Tropical Stingrays in Chapter 8: Threatened Groups of The Global Status of Sharks, Rays, and Chimaeras
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Preliminary findings regarding the reproduction of three neotropical freshwater
stingray species, Potamotrygon henlei, P. motoro, and P. orbignyi, are presented herein. These species are characterized by the presence of a singular functional left ovary and two operational uteri. Two distinct fertility models have been identified: fecundity dependent, and fecundity independent of maternal size. Furthermore, this study briefly outlines the process of capture-induced parturition and its implications for the fate of the litter. Additionally, it sheds light on novel embryonic behavioral patterns, particularly highlighting the active swimming exhibited by embryos within the uterine environment, notably during the latter stages of development. Consequently, it is suggested that these movements, coupled with oral activities and active biting, facilitate branchial ventilation and potentially intrauterine foraging - a phenomenon previously documented only in sharks belonging to the order Lamniformes.
Rhino rays (Order Rhinopristiformes) are one of the most threatened vertebrate groups. Despite overfishing being recognised as the greatest threat to this group, a comprehensive assessment of the fisheries and types of fishing gear that rhino rays interact with is lacking. We reviewed published and grey literature ( n = 116 references) to evaluate interactions between rhino rays and fisheries, determine gear types most responsible for their capture, and species for which most interactions occur by region. Interactions ( n = 420) were reported from 37 of 88 range states, for 52 of 68 known species. Combined, 59% of the literature reported interactions from trawlers and gillnets. Wedgefishes (Rhinidae) were the most reported family (29%) and bowmouth guitarfish ( Rhina ancylostomus ) the most reported species (9.5%). Asia accounted for half of interactions ( n = 211), with 67% of the literature from unspecified gear (e.g., mixed landings), masking gear-specific interactions. Data quality was variable, with the highest resolution in countries with lower species diversity and where species are least imperiled (e.g., Australia). Discard mortality and physiological effects of capture are poorly known with data available for 25% of species (almost exclusively from Australia and the Americas). While additional data are urgently required to quantify the true extent of rhino ray catch globally, reducing fisheries mortality is a priority and key to address declining populations. Recommendations include prioritizing spatial management in critical habitats (e.g., nursery areas), expansion in the use of proven bycatch reduction devices, encouraging safe release and handling, and addressing drivers of retention and trade.
The pet trade is known to be one of the most important pathways of aquatic non-native species introduction and Indonesia is a significant trade partner. Popular ornamental South American river stingrays (Potamotrygon spp.) were introduced to Indonesia in the 1980s and the culture was established. Here we present a detailed Indonesian market and aquaculture survey, the volume of trade between January 2020 and June 2022, and the list of customer countries with the total amount of imported stingrays. Climate similarities between the native range of P. motoro and P. jabuti and Indonesia were analysed. A significant number of areas of Indonesian islands were identified as suitable for the establishment of this species. This was confirmed by the first record of probably established populations in the Brantas River (Java). In total 13 individuals including newborns were captured. The culture of potamotrygonid stingrays is unregulated in Indonesia, and the risk of the establishment of this predator and its potential spread is alarming for wildlife. Moreover, the first case of envenomation caused by Potamotrygon spp. in the wild outside of South America was recorded. The current condition is predicted as the 'tip of the iceberg' and continuous monitoring and mitigation of risks are strongly recommended.
