1 Introduction

Shark and ray populations (Subclass: Elasmobranchii) are undergoing severe global declines, with many species now threatened with extinction because of intensive fishing pressure (Dulvy et al., 2021; Pacoureau et al., 2021; Dulvy et al., 2024). Among the most vulnerable are manta and devil rays (genus Mobula, hereafter referred to as mobulids), whose slow growth rates, delayed maturity, and exceptionally low reproductive output constrain their ability to recover from overexploitation (Notarbartolo‑di‑Sciara 1988; White et al., 2006; Marshall and Bennett 2010; Dulvy et al., 2014; Pardo et al., 2016; Lawson et al., 2017; Barrowclift et al., 2025;). The family currently includes ten recognised species (White et al., 2018; Hosegood et al., 2020; Bucair et al., 2025; Stevens et al., 2025), seven of which occur in the Indo‑West Pacific: the oceanic manta ray (Mobula birostris), reef manta ray (M. alfredi), spinetail devil ray (M. mobular), bentfin devil ray (M. thurstoni), sicklefin devil ray (M. tarapacana), shorthorned pygmy devil ray (M. kuhlii), and longhorned pygmy devil ray (M. eregoodoo) (Notarbartolo‑di‑Sciara et al., 2020; Stevens et al., 2025). While M. alfredi is classified as Vulnerable on the International Union for Conservation of Nature’s (IUCN) Red List, the oceanic devil ray species (M. mobular, M. thurstoni, and M. tarapacana) were uplisted to Critically Endangered in 2025. The remaining mobulid species are all listed as Endangered, reflecting evidence of population declines across most mobulid species driven by fisheries and trade (Laglbauer et al., 2026).

Mobulids frequently aggregate to feed, migrate, reproduce, and engage in other social behaviours (Couturier et al., 2012; Palacios et al., 2023; Venables et al., 2025). These behaviours, combined with the use of non‑selective fishing gears, heighten their susceptibility to capture in both artisanal and industrial fisheries (White et al., 2006; Lewis et al., 2015; Laglbauer et al., 2026). While some communities have historically targeted mobulids with harpoons (Dewar 2002; Acebes and Tull 2016; Booth et al., 2021), most catches throughout the Indo‑Pacific are incidental and are retained as bycatch where the sale of meat, cartilage, skin, and dried gill plates, provides economic incentive (O’Malley et al., 2017; MacNeil et al., 2025; Palacios et al., 2025). The emergence of demand for gill plates in Chinese medicines has intensified fishing pressure since the early 2000s (Heinrichs et al., 2017; O’Malley et al., 2017). Demand by international markets, centred in Guangzhou, Hong Kong, and Singapore, continues to drive trade, with gill plates fetching hundreds to over a thousand U.S. dollars per kilogram depending on size and species (Yu et al., 2016; Palacios et al., 2025). For a decade, until the end of 2025, all mobulid species had been listed on the Convention on International Trade of Endangered Species of Wild Fauna and Flora (CITES) Appendix II. These listings required strict trade control and export permits, if positive Non-Detriment Findings (NDFs) were obtained. Yet compliance with these CITES requirements by many range states was limited, with few NDFs submitted and continued reports of unregulated or illegal international trade (Lawson et al., 2017; Clark‑Shen et al., 2023; CITES 2024; Palacios et al., 2025). Consequently, and in the face of continuing population declines (Laglbauer et al., 2025), all mobulid species are now listed on CITES Appendix I, effectively banning all international commercial trade in these species. While this represents a critical step toward reducing exploitation, the success of this measure will depend on stronger national level policies, improved monitoring, and targeted awareness initiatives to curb local catch and consumer demand.

Malaysia plays an increasingly important role in the capture and trade of mobulids in Southeast Asia, particularly in Sabah (northeastern Borneo) (MacNeil et al., 2025). Mobulids are landed in coastal fisheries operating in areas including Semporna, Sandakan, Kudat, and Lahad Datu, typically as incidental catch in drift gillnets, longlines, and handlines (CITES 2016; Shark Stewards 2018; Clark-Shen et al., 2023). Although not targeted by industrial fleets domestically, mobulids landed in Malaysia are usually sold locally for meat, with gill plates trafficked informally into international trade networks (Boon 2017; O’Malley et al., 2017). Surveys in Hong Kong and Guangzhou have documented Malaysian-sourced gill plates in market supply chains (Yu et al., 2016; O’Malley et al., 2017), yet no species‑specific customs codes currently exist to monitor these exports, and Malaysia has yet to submit any CITES NDFs for mobulids (CITES 2024).

Although Malaysia is a signatory to both CITES and the Convention on the Conservation of Migratory Species (CMS), these international agreements do no necessarily translate into domestic legal protection. Mobulid rays were listed under CMS Appendices I and II in 2014 and under CITES Appendix II in 2017, obligating range states to regulate international trade and support conservation of migratory species. However, despite evidence of Malaysia's involvement in the international trade of mobulids (MacNeil et al., 2025; Palacios et al., 2025), no national legal protection was in place until 2019. Amendments to the Fisheries Act 1985, initiated by the state of Sabah, conferred full protection on only two species, M. birostris and M. alfredi, while other mobulid species remain unprotected under national legislation. Additionally, there are presently no bycatch mitigation or handling regulations in place (Cronin et al., 2025; Laglbauer et al., 2026). Species-specific landings are also not recorded in national or Food and Agricultural Organization (FAO) datasets (CITES 2024), highlighting a significant data gap in monitoring. Moreover, vendor surveys in Hong Kong and Guangzhou identified Malaysia as a source of gill plates and positions Malaysia as both a source country and transit hub within the wider regional mobulid trade networks. The lack of reliable national statistics also hinders the understanding of the extent of mobulid fishing and trade (Clark‑Shen et al., 2023).

