Reproductive biology of Hilsa Shad Tenualosa ilisha (Hamilton, 1822) in Labuhanbatu waters, Northern Sumatra, Indonesia: a basis for fishing managementAbstract
Hilsa shad Tenualosa ilisha, locally known as terebuk fish, is found in Labuhanbatu waters, Northern Sumatra, Indonesia. The population of this species has decreased significantly due to overexploitation. Despite its commercial importance, there is currently no information on the reproductive biology of this species in the region. This information is important for the development of sustainable fishing and conservation strategies. Therefore, this study aimed to examine the reproductive biology of T. ilisha in Labuhanbatu waters, Northern Sumatra, Indonesia. Observations were conducted for a year, from January to December 2019, in the Barumun River, Labuhanbatu Regency. Fish samples were caught using gill nets with three mesh sizes (2, 3, and 4 inches) operated by a boat. The samples were analyzed for sex, gonad development stage (GDS), gonadosomatic index (GSI), and fecundity. A total of 1,914 fish were recorded, including 1,428 (74.5%) males and 486 (25.4%) females. Samples with GDS III and IV were predominant among both males and females throughout the year. The highest GSI was recorded in January and March with values of 1.6% and 11.3%, respectively. The fecundity of female fish ranged from 81,450 to 245,267 eggs/fish, whereas the number of males was predominant in every month of sampling. Tenualosa ilisha spawns throughout the year, with the peak spawning season occurring from January to March. Strengthening the management implications of seasonal fishing bans during the spawning peak (January–March) may protect females.
Keywords:
fecundity; gonad development stage; gonadosomatic Index; Terebuk
Resumo
A sardinha Tenualosa ilisha, conhecida localmente como peixe terebuk, é encontrada nas águas de Labuhanbatu, Norte de Sumatra, Indonésia. A população dessa espécie diminuiu significativamente devido à sobre-exploração. Apesar de sua importância comercial, atualmente não há informações sobre a biologia reprodutiva dessa espécie na região. Essas informações são importantes para o desenvolvimento de estratégias de pesca sustentável e conservação. Portanto, este estudo teve como objetivo examinar a biologia reprodutiva de T. ilisha nas águas de Labuhanbatu, Norte de Sumatra, Indonésia. As observações foram realizadas ao longo de um ano, de janeiro a dezembro de 2019, no rio Barumun, Regência de Labuhanbatu. As amostras de peixes foram capturadas com redes de emalhar com três tamanhos de malha (2, 3 e 4 polegadas) operadas por uma embarcação. As amostras foram analisadas quanto ao sexo, estágio de desenvolvimento gonadal (GDS), índice gonadossomático (GSI) e fecundidade. Um total de 1.914 peixes foi registrado, incluindo 1.428 (74,5%) machos e 486 (25,4%) fêmeas. Amostras com GDS III e IV foram predominantes tanto entre machos quanto entre fêmeas ao longo do ano. Os maiores valores de GSI foram registrados em janeiro e março, com 1,6% e 11,3%, respectivamente. A fecundidade das fêmeas variou de 81.450 a 245.267 ovos/peixe, enquanto o número de machos foi predominante em todos os meses de amostragem. Tenualosa ilisha desova ao longo de todo o ano, com pico de desova entre janeiro e março. O fortalecimento das implicações de manejo, como a proibição sazonal da pesca durante o pico de desova (janeiro–março), pode proteger as fêmeas.
Palavras-chave:
fecundidade; estágio de desenvolvimento das gônadas; Índice gonadossomático; Terebuk
1. Introduction
Hilsa shad Tenualosa ilisha (Hamilton, 1822), locally known as terebuk fish, belongs to the family Dorosomatidae and order Clupeiformes (Fricke et al., 2025a,b). This family is composed of fish with small body sizes less than 50 cm, inhabiting coastal waters, and often forming groups. Hilsa shad is an anadromous species that migrates from freshwater to saltwater and returns to freshwater to spawn (Nuitja, 2010). The distribution of the species has been reported to extend across the Indian Ocean to the Persian Gulf (Chattopadhyay et al., 2024). Furthermore, Hilsa shad are one of the commercial fish groups worldwide (Hossain et al., 2020), and a total of 160 species belonging to 50 genera have been described (Sarma, 2019). There are five species frequently caught in the world: Tenualosa ilisha, T. macrura (Bleeker, 1852), T. reevesii (Richardson, 1846), T. thibaudeaui (Duran, 1940), and T. toli (Valenciennes, 1847) (Blaber et al., 2003; Froese and Pauly, 2025; Fricke et al., 2025b). However, these species are threatened with extinction due to intensive exploitation (Vidthayanon, 2011; Freyhof, 2014; Di-Dario, 2018a; Di-Dario, 2018b; Mohd-Arshaad et al., 2018). According to several studies, the population of Tenualosa Fowler, 1934 population is also threatened by habitat degradation (Ahmad et al., 1995; Efizon, 2001).