Intertidal habitats (i.e. marine habitats that are (partially) exposed during low tide) have traditionally been studied from a shorebird‐centred perspective. We show that these habitats are accessible and important to marine predators such as elasmobranchs (i.e. sharks and rays). Our synthesis shows that at least 43 shark and 45 ray species, of which 54.5% are currently threatened, use intertidal habitats. Elasmobranchs use intertidal habitats mostly for feeding and as refugia, but also for parturition and thermoregulation. However, the motivation of intertidal habitat use remains unclear due to limitations to observe elasmobranch behaviour in these dynamic habitats. We argue that elasmobranch predators can play an important role in intertidal food webs by feeding on shared resources during high tide (i.e. ‘high‐tide predators’), which are accessible and also consumed by terrestrial or avian predators during low tide (i.e. ‘low‐tide predators’). In addition, elasmobranchs are able to change the bio‐geomorphology of intertidal habitats by increasing habitat heterogeneity due to feeding activities and may also alter resource availability for other consumers. We discuss how the ecological role of elasmobranchs in intertidal habitats is being affected by the continued overexploitation of these species, and conversely, how the global loss of intertidal areas poses an additional threat to an already vulnerable taxonomic group. We conclude that studies on intertidal ecology should include both low‐tide (e.g. shorebirds) and high‐tide (e.g. elasmobranchs) predatory guilds and their ecological interactions. The global loss of elasmobranch predatory species and intertidal habitat provides additional compelling arguments for the conservation of these areas.
The 6.7-million-hectare Raja Ampat archipelago is home to Indonesia’s largest reef manta ray (Mobula alfredi) population and a representative network of nine marine protected areas (MPAs). However, the population dynamics of M. alfredi in the region are still largely unknown. Using our photo-identification database, we fitted modified POPAN mark-recapture models with transience and per capita recruitment parameters to estimate key demographic characteristics of M. alfredi from two of Raja Ampat’s largest MPAs: Dampier Strait and South East (SE) Misool. A total of 1,041 unique individuals were photo-identified over an 11-year period (2009–2019) from Dampier Strait (n = 515) and SE Misool (n = 536). In our models, apparent survival probabilities and per capita recruitment rates were strongly linked with El Niño–Southern Oscillation (ENSO) events. Our models also estimated high apparent survival probabilities and significant increases in (sub)population sizes in both MPAs over a decade. In Dampier Strait, the estimated population size increased significantly (p = 0.018) from 226 (95% CI: 161, 283) to 317 (280, 355) individuals. Likewise, the estimated population size in SE Misool increased significantly (p = 0.008) from 210 (137, 308) to 511 (393, 618) individuals. Regardless of variation in the percentage change in population size between years throughout the study, the estimated overall population change shows a compound growth of 3.9% (0.7, 8.6) per annum in Dampier Strait and 10.7% (4.3, 16.1) per annum in SE Misool. Despite the global decline in oceanic sharks and rays due to fishing pressure in the last five decades, our study demonstrates the positive impact of a suite of long-term conservation efforts, coupled with the influence of ENSO events, on increasing M. alfredi abundance in Raja Ampat MPAs. Our study also underscores the importance of long-term monitoring to evaluate the effectiveness of conservation management measures on manta ray populations. Our modification of the standard POPAN model by incorporating per capita recruitment and transience parameters represents an important advance in mark-recapture modelling that should prove useful when examining other manta ray populations and other highly migratory species that are likely to have a substantial percentage of transient individuals.