Addressing these knowledge gaps requires fisheries‑dependent monitoring as a foundation for evidence-based management. In this study, we present the first systematic documentation of mobulid landings in Semporna, a small town in Sabah, collected between 2013 and 2014. We (1) characterised the species and size compositions of landed mobulids, (2) estimated sex ratios and proportions of mature individuals, and (3) evaluated seasonal trends in catch rates. By situating Semporna landings within the broader context of exploitation and trade, we highlight the importance of improved monitoring and regulatory measures for the conservation of mobulids in Malaysia and the region.

2Methods

2.1 Data collection

Semporna (Figure 1, 4°28’N 118°36’E) is located on the east coast of Sabah, Malaysia, and supports one of the country’s most active coastal fisheries, including targeted and incidental catches of mobulid (Shark Stewards, 2018). Opportunistic surveys were conducted at the Semporna Fish Market between May 2011 and November 2014.

Figure 1 Map of eastern Sabah, Malaysia, showing the Semporna Fish Market, where mobulid landings were recorded, and three probable capture locations identified by fishers: Pulau Gaya, Denawan Island, and Si Amil Island. The inset indicates the position of Sabah (blue square) within Southeast Asia

For each landed individual, researchers recorded species identity, sex, disc width (DW, cm), and body length (BL, cm). Reported gear type and fishing location, as relayed by fishers and traders, were also noted. Photographs were taken of specimens to support species identification, which followed regional diagnostic keys (Stevens et al., 2025). Specimens were sexed based on the presence (in males) or absence of claspers, where available. Owing to study limitations, maturity stages could not be consistently assessed in the field, but observations of external morphology were retained and compared with species-specific maturity thresholds according to Stevens et al (2025) where relevant.

2.2 Data analysis

Data were digitized, cleaned, and standardized before analysis in R version 4.5.0 (R Core Team 2025). Data processing used the tidyverse workflow (Wickham et al., 2019). Taxonomy was updated to reflect current literature, with Mobula mobular replacing M. japanica and Mobula birostris replacing Manta birostris (White et al., 2018). Specimens with missing morphometric information were excluded from analyses requiring size data. Each record was assigned to one of four seasonal categories: Northeast monsoon (NEM; November to March), Inter-monsoon 1 (IM1; March to April), Southwest monsoon (SWM; May to September) and Inter-monsoon 2 (IM2; October) (Bahrom et al., 2025; Yeakub et al., 2025).

Species composition was summarized as counts and proportions by survey period. Sex ratios were calculated for each species, with exact binomial 95% confidence intervals. Deviations from a 1:1 ratio were tested with exact binomial tests, with Holm correction across species, and the independence of sex and species was assessed using Fisher’s exact test. Sex-based analyses were restricted to individuals recorded as male or female. Size-based analyses were restricted to individuals with finite disc width (DW) measurements.

Size comparisons across species and sexes were assessed using a two-factor additive ANOVA model (DW ~ Species + Gender; Table S1). A sensitivity analysis including the interaction term (DW ~ Species × Gender) was conducted using only species for which both sexes were represented (Table S2). Model assumptions were evaluated using residual and Q-Q plots. Because some species had unequal sample sizes, additional robustness checks were conducted using non-parametric Kruskal-Wallis tests and Welch’s one-way tests for species with sample sizes ≥5 (Table S3). Standardized residuals were inspected to identify potential outliers; these observations were retained in the analysis to preserve biological variability.

Published species- and sex-specific size-at-maturity thresholds were encoded from Stevens et al., (2025) and applied to the dataset Table S4). Individuals were flagged as mature when DW was greater than or equal to the reported threshold. Proportions mature and Wilson 95% confidence intervals were estimated by species, sex, and season. For groups with data spanning both warm and cool seasons, seasonal differences in maturity proportion were tested with Fisher’s exact tests. Where n ≥ 3 for a given sex-species group, one-sample t-tests compared mean DW against the maturity threshold.

Landings per survey effort (LPSE) was defined as the number of mobulids recorded per survey day (unique Date × Survey_ID). LPSE was calculated from survey days with at least one recorded mobulid landing (unique Date × Survey_ID combinations). Species-level LPSE was summarized as mean ± SD across recorded survey days. Landings were modeled with Poisson generalized linear models (GLMs), including models of LPSE ~ Season and LPSE ~ Season × Species. Overdispersion was assessed using Pearson residuals relative to model degrees of freedom; if the ratio exceeded 1.5, models were re-fit with a negative binomial distribution. Model-based means and confidence intervals on the response scale were estimated using the emmeans package (Lenth 2024).

Gear type and reported fishing locations were summarized descriptively. Nearly all mobulids were documented as net captures from the Near Si Amil/Denewan fishing grounds. Due to the dominance of this category, no formal comparative analyses were possible for fishing method or ground. Market observations were transcribed where available, including details on price per strip, partial carcass sales, and whole-animal transactions.

3 Results

3.1 Demographic characteristics of landed specimens

Across 79 survey days conducted between May 2011 and November 2014, mobulids were recorded on 52 days (66% of surveys), yielding a total of 126 individuals across five species (Figure 2). Data on species composition, fishing methods, and morphometrics were collected for the landed specimens.