Three Tenualosa species have been reported in Indonesian waters: T. ilisha, T. macrura, and T. toli. Specifically, T. ilisha is found in the Barumun River, Labuhanbatu, North Sumatra Province, T. macrura in the Bengkalis estuary, Riau Province, and T. toli in the Pemangkat coastal waters, West Kalimantan Province (Suwarso and Merta, 2003). Similar to other regions globally, a decline in the Hilsa shad population has also been observed in Indonesian waters.
Tenualosa ilisha, also known as terebuk, is a protandry hermaphrodite, starting life as a male and transforming into a female at a certain size (Halim et al., 2020). The sexual maturation process occurs in sea waters, whereas spawning occurs in river waters. After hatching, fish larvae migrate to lower river areas as a nursery ground, and as juveniles migrate to sea waters for feeding, growth, and sexual maturation (Blaber et al., 2003). T. ilisha is a total spawner (synchronous), releasing all the eggs in one spawning session (Ray et al., 2022).
Several studies of T. ilisha in Indonesia have been reported, including that by Ahmad et al. (1995), who studied reproduction in Bengkalis waters, Riau Province, Indonesia. The fish first matured at 25 cm in length and 103 g body weight. Machrizal et al. (2019) also studied the distribution and length-weight relationships of Hilsa shad in the Bilah River, Labuhanbatu Regency, North Sumatera Province, Indonesia. Blaber et al. (1999) examined the life history of T. macrura in the Bengkalis waters, whereas Brewer et al. (2001) explored the feeding habits of T. macrura in the same area. Labuhanbatu is a natural distribution area of T. ilisha, but overexploitation persists, with fishing pressure intensifying during peak reproductive periods to harvest fish eggs. Therefore, reproductive biology are required to support sustainable management, especially during the peak spawning period.
Siregar et al. (2024) revealed that T. ilisha in Labuhanbatu waters, North Sumatra, Indonesia, has been overexploited using the Gordon-Schaefer Model and the maximum sustainable yield analysis approach. Therefore, management plans for the conservation of T. ilisha need to be initiated. Data on reproductive biology are one of the key types of information needed to devise a better fisheries management plan (Pukazhenthi and Wildt, 2004; Tara-Marshall, 2016; Efizon et al., 2021). This study addresses the lack of reproductive data on T. ilisha in Labuhanbatu to inform local management.
2. Materials and Methods
2.1. Time and site
This study was conducted from January to December 2019 in the Barumun River, Labuhanbatu Regency, Northern Sumatra Province, Indonesia. The sampling was carried out at five sampling stations along the estuary (Figure 1). The reproductive biology of the fish samples was analyzed in the Laboratory of Aquatic Biology, Faculty of Fisheries and Marine Sciences, Riau University, Pekanbaru, Indonesia.
Map of Labuhanbatu Regency showing the sampling locations (red dots indicate sampling locations).
The sampling locations are as follows: Station I (02°39′59.5″N; 100°07′13.7″E) was an estuary area heavily influenced by the Malacca Strait’s seawater, while Station II (02°38′11.0″N; 100°06′27.9″E) was close to residential areas with various human activities, such as fishing ports and industries. Station III (02°30′42.2″N; 100°08′16.9″E) was similar to Station II, but it was also influenced by plantation areas and freshwater flow from the Barumun River. In addition, Station IV (02°30′17.74″N; 100°10′4.93″E) was an estuary area affected by domestic and industrial activities as well as freshwater flow from the upper reaches of the Bila River. Station V (02°27′53.0″N; 100°09′21.5″E) was a watershed impacted by the palm oil processing industry and freshwater flow from the upper Barumun River.
2.2. Sampling procedures
Gill nets of 2–4 inches were used to capture a size range representative of mature and immature fish, operated by a boat. The gill nets were operated from 5 PM to 5 AM (24 h) once per month for 12 months. The sampled fish were counted, measured for standard length (SL) and weight (g), preserved in an ice-cool box (4oC), and transported to the laboratory for further analysis. This study adhered to the principles of care and animal use (Section on Animal Care and Use in Research). Chapter 6, Article 11–13 (LPPM USK, 2015).
2.3. Analysis of gonad development stage and gonadosomatic index
The fish samples was dissected abdominally using a scissor, and the gonad was removed and cleaned with tissue paper. Subsequently, the gonads were weighed and observed for morphology, color, surface texture, and the presence of milt or oocytes. Gonad maturity was grouped into five stages based on Haryono et al. (2015) (Table 1), and the number of fish samples was calculated based on the GDS classification. The GSI was calculated according to the method of Muchlisin et al. (2010) as follows: GSI (%) = gonad weight (g)/total body weight (g) × 100.