Scarce information exists on the life history of the bowmouth guitarfish Rhina ancylostomus (=R. ancylostoma) landed by trawlers and gillnetters that operate in the eastern Arabian Sea off Karnataka, on the south-west coast of India. This study was carried out to update information on this species by analysing the size distribution, sex ratio, length-at-maturity (Lm50%), length–weight relationship and diet using data collected during 2016-2019. Three hundred and sixty-nine individuals were collected with a total length (TL) range of 44.0 to 295 cm and total weight (TW) range of 0.2 to 127 kg. The length-weight relationship (LWR) of females and males did not differ significantly and therefore, a common equation was derived as TW = 0.006604TL 3.027504(r2=0.979). The length-at-maturity (Lm50%) for females and males were estimated to be 183.0 and 164.0 cm TL, respectively. R. ancylostoma has two functional ovaries and the ovarian cycle and gestation run concurrently. The number of embryos ranged from 2 to 8 and size at-birth was estimated to range between 44.0 and 50.0 cm TL. Overall sex ratio indicated dominance of females. Dietary analysis of stomach contents (%IRI) revealed that R. ancylostomus fed primarily on teleosts (73.3%), crustaceans (20.5%) and molluscs (6.2%). Gravid females enter coastal waters possibly for parturition and feeding in some seasons, where they become vulnerable to trawl and gillnets used along Karnataka coast. The species is categorised in IUCN Red List as “Critically Endangered” and the present study revealed the urgent need to construct an effective management plan to conserve this species in the region.Keywords: Bycatch, Elasmobranchs, Life history, Shark ray
Some sharks and rays are subject to fisheries catch and international trade regulations. However, the Guitarfishes (family Rhinobatidae) are a highly threatened group with minimal regulations. The true volume of Guitarfishes in international trade is underestimated and masked by substantial underreporting of catch and the use of broad commodity codes for traded products. Here, we begin to document the extent of trade by collating international trade information for Guitarfishes that have not readily been documented in trade, possibly due to poor resolution of molecular genetic markers. We also assess the shortfall in fisheries management (M-Risk) for all species of Guitarfish based on 99 assessments across 28 countries. Globally, Guitarfishes are inadequately managed, with an average M-Risk of 45% of an ideal score, resulting in 76% of species being threatened globally. The high and unregulated catch and trade volume, paired with the management shortfalls, require global integrated improvement in fisheries management, supported by regulating international trade to sustainable levels.
The present study characterized complete mitochondrial genome of Blue-spotted maskray, Neotrygon indica and studied the evolutionary relationship of the species within the Dasyatidae family. The total length of the mitogenome was 17,974 bp including 37 genes and a non-coding control region. The average frequency of nucleotides in protein-encoding genes was A: 29.1 %, T: 30.2 %, G: 13.0 % and C: 27.7 % with AT content of 59.3 %. The values of AT and GC skewness were −0.018 and −0.338, respectively. Comparative analyses showed a large number of average synonymous substitutions per synonymous site (Ks) in gene NADH4 (5.07) followed by NADH5 (4.72). High values of average number of non-synonymous substitutions per non-synonymous site (Ka) were observed in genes ATPase8 (0.54) and NADH2 (0.44). Genes NADH4L and NADH2 showed high interspecific genetic distance values of 0.224 ± 0.001 and 0.213 ± 0.002, respectively. Heat map analysis showed variation in codon usage among different species of the Dasyatidae family. The phylogenetic tree showed sister relationship between the Dasyatinae and Neotrygoninae subfamilies. Neotrygon indica formed as a sister species to the clade consisting of N. varidens and N. orientalis. Based on the present results, Neotrygon indica could have diverged from the common ancestor of the two latter in the Plio-Pleistocene. The present study showed distinct characteristics of N. indica from its congeners through comparative mitogenomics.
Overfishing is recognized as the most pervasive threat to sharks and rays globally. While there is increasing emphasis on ecological aspects of shark and ray fisheries, socio-economic considerations are often poorly incorporated into management policies. Here, we assess the utilization and trade of sharks and rays across the Andaman Islands by conducting semi-structured interviews with 87 fishers and eight traders. Sharks and rays were exported to supply the meat market in peninsular India and contribute to the international trade in products such as fins, gill plates, and liver oil. A large proportion of fishers (n = 38, 43.67%) consumed sharks and rays due to declines in reef fish, as an accessible and cheap protein source. Small-sized sharks (<1 m total length), juvenile hammerheads, and uniformly coloured rays were preferred for local consumption. Fishers (n = 43, 49.42%) noted the difficulty of relying on profits from shark fishing due to declines in shark populations. However, it was easier to fish and trade rays due to their perceived abundance, few regulations, and increased demand for their products. Traders (n = 7, 87.5%) mentioned a rising demand for ray meat from peninsular India, leading to the development of a targeted ray fishery. Expanding and targeted shark and ray fisheries benefit the stakeholders who have the resources to invest, while affecting the livelihoods of others due to declining local fisheries resources. Our results highlight the need to revise and improve legal frameworks to consider the conservation needs of threatened species and likely impacts on local communities.