Mobula thurstoni was the most abundant species (n = 48, 36.6 %), followed by M. mobular (n = 47, 35.9 %), M. kuhlii (n = 19, 14.5 %), M. tarapacana (n = 7, 5.3 %), and M. birostris (n = 3, 2.3 %) (Table 1). Disc width of all mobulids ranged from 92 to 490 cm (mean = 174 cm, SD = 49.5), while body length ranged from 50 to 238 cm (mean = 86 cm, SD = 27.0) (Table 2).

Landings per survey effort averaged 1.73 mobulids per survey day, corresponding to an annualized estimate of approximately 632 individuals. Species-level mean landings per survey day (Table 1) were 1.52 ± 1.15 for M. mobular, 1.71 ± 0.94 for M. thurstoni, 1.17 ± 0.41 for M. tarapacana, 1.00 ± 0.00 for M. birostris, and 4.75 ± 6.85 for M. kuhlii. Species-specific totals can exceed the overall estimate because multiple taxa were landed on the same survey days.

Figure 2 Mobulid species landings (ventral, dorsal and head images) observed at Semporna Fish Market (2011-2014). (a-c) oceanic manta ray (Mobula birostris), (d-f) Spinetail devil ray (M. mobular), (g-i) sicklefin devil ray (M. tarapacana), (j-l) bentfin devil ray (M. thurstoni), and (m-o) shorthorned pygmy devil ray (M. kuhlii)

Table 1 Landings of mobulids (Mobula spp.) at the Semporna Fish Market, Sabah, Malaysia (2011-2014). Observed counts and percentages are shown by species and year, alongside descriptive landings per survey effort (LPSE; mean ± SD individuals per survey day, n = 126)

Table 2 Disc width (cm) of mobulids (Mobula spp.) by species and sex. Values show sample size (n), mean, standard deviation (SD), and observed minimum and maximum

Sex was determined for 112 specimens (88.8%), comprising 64 females and 48 males. The overall proportion of females was 57.1% (95% CI: 47.4-66.5), not significantly different from parity (binomial test, p = 0.16). No species exhibited a significant deviation from a 1:1 sex ratio after Holm correction: M. kuhlii (15 females, 4 males; 78.9% female, 95% CI: 54.4-93.9, p = 0.019, Holm p = 0.115), M. mobular (22 females, 17 males; 56.4% female, 95% CI: 39.6-72.2, p = 0.52), M. thurstoni (24 females, 22 males; 52.2% female, 95% CI: 36.9-67.1, p = 0.88), M. tarapacana (2 females, 3 males; 40.0% female, 95% CI: 5.3-85.3, p = 1.0), and M. birostris (0 females, 2 males; 0% female, 95% CI: 0-84.2, p = 0.50). Species and sex were independent (Fisher’s exact test, p = 0.118).

3.2 Size and maturity of landed specimens

Figure 3 Observed disc widths (cm) for four mobulid species landed at the Semporna Fish Market, Sabah, Malaysia, between May 2011 and November 2014. Boxplots show distributions by sex, with points indicating individual observations. Point shapes indicate field-observed maturity status based on external morphology. Mobula tarapacana was excluded from this figure because only one usable disc width observation was available: a mature male measuring 249 cm DW

Disc width differed significantly among species (ANOVA F4,774,77 = 79.2, p < 0.001) but not between sexes (F1,771,77 = 1.44, p = 0.23). When restricted to species with both sexes represented, the species effect remained significant (F1,721,72 = 37.7, p < 0.001), while neither sex (p = 0.23) nor the Species × Sex interaction (p = 0.30) were significant.

Based on published size-at-maturity thresholds from Stevens et al., (2025), most landed rays were below maturity size (Table 4). The proportion meeting threshold maturity was low for M. mobular (females 19.0% [4/21], males 26.7% [4/15]) and moderate for M. thurstoni (females 40.9% [9/22], males 50.0% [9/18]). In contrast, threshold maturity was higher for the limited M. kuhlii measurements (75.0% [3/4]) and complete for the few measured M. birostris (2/2) and M. tarapacana (1/1). Seasonal differences in threshold-based maturity proportions were not significant for any species-sex group with data in both seasons (all Fisher tests p ≥ 0.187).

Table 4 Maturity status of mobulids (Mobula spp.) by species and sex. Values include mean disc width (DW, cm ± SD), species- and sex-specific size-at-maturity thresholds from Stevens et al., (2025), and the number and proportion of individuals meeting published maturity criteria, defined as DW ≥ threshold. Proportions mature are calculated using only individuals with measured disc width and are presented with Wilson 95% confidence intervals

Observed field-based maturity classifications were largely concordant with size-at-maturity thresholds (75 of 83 individuals; 90.4% agreement). Agreement was highest for M. thurstoni (95.0%; 38 of 40) and lowest for M. mobular (83.3%; 30 of 36), with complete agreement for the limited samples of M. birostris, M. kuhlii, and M. tarapacana.

Discordance reflected individuals classified as mature in the field but falling below published minimum mature size thresholds. Discordance reflected individuals classified as mature in the field but falling below published size-at-maturity thresholds. This occurred in six male M. mobular, which were on average 6.3 cm below the male threshold of 206 cm DW (range: 2-14 cm), and two female M. thurstoni, which were on average 5.0 cm below the female threshold of 164 cm DW (range: 4-6 cm). No individuals exceeded published size thresholds while being classified as immature in the field. No individuals exceeded published size thresholds while being classified as immature in the field.

