September 2020

We often want to know about the distribution of animals, how they are distributed on earth, what are the environmental factors affecting their distribution, and how human activity affects their habitat and population?

The habitat of Oceanic manta rays, Spinetail devil rays, and Sicklefin devil rays in the Lesser Sunda Seascape in Indonesia was successfully discovered by a group of mobulid scientists using an ecological niche modelling approach, which also evaluates how fisheries in this area are threatening mobulid habitats and populations. The results of our study were recently published in the journal Ocean and Coastal Management.

Multiple data sources of mobulid occurrence and environmental factors were combined to create an ecological niche model using maximum entropy. The maximum entropy model uses a machine-learning algorithm, which is part of the application of artificial intelligence that provides systems the ability to automatically learn the environmental signature of the examined animal and predict the areas that have similar suitable environment for the species. This approach allows us to understand key environmental variables driving mobulid ray distributions, generate spatial predictions of their likelihood distribution, and further can be used to identify their habitat overlaps with existing marine protected areas (MPAs) and fishing grounds in the lesser Sunda Seascape. This constitutes critical information for their management and conservation.

Spatial overlaps between potential distributions of mobulid ray species with past and present known commercial fishing areas interacting with mobulid ray species. (Putra, Mochamad Iqbal Herwata, et al. "Predicting mobulid ray distribution in coastal… — Spatial overlaps between potential distributions of mobulid ray species with past and present known commercial fishing areas interacting with mobulid ray species. (Putra, Mochamad Iqbal Herwata, et al. "Predicting mobulid ray distribution in coastal areas of Lesser Sunda Seascape: Implication for spatial and fisheries management." Ocean & Coastal Management 198 (2020): 105328)

These mobulids not randomly distributed in the coastal area of Lesser Sunda Seascape. We found that their distribution was highest close to the 200-m isobath. This preferred habitat also characterized by steeper sea-bottom slopes, which further support stronger upwelling intensity and in turn enhance the primary production, which drive the presence of mobulids' prey. Thus, coastal upwelling zones are a crucial feature determining mobulid ray habitats in this region.

In addition, because mobulid rays need adequate thresholds of prey density in oligotrophic waters, coastal zones with upwelling conditions in this area are likely to be crucial foraging grounds, as they contain dense euphausiid and crab patches, the primary food source of many mobulid rays.

Interestingly, an overlay of our model with reef manta movement based on satellite tracking and photo ID data in Komodo National Park and Nusa Penida, Bali suggests their movements spatially matched our predictive coastal habitat for oceanic manta rays. Taken together, this suggests that both manta ray species share similar niche in this region. Then, this approach may represent a useful tool to identify potential new aggregation sites and migration corridors between these areas.

Mobulids group in this region facing serious population decline as a result of fisheries activity that are targeting them as a primary target, as well as bycatch. As part of this area, Lamakera is known as the world 's largest mobulid fishing group, which for centuries has been practicing targeted mobulid fishing. Mobulid fishing has traditionally been carried out on small, non-motorized craft vessels using spears, mainly for the purpose of supplying meat for local subsistence. Under these conditions, fishing levels remained relatively low, due to limited capacity. However, the expansion and modernization of fishing fleets and the emergence of a globally commercialized industry led to a dramatic increase in mobulids fishing in Lamakera, which caused their population to decline by 75 percent. In addition, an increase in the gillnet fishing fleet in Lamakera has increased six times the catch of the devil ray. Gillnets, longlines, and purse seines are commonly used in small-scale artisanal fisheries, which often involve fleets using multiple gears in Indonesia. Drift gillnets in particular often capture mobulid rays since these are non-selective.

The Manta Caribbean Project and Océanos Vivientes A. C. teams collaborating on the tagging and biopsy work.

Unfortunately, this information only came from Lamakera. Whereas the whole impact of fishing pressures on mobulid ray populations in the wider region is still largely unknown due to limited monitoring in many small-scale fisheries, and the lack of species-specific data.

This study found that 52% of the predicted mobulid rays habitat overlap with fisheries activities which may represent crucial habitats for these species. However, only 31% of the habitats are exposed to mobulid fisheries within the MPA, which allow the MPA authority to develop mobulid fisheries management. While, most of the mobulids habitat that is exposed to fisheries is unprotected and unregulated.

Given the high number of blind spots that are currently unmonitored, fishing pressure on mobulid ray populations is likely greater than our current knowledge. This study suggests to increase the numbers of MPAs or strengthen the fisheries management actions in the areas with mobulid fisheries exposures, such as the coastal areas of south Flores.

Thus, we highlight a critical need for species-specific and population-level management measures for mobulids, whereas current MPA design has focused on a broad-scale coastal ecosystem management approach, which may have limited effectiveness in practice, especially managing highly mobile species such as mobulids.

The research was supported by Vulcan Philanthropy, MacArthur Foundation, Shark Conservation Fund, and WildAid Canada Society.