Biology subjects

Stevens, G. M. W.

Publications and source records attributed to Stevens, G. M. W..

6 recordsLinked to original sources

bioRxiv2026

Why are fishers retaining manta and devil ray bycatch?

Increasing fishing pressure, including from small-scale fisheries, has caused declines in more than one-third of all elasmobranch species. Tackling conservation issues in small-scale fisheries requires interdisciplinary approaches due to the coastal community's interdependence on marine resources. To support inclusive policy change and fisher engagement, an understanding of the motivations driving fishers' operational choices (i.e., the retention of elasmobranch bycatch) is needed. We assess the motivational drivers behind bycatch retention of one of the slowest-growing and most vulnerable elasmobranch groups, manta and devil rays (collectively, mobulids), through a case study in India, their largest fishery in the world. We conducted a best-worst scaling survey in the fishery-intensive states of Tamil Nadu and Andhra Pradesh, which make significant contributions to mobulid landings on India's east coast. Our results suggest that fishers exhibit varied motivations for retaining mobulid bycatch across states. Financial motivation to sell mobulids for additional revenue was the most important motivator for bycatch retention in both states. In Tamil Nadu, the top three motivators were all financially driven, whereas in Andhra Pradesh, the top three motivators included both financial and non-financial attributes, such as nutritional importance and storage optimisation. As the first socio-economic study of mobulid fisheries in India, we show that motivations underlying bycatch retention decisions vary geographically and may be influenced by cultural differences between states and the socio-economic characteristics of decision makers. Based on identified fisher motivations, we provide context-specific recommendations to align conservation strategies with the values fishers derive from the mobulid fishery and encourage participation in conservation. These include subsidies for net repair to encourage mobulid release; promotion of a minimum price measure for sustainably sourced alternative species; quality improvement of target species; and increased awareness of national and international regulatory obligations (e.g., CITES, CMS, IOTC).

ecology↗

bioRxiv2026

The emerging medium-scale: Prioritising fisheries management for elasmobranch conservation.

Fishing pressure has substantially increased over time and elasmobranchs are among the most vulnerable marine taxa impacted. 'Small-scale' and 'large-scale' are terms frequently used to describe fisheries, yet definitions used across the globe are inconsistent. The global impact of small-scale fisheries on elasmobranch decline is increasingly evident. However, ambiguous vessel-capacity classifications undermine effective conservation management. Technological advancements have led to the emergence of medium-capacity vessels that sit between small and large-scale classifications, creating regulatory gaps that increase bycatch impacts on vulnerable elasmobranchs. Beyond vessel size, the gear specificities and practices also affect interaction with species caught incidentally, and can alter bycatch risk for vulnerable elasmobranchs. Manta and devil rays (collectively, mobulids) represent a pelagic ray group that includes species listed as Critically Endangered with high risk of extinction, largely due to bycatch threats. To prioritise fisheries management for conservation, we assess the contribution of fishing gear specifications and practices to mobulid bycatch as a case study in their largest fishery globally. We use a Huber-robust extension to a Hurdle negative binomial model to assess the impact of fishery characteristics and operational specificities on mobulid bycatch risk. We find that within the Food and Agriculture Organization (FAO) of the United Nations' broad international fleet classification of small-scale vessels based on overall length ([≤]24 m), the 'medium-capacity vessels' in India are twice as likely to catch mobulids than small-scale vessels. Further, increased fishing intensity, measured in five-day increments, doubles mobulid bycatch risk. Our results indicate the emergence of a distinct medium-capacity fishery class in rapidly developing blue-economy nations, shaped by rising technological capacity and exemptions from large-scale management measures. We propose a comprehensive re-conceptualisation of small-scale fisheries to ensure that advanced medium-capacity vessels are subject to management measures commensurate with their bycatch impact. Our recommended re-conceptualisation is complementary to FAO's SSF characterisation matrix and would promote fishery-level sustainability and enable bycatch risk mitigation for elasmobranchs, including manta and devil rays. Further, for congruence between elasmobranch demography and regulations, we recommend that fisheries management measures distinguish vessel capacities for elasmobranch bycatch risk and apply biologically relevant regulations to the associated fisheries scale.

ecology↗

bioRxiv2026

No single measure is enough: Recovery of the Critically Endangered Mobula mobular requires integrated maximum bycatch mitigation and nursery area protection.

