Deep-water feeding and behavioral plasticity in Manta birostris revealed by archival tags and submersible observations

Abstract

Foraging drives many fundamental aspects of ecology, and an understanding of foraging behavior aids in the conservation of threatened species by identifying critical habitats and spatial patterns relevant to management. The world's largest ray, the oceanic manta (Manta birostris) is poorly studied and threatened globally by targeted fisheries and incidental capture. Very little information is available on the natural history, ecology and behavior of the species, complicating management efforts. This study provides the first data on the diving behavior of the species based on data returned from six tagged individuals, and an opportunistic observation from a submersible of a manta foraging at depth. Pop-off archival satellite tags deployed on mantas at the Revillagigedo Archipelago, Mexico recorded seasonal shifts in diving behavior, likely related to changes in the location and availability of zooplankton prey. Across seasons, mantas spent a large proportion of their time centered around the upper limit of the thermocline, where zooplankton often aggregate. Tag data reveal a gradual activity shift from surface waters to 100-150m across the tagging period, possibly indicating a change in foraging behavior from targeting surface-associated zooplankton to vertical migrators. The depth ranges accessed by mantas in this study carry variable bycatch risks from different fishing gear types. Consequently, region-specific data on diving behavior can help inform local management strategies that reduce or mitigate bycatch of this vulnerable species.

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…Moreover, their low annual reproductive output and conservative demographic characteristics make them highly susceptible to fisheries-induced population declines (Dulvy et al 2014, Pardo et al 2016, even when catch rates are low. Understanding horizontal and vertical habitat use, which are likely driven by foraging in many cases, can help determine when mobulid rays are most vulnerable to incidental capture in fisheries and can aid in the development of bycatch mitigation measures (Stewart et al 2016b).…”

“…Several studies have examined the horizontal and vertical movements of mobulid rays using satellite telemetry (Canese et al 2011, Croll et al 2012, Braun et al 2014, Jaine et al 2014, Thorrold et al 2014, Stewart et al 2016b, and others have used direct observations to identify feeding patterns and prey sources (Notarbartolo di Sciara 1988). The results of these studies demonstrate differences in habitat use (e.g.…”

“…Croll et al 2012, Thorrold et al 2014, vertical movements (e.g. Canese et al 2011, Croll et al 2012, Braun et al 2014, Jaine et al 2014, Thorrold et al 2014, Stewart et al 2016b, and in some cases prey sources between mobulid species (Notarbartolo di Sciara 1988). However, few studies have examined the feeding ecology of multiple mobulid species in a given region or within a single species at multiple locations (see Notarbartolo di Sciara 1988, Sampson et al 2010.…”

“…Anderson et al 2011) and occasional opportunistic observations (e.g. Stewart et al 2016b), direct observations of feeding behavior in mobulids are rare and largely restricted to surface feeding and daylight hours, complicating efforts to characterize the trophic ecology of this family. These characteristics make stable isotope analysis a powerful tool for studying the trophic ecology of mobulid rays.…”

“…M. japanica and closely related Mobula mobular appear to spend the majority of their time in near-surface habitats, shallower than 50 m depth (Canese et al 2011, Croll et al 2012, Francis & Jones 2017. M. birostris makes deeper foraging dives and appears to spend substantial amounts of time below 100 m (Stewart et al 2016b). Mobula tarapacana routinely undertakes deep dives below 800 m -in some cases over 1800 m -for periods of 1 to 4 h, presumably to access bathypelagic scattering layers of fishes (Thorrold et al 2014).…”