One-sample t-tests comparing mean disc width against species- and sex-specific maturity thresholds indicated that mean DW was significantly below the reported threshold for female M. mobular (mean 181.5 cm vs 220 cm; t = −5.15, p = 4.88 × 10⁻⁵), male M. mobular (mean 194.9 cm vs 206 cm; t = −2.89, p = 0.0119), and female M. thurstoni (mean 151.5 cm vs 164 cm; t = −2.28, p = 0.0329). Mean DW did not differ significantly from the threshold for male M. thurstoni (mean 152.8 cm vs 154 cm; p = 0.748) or female M. kuhlii (mean 121 cm vs 116 cm; p = 0.641). Sample sizes for male M. birostris (n = 2) and male M. tarapacana (n = 1) were insufficient for formal testing.

3.3 Seasonal patterns in landings

Survey coverage varied among monsoon periods, with 27 survey days during the Southwest Monsoon (SWM), 15 during the First Inter-Monsoon (IM1), nine during the Northeast Monsoon (NEM), and two during the Second Inter-Monsoon (IM2).

Generalized linear models indicated no overall effect of monsoon season on total landings per survey day (analysis of deviance: χ² = 3.93, df = 3, p = 0.27). Because the Poisson model exhibited moderate overdispersion (dispersion = 1.77), a negative binomial model was also fitted, which produced similar results. Using the Northeast Monsoon as the reference category, predicted landings during the Southwest Monsoon were lower (β = −0.53 ± 0.30, z = −1.79, p = 0.073), although this difference was not statistically significant. Landings during the First Inter-Monsoon (β = 0.20 ± 0.30, p = 0.50) and the second Inter-Monsoon (β = 0.08 ± 0.56, p = 0.89) did not differ from the NEM.

Species identity significantly influenced landings (χ² = 18.38, df = 5, p = 0.0025), while the interaction between species and season was not statistically significant (χ² = 15.57, df = 9, p = 0.076), indicating broadly similar seasonal patterns among taxa.

Predicted landings per survey day varied among species and monsoon periods (Table S5; Figure 4). Mobula mobular showed the highest predicted landings during the second Inter-Monsoon (4.00 individuals per survey day, 95% CI 1.50-10.66), intermediate values during the Northeast Monsoon (2.00, 1.23-3.26), and lower estimates during the First Inter-Monsoon (1.50, 0.85-2.64) and the Southwest Monsoon (1.07, 0.65-1.78). Mobula thurstoni exhibited relatively stable predicted landings across seasons, ranging from 1.60 to 1.79 individuals per survey day across the NEM, IM1, and SWM. Predicted landings of M. kuhlii were highest during the First Inter-Monsoon (8.50 individuals per survey day, 95% CI 5.28-13.67), although this estimate reflects a small number of survey days and substantial variance. Predictions for M. birostris and M. tarapacana were highly uncertain because of few observations.

Figure 4 Observed counts of mobulid species landed at the Semporna Fish Market, Sabah, Malaysia between May 2011 and November 2014. Estimates are shown for the Northeast Monsoon (NEM; November–March), First Inter-Monsoon (IM1; April-May), Southwest Monsoon (SWM; June-September), and second Inter-Monsoon (IM2; October)

3.4 Seasonal variation in maturity proportions

Seasonal splits of threshold-based maturity proportions showed variable patterns among species, although sample sizes were small in several monsoon periods. Among female M. mobular, individuals meeting the published maturity threshold occurred in 10% of cases during the Northeast Monsoon (1/10), 37.5% during the Southwest Monsoon (3/8), and were not observed during either inter-monsoon period (0/1 in IM1; 0/2 in IM2). Male M. mobular met the threshold in 50% of NEM observations (3/6), 33.3% during the SWM (1/3), and none during either inter-monsoon period.

For M. thurstoni, maturity proportions varied among seasons but showed no consistent directional pattern. Female maturity proportions were highest during the Northeast Monsoon (100%, 3/3), followed by the First Inter-Monsoon (40%, 2/5) and the Southwest Monsoon (28.6%, 4/14). Male maturity proportions ranged from 33.3% during the Northeast Monsoon (1/3) to 50% during the First Inter-Monsoon (4/8) and 57.1% during the Southwest Monsoon (4/7). Limited observations of M. kuhlii indicated maturity in all individuals during the Northeast Monsoon (1/1) and Southwest Monsoon (1/1), and in half of individuals during the First Inter-Monsoon (1/2). The few available observations of M. birostris and M. tarapacana also met maturity thresholds.

Fisher’s exact tests detected no significant differences in maturity proportions among monsoon periods for any species–sex group with observations across multiple seasons (all p ≥ 0.11).

3.5 Catch and market information

Gillnets dominated recorded landings around the Si Amil and Denawan grounds, accounting for 91 of 126 total surveys (72%). A further 12 surveys (10%) used non-net gear, while 23 surveys (18%) lacked gear information. When restricting to surveys with recorded gear type only (n = 103), gillnets represented 91 records (88%).  In this area, M. thurstoni (41%) and M. mobular (36%) were the most frequently landed species, followed by M. kuhlii (19%), M. tarapacana (3%), and a single unidentified Mobula specimen (1%). Additional captures occurred near Pulau Denawan, where M. thurstoni (57%), M. kuhlii (29%), and M. mobular (14%) were landed, and near Pulau Gaya, where trawlers landed M. tarapacana (50%), M. birostris (25%), and M. mobular (25%). One M. birostris individual was trawled far offshore from Semporna.