As anthropogenic threats have intensified over the past 500 years, we find ourselves in the midst of a sixth mass extinction, with continued losses of biodiversity threatening ecosystem stability. This biodiversity loss has caused species extinctions across taxa, and placed several others at high risk of functional extinction. These disturbance-driven impacts represent one of the most acute biodiversity crises facing global marine systems. Species exhibiting slow life histories characteristically have low resilience to disturbance. Here, we assess the risk of functional extinction and identify policy pathways for population recovery of the slow-living, Critically Endangered elasmobranch, the spinetail devil ray (Mobula mobular). We develop a stochastic, state-structured Integral Projection Model (IPM) parameterised with demographic data collected from fishery landings data in India, the world's largest mobulid fishery, and supplemented with data on vital rates from published literature. Using the IPM, we estimate that the population is declining at approximately 12% annually, experiencing substantial limiting pressure from fisheries overexploitation and failing to approach its biological maximum growth potential. Our results indicate that populations of M. mobular will be at high risk of functional extinction if 'business as usual' harvest scenario persists for another decade. We further show that long-term population recovery is only possible if survival increases significantly across all size classes, especially among large reproductive females, alongside a concurrent increase in fecundity. We conclude that no single policy measure is sufficient to recover population of M. mobular along the southeastern coast of India. Instead, combined protection through maximum bycatch mitigation and protection of nursery areas in no-take zones will be required for population recovery. This research demonstrates that recovery of overexploited populations often requires integrated resource management across life stages, and that the Critically Endangered M. mobular warrants urgent conservation action to avoid functional extinction.

ecology↗

bioRxiv2026

Remote underwater photographs reveal environmental correlations and patterns in reef manta ray habitat use in Laamu Atoll, Maldives

Reef manta rays (Mobula alfredi) are threatened by fishing and other anthropogenic threats. Which, when coupled with conservative life history traits, have made this species vulnerable to extinction. Spatiotemporal ecological knowledge, such as site fidelity and visitation patterns to key aggregation sites, are imperative for effective conservation management of M. alfredi. A novel method of environmental sensing, remote underwater photo systems (RUPs), was employed to understand drivers of M. alfredi habitat use and resighting patterns. RUPs were deployed at four cleaning sites around Laamu Atoll, Maldives. Between March 2021 and May 2023, 455,458 photos were analysed. Generalised linear models revealed increases in M. alfredi presence in response to high chlorophyll-a concentrations, low illumination moon states, the Southwest Monsoon, and in the morning, while human presence had no effect. Branchial spot patterns allowed for 81 M. alfredi individuals to be identified, from 629 sightings, representing 51.59% of Laamu Atolls previously identified population (n = 157). Cleaning stations are visited more intensively during periods of increased productivity of the Southwest Monsoon, likely in response to greater foraging opportunities near the study areas. Additionally, moon state, used as a proxy for tidal strength, was associated with increased visitation during new moon periods, suggesting that weaker tidal states may facilitate presence. These data support integrating RUPs with observational surveys to improve inferences about habitat use and our understanding of cleaning sites frequented by M. alfredi. This study aims to inform the implementation of Laamu Atolls first marine protected area management plan.

ecology↗

bioRxiv2025

Valuing conservation and natural wealth: The blue economy of manta ray watching in the Maldives

Amid declining manta ray populations globally, the well-established and growing manta ray tourism industries generate substantial economic benefits and aid protective legislation for these threatened elasmobranchs. As flagship species, manta rays are a drawcard for marine wildlife tourism and a gateway for engaging the public and communities in conservation. Healthy marine ecosystems are the key drivers of employment and economic sustainability for island nations such as the Maldives. However, there are many stakeholders competing for these shared resources, which can result in environmental degradation. Economic evaluations are a powerful tool for justifying the conservation efforts of threatened species and natural areas, especially in light of competing stakeholders. Using tour operator surveys (n=106) and data mining, this study provides an updated assessment of manta ray watching tourism in the Maldives, and represents the first national evaluation of its total economic and socio-economic benefits. In 2021, manta ray tourism in the Maldives generated an estimated US$227.3 million, including US$39 million on manta ray focused diving and snorkelling excursions, and US$188.3 million in related tourist expenditure. This represented 4.8% of the national Gross Domestic Product. This industry appears to have grown around 380% since 2008 (US$8.1 million) and manta ray watching is now offered by 80% of tourism operators nation-wide. Our findings revealed that manta rays hold intrinsic value and cultural significance within local communities. Acknowledging this, the flow-on benefits to the community extend beyond this industry, reaching local businesses, employed staff, and the government with the total economic benefits of the manta ray tourism industry were estimated at over US$311 million per year. Such value highlights the significance of manta rays for this nation and the need for effective management centred on manta ray conservation to safeguard future prosperity and mitigate the potential impact of tourism on manta ray populations.

ecology↗

bioRxiv2023

Comparative population genomics of manta rays has global implications for management

Understanding population connectivity and genetic diversity is of fundamental importance to conservation. However, in globally threatened marine megafauna, challenges remain due to their elusive nature and wide-ranging distributions. As overexploitation continues to threaten biodiversity across the globe, such knowledge gaps compromise both the suitability and effectiveness of management actions. Here, we use a comparative framework to investigate genetic differentiation and diversity of manta rays, one of the most iconic yet vulnerable groups of elasmobranchs on the planet. Despite their recent divergence, we show how oceanic manta rays (Mobula birostris) display significantly higher heterozygosity than reef manta rays (Mobula alfredi) and that M. birostris populations display higher connectivity worldwide. Through inferring modes of colonisation, we reveal how both contemporary and historical forces have likely influenced these patterns, with important implications for population management. Our findings highlight the potential for fisheries to disrupt population dynamics at both local and global scales and therefore have direct relevance for international conservation of marine species.

evolutionary biology↗