2.4. Sex dimorphism, sex ratio, and fecundity
Sex dimorphism was observed through morphology and body color to distinguish between males and females. The sex ratio was calculated based on Adenike (2013) and Muchlisin et al. (2010) as follows: sex ratio = total male fish/total female fish.
The relative and total fecundities were examined using gravimetry based on Muchlisin et al. (2011) and Muchlisin (2014) as follows: TF = n. (Wt/Ws), while RF = TF/BW. Here, TF is the total fecundity (oocytes per spawning season), RF is the relative fecundity (oocytes g−1 body weight), n is the total number of oocytes in a sub-sample of gonads (oocytes were counted from a sub-sample of the gonads whose weight was measured as Ws), Wt is the total weight of gonads (g), Ws is the weight of the gonad sub-sample (g) from n, and BW is the total body weight of fish (g).
The fecundity data were analyzed to determine the correlation between the SL and body weight of the fish. Fecundity, SL, and body weight data were transformed into logarithmic form to achieve a normal distribution of data, and then linear regression analysis was performed. Data transformation into logarithmic form and linear regression analysis were performed using Microsoft Excel for Mac Ver. 16.77.1.
2.5. Gonad histological analysis
Histological analysis was performed on 3 male gonads, 3 samples from the transition period gonads, and 3 samples of GDS I, II, III, and IV from female gonads. The histological procedure was based on Roberts (2012), as follows: gonadal samples were fixed for 24 h using a fixation solution (10% formalin) after they were previously frozen. Subsequently, the sample was moved to 70% alcohol to remove the fixative solution, and the dehydration process was carried out using a series of alcohol concentrations of 70%, 80%, and 96% for 30 min each. This was followed by a clearing process using a xylene solution to purify the tissue. The impregnation and embedding processes were performed by removing the xylene from the tissue and adding a paraffin solution before the cutting process. Furthermore, the sample was cut using a microtome machine (Microtomo SLEE Modelo CUT 4062) with a thickness of 4–5 µm. The staining process was conducted by removing paraffin using graded alcohol (96%, 80%, and 70%), and then the sample was stained with hematoxylin and eosin solutions for 15 min. Subsequently, predehydration was performed using 70% alcohol and xylene. The sample was dried, dripped with xylene (Merck, Germany), and then covered with a glass slide. The histological preparations were observed under a stereomicroscope (Olympus CX23, China) at 400× magnification.
3. Results
3.1. Sex ratio and GDS score
A total of 1914 fish samples were collected during the 12 months of sampling, comprising 1,428 (74.5%) males and 486 (25.4%) females (Table 2). Five GDS stages were detected during the study, namely GDS I, II, III, IV, and V (Table 3). GDS values began to increase from November to April, with the majority reaching GDS III and GDS IV in January–April, while the lowest occurred from July to August (Table 4).
3.2. GSI
The GSI value ranged between 0.4–2.0% in males and 0.3–25.0% in females (Table 5). The highest mean GSI of males occurred in January (1.6%), July (1.4%), and December (1.4%), whereas the highest GSI of females was recorded in January (10.8%), March (11.3%), and December (10.3%). This result suggested that the peak of the spawning season was in January and December when the GSI of both females and males was higher.
3.3. Fecundity and egg diameter
The total fecundity ranged from 81,450 to 245,267 eggs/fish with a standard female length of 28.5–49.5 cm and body weight of 390–1,238 g (Figure 2). Two GDS stages of the fish sample were recorded during the study, namely, III and IV, where the egg diameters ranged from 0.3 to 0.4 mm and 0.5–0.8 mm, respectively (Figure 3). In addition, the chi-square test showed that X2count < X2table, suggesting that the egg diameter was homogeneous.
Relationship between (a) fecundity and standard length and (b) fecundity and body weight of T. ilisha.
3.4. Histology of the gonad
Histological analysis showed that T. ilisha was a protandrous hermaphrodite, containing spermatocytes at a length of 16.0–26.0 cm (Figure 4a). Subsequently, at a length of 27.0–32.5 cm, the gonads contained oocytes. At a length of 26.5–26.7 cm, a transition occurred in the gonads from male to female. During this transitional period, spermatid remnants were still present, while vitellogenic eggs began to appear (Figure 4b).
Gonad histology of terubuk, where (a) sperm, (b) transition period, (c) females in GDS I, (d) females in GDS II, (e) females in GDS III, and (f) females in GDS IV, where primary oocytes (Po), lipid vesicles (Lv), cortical alveolus stage (Cas), vitellogenic (V), advanced vitellogenic (Av), migrating (M), spermatocyte (Sc), and spermatide (Sd) stages.