Non-net events included trawler catches on 5 May 2014 near Pulau Gaya: one M. mobular male (204 cm DW) and one M. birostris male (490 cm DW), along with two additional M. tarapacana from the same trawl without size data. Line gear was also recorded on 18 May 2012 near Pulau Denawan, capturing M. thurstoni (178, 171, 151, and 140 cm DW), one M. mobular female (175 cm), and M. kuhlii females (130 and 92 cm).

At the Semporna Fish Market, landing and handling of whole and partially processed mobulids were observed (Figure 5a-c). Carcasses were laid in a range of processing states, from whole or partially landed individuals to bodies cut into sections and arranged for sale (Figure 5d-e). Sale records indicated that nearly all mobulids were sold in strips of tissue (Figure f–g for a flat price of RM5 (~$1.20 USD) per strip (>1 kg), regardless of species, size, or condition. Notes frequently recorded only partial carcasses (e.g. half animals, quarters, or sections already cut for market). In addition to meat sales, gill plates were frequently extracted and sold separately (Figure 5h-i), although specific prices for these products were not recorded. The only exceptions were two whole M. birostris that were reported as being sold for substantially higher sums [RM2000 (~$500 USD) and RM600 (~$143 USD), respectively] and one M. mobular male for RM300-400 (~$85 USD).

Figure 5 Mobulid landing, processing and product forms observed at Semporna Fish Market, Sabah, Malaysia, 2011-2014: (a-c) landing and handling of whole and partially processed mobulids; (d-e) carcasses cut into sections, laid out for sale; (f-g) strips of meat prepared for trade; and (h-i) gill plates extracted and displayed separately for sale

4 Discussion

This study provides important insights into the catch composition, size, structure, and demographic characteristics of mobulids landed in Semporna, Sabah, Malaysia. The status of mobulids in Malaysia is best understood in comparison with patterns emerging from analogous fisheries across the Eastern Indian Ocean and Indo-West Pacific. The predominance of M. mobular and M. thurstoni in the catch between 2011 to 2014, accounting for over 75% of individuals recorded, reflects catch patterns similar to those observed elsewhere across the region (e.g. India, Sri Lanka, Indonesia, Philippines), but with fewer manta rays landed (White et al., 2006; Fernando and Stewart, 2021; Laglbauer et al., 2025; Chopra et al., 2026; Laglbauer et al., 2026). This supports the inference that ecological factors, such as habitat overlap, schooling behavior, and wide-ranging movements, when combined with fisheries employing non-selective gear like drift gillnets targeting pelagic species, drive these catch patterns (White et al., 2006; Mohanraj et al., 2024; Palacios et al., 2023).

4.1 Understanding juvenile-dominated landings

One of the most striking and consistent findings across regional studies in the Indo-West Pacific and Indian Ocean, including this investigation of the Semporna mobulid fishery, is the overwhelming prevalence of juvenile mobulid rays in landed catches, accompanied by a marked scarcity of mature individuals (Fernando and Stewart, 2021; Laglbauer et al., 2025; Chopra et al., 2026). In Semporna, most M. mobular (77.8%) and M. thurstoni (55.0%) were below size-at-maturity thresholds (Stevens et al., 2025), with very few mature individuals recorded. Field observations from the Sabah Shark and Ray Catch Report (SSRC) (2018) reinforce these findings, noting that most mobulids landed at Semporna jetties and informal markets were small-bodied juveniles, which were often retained for immediate meat consumption, while larger individuals were more likely to be processed into dried trade products such as gill plates for international trade.

This demographic pattern is fundamentally tied to the life history traits common to mobulid rays; long-lived, slow-growing, late-maturing, and producing very few offspring (Lawson et al., 2017; Barrowclift et al., 2025). The predominance of immature mobulids, likely age-class truncation, and small mean sizes dominated by juveniles observed in Malaysia mirror alarming patterns described regionally, signaling severe recruitment overfishing (Laglbauer et al., 2025; Chopra et al., 2026). Vendor surveys in Hong Kong and Guangzhou have explicitly identified Malaysia as a source of gill plates, linking juvenile-dominated landings in Semporna to international trade networks, while also reflecting Malaysia’s role as a regional importer and re-exporter of gill plates (Yu, Ho, and Shea, 2016; Boon, 2017; Palacios et al., 2025).

Similar trends are also reported from Indonesia, where artisanal drift gillnet fisheries have recorded high juvenile bycatch across multiple mobulid species, leading to severe population declines as inferred from catch trends and fishers’ testimony (White et al., 2006; Lewis et al., 2015; Laglbauer et al., 2025). Notably, aggregation grounds such as the Bali Strait show pronounced seasonal pulses of juvenile capture, whereas Semporna demonstrates year-round exploitation, exacerbating the demographic bottleneck (Shark Stewards, 2018). In India, Thoothukudi fisheries exhibited a 12% reduction in the proportion of adult mobulids and a shift towards smaller mean disc widths over a decade, consistent with heavy fishing pressure and demographic truncation (Chopra et al., 2026).

