Gone with the wind: Seasonal distribution and habitat use by the reef manta ray (Mobula alfredi) in the Maldives, implications for conservation

Abstract

Reef manta rays (Mobula alfredi) are one of the ocean's largest and most charismatic species. Pressure from targeted and bycatch fisheries coupled with their conservative life‐history traits including slow growth, late maturity, and low fecundity has led to catastrophic declines of the global population. The species is now listed as Vulnerable to Extinction on IUCN Red List of Threatened Species. The global M. alfredi population is widely distributed in highly fragmented subpopulations. The Maldives supports the world's largest known subpopulation that undergoes seasonal migrations which are thought to be linked to peaks in ocean productivity induced by the South Asian Monsoon. Although the species is protected from targeted fisheries in the region, increasing pressures from habitat degradation and unsustainable tourism activities mean their effective conservation relies upon knowledge of the species' habitat use, seasonal distribution, and the environmental influences on such movements. Photo‐ID sighting records collected between 2005 and 2017 were used to identify key aggregation sites throughout the archipelago, and multiple linear regression and prediction analysis identified the environmental variables affecting variations in the intra‐annual sighting frequency of M. alfredi. Mobula alfredi were recorded at 273 different sites, 48 of which, with >100 sightings at each, were classified as key areas of habitat use. South‐west monsoon winds and chlorophyll‐a concentration predominantly affected the monthly percentage of M. alfredi sighted on the down‐current side of the atolls. In a country where climate change and touristic pressure are increasingly threatening this species and its habitat, the identification of key areas of habitat use and temporal changes in the use of these sites highlight the areas that should be prioritized for protection enabling more effective conservation management.

Sightings trends and behaviour of manta rays in Fernando de Noronha Archipelago, Brazil

“…In general, manta rays demonstrate site affinity to feeding areas, cleaning stations, and reproductive sites (Couturier et al 2011;Kashiwagi et al 2011;Harris et al 2020). Preliminary indications are that young of the year and juveniles may display more intense site fidelity, even permanent use of specific habitats, during the first years of their lives (Heupel et al 2007;Stewart et al 2018b;Pate and Marshall 2020).…”

Fine‐scale oceanographic drivers of reef manta ray (Mobula alfredi) visitation patterns at a feeding aggregation site

“…The final time-series contained 28,654 × 5-min bins of presence and absence observations. Nine predictor variables representing temperature (1-2), zooplankton biomass (3), ocean currents (4-8), and tide (9), all of which have been shown to influence M. alfredi occurrence (Anderson et al, 2011;Harris et al, 2020;O'Shea et al, 2010), were selected for inclusion (Table 1). Temperature variables included the following: temperature at 2 m above the seabed (temp 2 m) (1), and 50 m above the seabed (temp 50 m) (2), sampled every 5 s using RBR Concerto CTDs.…”

“…| 4589 limited home ranges, typically centered around coral reef ecosystems (Couturier et al, 2018;Kessel et al, 2017;McCauley et al, 2014;Setyawan et al, 2018). Aggregation behavior is characteristic of the species, whereby subpopulations will concentrate the majority of their activities at certain "hotspot" locations (Couturier et al, 2018;Harris et al, 2020;Setyawan et al, 2018). These aggregations typically occur within particular discrete habitats (Harris et al, 2020;Stevens, 2016) such as cleaning stations (O'Shea et al, 2010), and locations where they engaged in social (Perryman et al, 2019;Stevens, 2016) or reproductive activities (Stevens et al, 2018).…”

“…Aggregation behavior is characteristic of the species, whereby subpopulations will concentrate the majority of their activities at certain "hotspot" locations (Couturier et al, 2018;Harris et al, 2020;Setyawan et al, 2018). These aggregations typically occur within particular discrete habitats (Harris et al, 2020;Stevens, 2016) such as cleaning stations (O'Shea et al, 2010), and locations where they engaged in social (Perryman et al, 2019;Stevens, 2016) or reproductive activities (Stevens et al, 2018). Large feeding aggregations also occur and are often associated with the species' reliance on dense assemblages of prey (Armstrong et al, 2016) in a largely oligotrophic environment (Morel et al, 2010).…”

“…Mobula alfredi are particularly vulnerable to exploitation and changes in climate at feeding aggregation sites. For example, anthropogenic disturbance may reduce individual M. alfredi fitness by driving them away from productive feeding areas (Murray et al, 2019;Venables et al, 2016), which has been highlighted as a major conservation concern for the species (Harris et al, 2020;Murray et al, 2019). Feeding behavior may also be disrupted by enhanced stratification driven by rising sea surface temperatures, which can decrease marine phytoplankton (Roxy et al, 2016), and with it zooplankton biomass (Richardson, 2008).…”