In GDS I and II, histological analysis revealed a predominance of primary growth eggs. This stage represented an early phase of development characterized by the absence of oil droplets, with egg sizes ranging from 0.03 to 0.07 mm, and no detected yolk (Figures 4c and 4d). In GDS III and IV, the gonad was dominated by advanced vitellogenic eggs, with fewer early vitellogenic eggs (Figure 4e and 4f). Yolk content increased, and the oocytes enlarged, resulting in a fuller appearance of the ovaries. The nuclei of the eggs started to migrate toward the periphery, and the diameter of the advanced vitellogenic eggs ranged from 0.5 to 0.6 mm (Figures 4e and 4f). At the GDS IV stage, the eggs were homogenous in size (Figure 4f).
4. Discussion
The results showed that males dominated the population of T. ilisha in the Labuhanbatu waters. Several scientists have observed a similar occurrence in the genus Tenualosa (Sihotang, 1991; Ahmad et al., 1995; Blaber et al., 1999; Merta et al., 1999; Efizon, 2013). This prevalence of males might be due to the sampling of predominantly young fish that have not yet changed sex to females. As protandrous hermaphrodites, Tenualosa are males at a young age and then change to females in older age (Rahman, 2020). Moreover, the overexploitation of female fish, primarily due to the higher-priced eggs, contributes to the skewed gender ratio. This study further showed that the fish were caught both during the day and night, suggesting migratory behavior during these periods.
The total fecundity of T. ilisha ranged from 81,450 to 245,267 eggs/fish, which is equivalent to 203.48 eggs/g body weight for relative fecundity. This value was greater than that of T. macrura found in the waters of Bengkalis, Riau Province, with a total fecundity of 60,000–200,000 eggs/fish (Efizon, 2013). The discrepancy was probably due to the larger body size of the Labuhanbatu population. However, the fecundity of T. ilisha was lower than that of T. toli from Sarawak Malaysia with a value of 1.2 million eggs/fish (Blaber et al., 1996). Fish with low fecundity have a small population and a higher threat of extinction (Muchlisin et al., 2011). Lower fecundity may increase the extinction risk under current fishing pressure (Hutchings and Reynolds, 2004). Thus, T. ilisha is likely to be more vulnerable to fishing pressure than T. toli.
A low correlation was found between total fecundity and SL (R2 = 0.26) and body weight (R2 = 0.35). Weak relationships suggest that other factors (e.g., age and environment) may influence fecundity. These environmental factors are associated with food abundance, temperature, fish density, and biomass index (Lambert, 2008). Biological factors that can influence fish fecundity include age, nutritional status, size, reproductive experience, and spawning frequency (Trippel, 1998).
Male T. ilisha measured between 16.0 and 25.0 cm, with 60.78% having matured gonads typically at 19.0 cm, whereas females matured at 27.0 cm. Milton (2010) stated that the gonads of male T. ilisha first matured at one year of age and changed to females at two years and older. In male T. macrura, the first gonads mature at a smaller size, 10 cm (Blaber et al., 2005), or at one year, and change into females in the second year (Blaber et al., 1996). At first maturity, the size of female T. ilisha in Labuhanbatu waters was smaller than that in Perak waters, Malaysia, and the Bay of Bengal, where the sizes were 30 cm and 31 cm, respectively (Halim et al., 2020; Hossain et al., 2019).
The female fish samples were between 27.0 and 48.5 cm in size with five GDS levels. The majority (56.79%) were at GDS III and IV, with the highest proportion occurring from January to March. A similar result was also reported in males, where the gonad maturity stage increased from November to April, with the highest peak occurring in January. However,T. ilishain the Persian Gulf, Iran, and Perak waters, Malaysia, spawned between March and May (Roomiani et al., 2014; Halim et al., 2020), and this species spawned from October to November in Bangladeshi waters (Das et al., 2022). Other species such asT. macrurain Sarawak waters (South China Sea) spawn in December (Blaber et al., 2005).
5. Conclusions
In conclusion, T. ilisha males were more predominant than females, and their fecundity ranged from 81,450 to 245,267 eggs/fish. Males first matured at 19.0 cm, whereas females matured at 27.0 cm. The spawning season was from January to March, with the peak occurring in January. Strengthening the management through seasonal fishing bans during the spawning peak (January–March) may protect females.
Acknowledgements
Thank you to the Department of Marine Affairs, Fisheries and Livestock, Labuhanbatu Regency, North Sumatra Province, Indonesia for funding and facilitating this research. The authors would like to thank Enago (www.enago.com) for the English language review. This research was funded through a collaboration scheme between the Faculty of Fisheries and Marine Sciences, University of Riau and the Department of Marine Affairs, Fisheries and Livestock, Labuhanbatu Regency, North Sumatra Province, Indonesia.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon request.
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