Specific evidence from Sri Lanka highlights an additional problem arising from increased juvenile bycatch. Records show that up to 75% of the males and over 80% of the females landed were below size-at-maturity thresholds, with numerous neonates and pups regularly observed (Fernando and Stewart, 2021). A higher percentage of female juvenile catch leads to the loss of mature females, which in turn, critically reduces population reproductive output, undermining replenishment efforts and increasing extinction vulnerability (White et al., 2006; Lawson et al., 2017). In the case of Semporna, where juvenile mobulid landings dominate, the cumulative effect is a truncated age structure that leaves local populations especially vulnerable to collapse, with little chance for replenishment under current fishing pressure (Clark-Shen et al., 2023; CITES, 2024).

4.2 Demographic and ecological consequences

Ecologically, the majority of mobulids heavily interact with highly productive coastal and shelf environments that also experience concentrated human activity and fishing effort (Marshall et al., 2019; Germanov et al., 2019). Familial traits, such as schooling and migratory behaviours are common in these productive continental shelf waters, which unfortunately coincide with the spatial and temporal footprint of small-scale and artisanal fisheries (White et al., 2006; Martin, 2020; Palacios et al., 2023; Mohanraj et al., 2024). Malaysia’s Semporna region exemplifies this overlap, sustaining continuous artisanal fishing with insufficient temporal or spatial restrictions (Shark Stewards, 2018; Martin, 2020; Clark-Shen et al., 2023). Si Amil, Denawan, and Pulau Gaya are identified as regular mobulid aggregation grounds, where nutrient-rich upwellings and coral reef cleaning stations draw animals close to shore, making them accessible to small-scale fleets (Shark Stewards, 2018).

Unlike the pronounced seasonal upwelling-driven aggregation and fishing peaks seen in Indonesia’s Bali Strait (Laglbauer, 2025), Malaysian fisheries maintain year-round pressure, minimizing natural population refuges (Shark Stewards, 2018; Martin, 2020). This pattern is also reflected in the findings of our study (Table 5; Figure 4), where mobulid landings did not show any significant seasonal variation across species, suggesting that fishing efforts likely overlap with aggregation sites throughout the year, intensifying the pressure on mobulid populations. This chronic exploitation without consistent respite further pressures mobulid recruitment, increasing local extinction risk (Martin, 2020). The non-selective nature of dominant fishing gear in the region, principally drifting and anchored gillnets targeting pelagic species such as tuna and mackerel, results in the frequent incidental capture of juveniles, which are retained both for meat consumption and for the lucrative dried gill plate trade (O’Malley et al., 2017; Martin, 2020; Palacios et al., 2024). In Semporna, informal landings at beaches and jetties mean that many of these captures bypass official records, echoing the broader underreporting trend flagged by CITES (2024), which lists Malaysia among countries with landings absent from FAO statistics (Palacios et al., 2024; Laglbauer et al., 2025; Laglbauer et al., 2026).

Recent population models and catch analyses from the region indicate fishing mortality rates frequently surpass maximum sustainable levels for mobulids, projecting continuing population declines and heightened extinction risk without immediate and coordinated management interventions (Pacoureau et al., 2021; Chopra et al., 2026). Given the elongated generation lengths of these rays—estimated at approximately 29 years for M. birostris and 13 years for pygmy devil rays—the observed rapid and extensive declines are particularly alarming (IUCN, 2022; Barrowclift et al., 2025; Laglbauer et al., 2026). Without targeted interventions, the combination of juvenile-dominated landings and chronic year-round exploitation represents one of the clearest signals of recruitment overfishing for mobulids in the Indo-Pacific (Laglbauer et al., 2025).

Addressing juvenile-dominated landings and ecological stress in Malaysia requires spatial–temporal protections that specifically reduce fishing pressure at areas of critical habitat use, such as migratory corridors, nursery grounds, and other aggregation sites. In Indonesia’s Raja Ampat MPA, restrictions on gillnets near known manta nurseries have reduced juvenile mortality and supported population recovery (Stewart et al., 2018; Laglbauer et al., 2025). Similarly, Sri Lanka’s recognition of mobulid nursery sites has spurred calls for seasonal closures where neonates and juveniles are most at risk (Fernando and Stewart, 2021). For Malaysia, seasonal closures at Semporna aggregation sites such as Pulau Si Amil and Pulau Gaya, combined with gear modifications like banning large-mesh drift nets during peak juvenile seasons, would directly limit the capture of immature mobulids. Establishing these zones at Key Biodiversity Areas (KBAs) or Other Effective Area-based Conservation Measures (OECMs) could enhance legal recognition and attract international support (Stewart et al., 2018; Germanov et al., 2019; Marshall et al., 2019; Venables et al., 2025; Laglbauer et al., 2026). Coupling such protections with community-led monitoring and alternative livelihood opportunities, including manta-watching tourism, as seen in the Maldives (Murray et al., 2024), would incentivize compliance while safeguarding the long-term reproductive potential of mobulid populations in Malaysia.

