibullah-Al-Mamun 2017; Tabinda et al. 2010) that exceeded the daily recommended levels (USFDA 1993; WHO 1993). Therefore, further studies on these trace elements considering the biological characteristics of hilsa shad (i.e., reproductive stage, sex and life stage etc.) may provide a comprehensive understanding of the dynamics of the trace elements. Similarly, we found a few studies focused on the other pollutants such as microplastics in hilsa shad, but their potential impact on both hilsa shad and humans is to be determined (Siddique et al. 2022). More recent studies also suggest that ongoing hilsa shad monitoring programs may especially focus on microplastics, given their high prevalence in fish (Kibria et al. 2021; Siddique et al. 2022).

Many articles describing the thematic areas “Conservation of hilsa shad” and “Management of hilsa fisheries in BoB region and surrounding countries, “hilsa shad conservation and legal framework” (thematic areas 2,3 and 8)” have indicated a lack of coordination in transboundary management of hilsa fisheries. This lack of coordination may be attributed to various factors such as different priorities and interests in the hilsa fishery in each country, limited understanding of the biology and behaviour of hilsa shad at the country level (i.e., Myanmar), insufficient institutional capacity to deploy the common mechanism to manage transboundary hilsa populations and other social and political conflicts between the nations. These limitations can mitigate with various strategies such as enhancing the knowledge and data on hilsa shad and investing in common monitoring programs. The Lack of institutional capacity to meet the goal of transboundary management of hilsa fisheries can be alleviated through technical assistance from donor agencies (i.e., FAO). Management of transboundary stocks and conservation of hilsa shad often conflict with the socio-economic factors of the fishermen. Therefore, providing alternative livelihoods, capacity building on sustainable fishery and social security for vulnerable fishery groups are recommended. Moreover, the effectiveness of the compensation provided for hilsa fishermen during the hilsa fishing banned period should also be studied through detailed socio-economic studies that address the questions such as: how well the compensations were handled by the fishermen? and does compensation meet the conservation goals? etc.

Integrating advanced technology is essential for the development of research under the thematic areas of “post harvesting of hilsa shad products (theme 9)” and “population dynamics of hilsa shads (theme 5)”. Currently, most studies on post harvesting of hilsa shad products focus on biochemical analysis. However, it is crucial to conduct more research on quality improvements, value addition and increasing the shelf life of hilsa shad products. Existing knowledge of the hilsa shad population structure and their feeding ecology is based on more traditional methods. To gain more in-depth insight into the population structure of hilsa shad, it is necessary to integrate these traditional methods with advanced technologies. For example, to compensate the knowledge gap in “feeding ecology of hilsa shad (theme 7)”, the use of stomach content analysis with stable isotope analysis/compound-specific stable isotope analysis would be helpful. Additionally, the use of traditional morphometric measurements with novel multiple genomic markers can provide a better understanding of the population structure of hilsa shad.

One of the key knowledge gaps identified in the thematic area: “genetics and physiology of hilsa shads (theme 04)” is the limited understanding of olfactory biology and migration physiology possibly of hilsa shad due to lack of funding and technical expertise. This gap can be addressed by collaborative research studies with western countries (i.e., Norway, UK) that have advanced knowledge on anadromous fish aquaculture. Under the “Nutritional profile of hilsa shad (theme 06)” recent research suggests that spatial variation of the nutritional profile and sensory properties of different size groups and riverine systems (De et al. 2019). Studies from this theme can be further extended by considering the seasonal variation of the nutritional profile, the impact of different feeding materials on the hilsa shad nutritional profile and also the bioavailability of the nutrients. Further, a large-scale nutritional study can be carried out to figure out how the nutritional profile of hilsa shad can help target populations (i.e., pregnant women and children) as previous studies reported that hilsa shad is rich in Fe (Alam et al. 2012; Ganguly et al. 2017) that has ability to prevent anaemia, especially during pregnancy.

Other than knowledge gaps based on thematic areas, we found that studies on the aquaculture of hilsa shad are in an infancy stage. Despite the development in artificial fertilization and larval development no significant improvements in broodstock management and grow-out rearing have been reported on hilsa shad aquaculture. Recent studies also suggest that collaborative efforts are needed for advancing the hilsa shad aquaculture (Sahoo et al. 2018). On the other hand, despite a large number of publications, literature on growth parameters (Linf, K, and Lc) of hilsa shad is scattered. To expand the current understanding of these parameters, and utilize them in management and conservation, it is also important to conduct a meta-analysis of articles on growth parameters.

Conclusion

The aim of the current study was to quantify the scientific research effort on hilsa shad that was conducted between 2000 and 2022 in WOS. The number of articles on hilsa shad found in the WOS was comparatively lower than the number of articles on other commercially important species in the region. Although, Myanmar was one of the important hilsa producers in the world, the number of publications found in WOS was very low. Scientometric analysis indicated that there were several thematic areas in hilsa shad research and many knowledge gaps exist within them. This study also highlighted the importance of addressing these knowledge gaps and the need for strong collaboration network in hilsa shad research. Therefore, more focus on these knowledge gaps and potential solutions for them would enhance the sustainable utilization and conservation of hilsa shad in future.