4.3 Declines in mobulid catch and landings

Evidence from Malaysia and across the Indo-Pacific and Indian Ocean highlights a sustained decline in mobulid catches and landings, suggesting ongoing population stress due to high fishing mortality and inadequate management (Fernando and Stewart, 2021; Thomas et al., 2022; Chopra et al., 2026; Laglbauer et al 2026). In Sabah, Malaysia, artisanal fishers in Semporna report that mobulid landings have become increasingly rare and dominated by juveniles, with mature specimens seldom encountered in landing sites such as Pulau Gaya and Denawan grounds (Shark Stewards, 2018; Clark-Shen et al., 2023). Observations from market surveys between 2015 and 2019 confirmed that landings were limited to a few individuals per month, contrasting with earlier reports of regular bycatch in small-scale tuna and mackerel gillnet fisheries (Boon, 2017). Local interviews and landing observations conducted by TRAFFIC and Shark Stewards in 2018-2019 indicated that less than 0.5% of total elasmobranch landings in Semporna were mobulids, a notable reduction from estimates exceeding 5% in the early 2000s (Shark Stewards, 2018). These declines coincide with regional market trends showing reduced mobulid product availability, particularly dried gill plates, which traders attribute to scarcity in local and neighboring Indonesian waters (Palacios et al., 2024).

Across the Indian Ocean, similar and often more severe declines have been documented, pointing to a transboundary depletion of shared populations (Fernando and Stewart, 2021; Thomas et al., 2022; Chopra et al., 2026; Laglbauer et al 2026). Indonesia experienced particularly steep declines. The Global Threat to Manta and Mobula Rays report (Heinrichs et al., 2011) estimated that landings in Lombok and Lamakera declined by more than 80% between 2002 and 2010, with traders reporting that animals “must be sourced from farther offshore” and are “smaller and fewer” than a decade earlier. In the Bali Strait, catches of M. alfredi and M. birostris fell from over 1,500 individuals annually in the mid-2000s to fewer than 150 by 2015, representing a 90% decline (White et al., 2006; IUCN, 2018). In the Philippines, sharp reductions in mobulid landings at Bohol and Pamilacan were reported, with fishers noting that “aggregations once occurring weekly now happen a few times per year.” (Rambahiniarison et al., 2018).

Regional assessments by the IUCN Shark Specialist Group (2018) and the Manta Ray of Hope Project (Heinrichs et al., 2011) found that Indo-Pacific landings of mobulids had declined by 70%-95% over the previous two decades, depending on location. Specifically, India, Indonesia, and Sri Lanka were identified as contributing over 95% of total mobulid landings recorded in the Indian Ocean in the early 2000s, but by 2017 these combined national totals had dropped by more than 80%, reflecting widespread stock depletion and increased targeting of juveniles (Fernando and Stewart, 2021; Laglbauer et al., 2025; Chopra et al., 2026). The IUCN strategy also highlighted that Malaysia and Thailand lacked consistent catch reporting, but the geographic overlap of fishing grounds with known mobulid aggregation sites (e.g. Denawan and Si Amil grounds) implies similar declines.

Complementary data from long-term monitoring in southern Mozambique reinforce these regional pattern where a documented decline of 99% in reef manta (M. alfredi), 92.5% in oceanic manta (M. birostris), and 81% in devil rays (M. kuhlii) sightings over a 20-year period (2003-2023), despite consistent survey effort was noted (Venables et al., 2025). These values represent one of the steepest declines recorded for any large elasmobranch aggregation globally. When coupled with global synthesis data which shows a ~71% decline in oceanic sharks and rays since the 1970s (Pacoureau et al., 2021), the evidence converges on a clear picture: local reductions in Malaysia mirror a broader Indo-Pacific collapse. With global population declines of 50%-99% across all mobulid species (Thomas et al., 2022; Laglbauer et al 2025; Chopra et al., 2026; Fernando and Stewart, 2028), the convergence of underreported domestic bycatch, juvenile-dominated landings, and persistent international demand for gill plates suggests that Malaysia’s declining mobulid presence is not isolated, but part of a coordinated, regional-scale population decline driven by chronic overexploitation and weak governance.

4.4 Legal and socio-economic complexity

Malaysia’s legal and regulatory framework for mobulids conservation currently trails regional advances. For instance, Indonesia implemented a ban on manta ray retention in 2014, reflecting a pioneering national commitment, while India only recently included manta rays in their national protection acts as of 2023 (Stewart et al., 2018; Laglbauer et al., 2026). However, despite similar biological and ecological vulnerabilities, devil rays remain without formal protection in Malaysia, leaving a significant gap in conservation coverage. More importantly, no national management plans have been developed for the protected manta rays, and without a coordinated response to the drivers of decline across the region and for all mobulids, simply extending the same legal protection to devil rays would likely have limited conservation impact. The artisanal fisheries dominating Malaysian coastal waters capture mobulids mostly as bycatch, and weak enforcement capacity, together with low awareness among fishers, further reduces the practical effectiveness of existing regulations. An ocean-basin approach to conservation management is therefore urgently needed (Lawson et al., 2017).

The economics of the fisheries further complicate effective conservation. Small-scale fishers rely on diverse catches to sustain livelihoods and lack alternatives, limiting their incentive to avoid mobulid bycatch or support conservation enforcement. Conversely, processors and middlemen disproportionately benefit from the lucrative international trade in mobulid gill plates, driving demand with minimal regulatory oversight (Shark Stewards, 2018; Palacios et al., 2024). This dynamic of limited fisher incentive, coupled with high external market demand, undermines local conservation efforts and facilitates ongoing extraction of vulnerable mobulid populations.

Recent concerted efforts supported by NGOs, regional fisheries bodies, and international conservation frameworks aim to strengthen these weak points. For example, targeted funding seeks to enhance legal frameworks and enforcement capacity within Sabah, Malaysia, alongside community engagement and awareness-building programs (Shark Conservation Fund, 2022). Internationally binding management measures adopted by Regional Fisheries Management Organizations (RFMOs), such as ICCAT’s 2023 recommendation 23-14 and its successor 24-12, promote prohibitions on mobulid retention and best handling practices, which Malaysia can integrate into domestic laws to improve compliance and conservation outcomes (ICCAT, 2023).

Realizing effective mobulid conservation in Malaysia thus requires a multifaceted approach: expanding the legal protection to all mobulid species; introducing and enforcing bycatch reduction policies informed by global best practices; improving traceability through port and market controls; and fostering fisher-centered solutions with economic incentives and alternative livelihood options. Integration with regional efforts under CITES, CMS, and subregional fisheries management bodies will ensure Malaysia benefits from shared knowledge, capacity building, and collaborative enforcement to safeguard its mobulid populations (Lawson et al., 2017; Stewart et al., 2018; Palacios et al., 2024).

4.5 Study limitations and next steps

This study contributes one of the first species-level analyses of mobulid landings in Semporna, Sabah, and provides valuable baseline information for a fishery that remains poorly documented in Malaysian waters (Laglbauer et al., 2025). However, because the data were collected in May 2011 and November 2014, they now represent a decade-old baseline rather than a current assessment. This makes the study especially useful as a benchmark for future work, since a repeat survey would allow comparison of landing composition, trade pathways, and fishing practices over time. Despite the insights generated, several limitations constrain the scope and certainty of conclusions. First, data collection encountered issues typical of artisanal fisheries research, including inconsistent sampling effort, zero-inflated count data, and fragmented temporal and spatial coverage (White et al., 2006; Stewart et al., 2018; Fernando and Stewart, 2021). These factors reduce statistical power and hinder robust modeling of population trends or fishing mortality rates. In particular, precise effort data linked to vessels or gear types were lacking, which prevents accurate catch-per-unit-effort (CPUE) calculations and limits the application of integrated stock assessments currently standard for well-monitored fisheries (Stewart et al., 2018; Laglbauer, 2025). As such, the LPSE values presented here should be viewed as descriptive indices of landings per survey day rather than standardized measures of fishing effort.

Species misidentifications and inconsistent maturity assessments further complicate demographic analyses, given the taxonomic similarity among some mobulid species and limited availability of validated reproductive biology data for Southeast Asian populations (Palacios et al., 2023; Chopra et al., 2026). Additionally, our data focus primarily on landing sites within Semporna and may not fully reflect catches from broader Malaysian waters or unreported landings, such as those from illegal or unmonitored fisheries (Shark Stewards, 2018).

Addressing these knowledge gaps requires multi-pronged next steps. First, establishing standardized, long-term fishery monitoring protocols across Malaysian and regional landing sites will improve data consistency and enable trend analyses across temporal and spatial scales (Laglbauer, 2025). Second, integrating fishing effort data through vessel monitoring systems or fisher logbooks will facilitate calculation of CPUE and potentially allow stock-assessment modeling (White et al., 2006). Third, expanding species-specific biological studies such as age, growth, and reproductive cycles with local samples will refine understanding of population dynamics and maturity thresholds crucial for management reference points (Dulvy et al., 2014; Chopra et al., 2026).

Complementing fishery-dependent data with fishery-independent surveys and using emerging genomic tools such as close-kin mark-recapture or eDNA-based population monitoring will further resolve abundance and population structure, especially for cryptic or low-density species (Stewart et al., 2018). Moreover, socio-economic research and stakeholder engagement will be vital to design locally acceptable management interventions, including bycatch mitigation technologies, temporal-spatial closures, and alternative livelihood development (Booth et al., 2021; Palacios et al., 2024). Future studies should also clarify the mobulid gill plate trade chain in Malaysia by quantifying how much product is consumed domestically, how much is re-exported, and whether any portion moves through illegal channels. This is especially relevant because gill plates from unprotected mobulids may still be sold legally in Malaysia, but Malaysia may also function as an importer and re-exporter, complicating attribution of trade sources and obscuring the extent of illicit movement (Yu, Ho, and Shea, 2016; Boon, 2017; Palacios et al., 2025).Through these advances, future studies can provide increasingly accurate assessments of mobulid population status and recovery potential in Malaysian waters, aiding evidence-based conservation and sustainable fishery management.

5 Conclusion

This study highlights the urgent conservation challenges facing mobulids in Semporna, Sabah, Malaysia, revealing juvenile-dominated landings and significant demographic truncation that mirror similar pressures across Southeast Asia. The slow growth, low fecundity, and habitat overlap with small-scale fisheries subject these rays to recruitment overfishing compounded by inadequate legal protection and socio-economic complexities, such as fisheries dependence and lucrative international gill plate trade. Addressing these challenges demands comprehensive legal reforms expanding protections to all mobulid species, improved fishery monitoring and data collection, bycatch mitigation innovation, stricter trade controls, and socially inclusive conservation strategies. Coordinated regional management under frameworks like CITES and CMS, combined with community engagement and alternatives for fishers, offers the best prospect for sustainable mobulid populations amidst mounting anthropogenic threats.

Author’s contribution

This study was originally conceptualized by GMWS and BJLL and supervised by NGD and GMWS. NGD contributed to the study design and coordination and drafted the original manuscript. SRT conducted the data analysis. SB contributed to the writing of the original manuscript. Fisheries data were collected by AH and DF. All authors contributed to the review of the manuscript.

Acknowledgements

The authors gratefully acknowledge The Manta Trust for funding the research and publication of this paper.

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