Abstract
Globally, reef manta rays (Mobula alfredi) are in decline and are particularly vulnerable to exploitation and disturbance at aggregation sites. Here, passive acoustic telemetry and a suite of advanced oceanographic technologies were used for the first time to investigate the fine‐scale (5‐min) influence of oceanographic drivers on the visitation patterns of 19 tagged M. alfredi to a feeding aggregation site at Egmont Atoll in the Chagos Archipelago. Boosted regression trees indicate that tag detection probability increased with the intrusion of cold‐water bores propagating up the atoll slope through the narrow lagoon inlet during flood tide, potentially transporting zooplankton from the thermocline. Tag detection probability also increased with warmer near‐surface temperature close to low tide, with near‐surface currents flowing offshore, and with high levels of backscatter (a proxy of zooplankton biomass). These combinations of processes support the proposition that zooplankton carried from the thermocline into the lagoon during the flood may be pumped back out through the narrow inlet during an ebb tide. These conditions provide temporally limited feeding opportunities for M. alfredi, which are tied on the tides. Results also provide some evidence of the presence of Langmuir Circulation, which transports and concentrates zooplankton, and may partly explain why M. alfredi occasionally remained at the feeding location for longer than that two hours. Identification of these correlations provides unique insight into the dynamic synthesis of fine‐scale oceanographic processes which are likely to influence the foraging ecology of M. alfredi at Egmont Atoll, and elsewhere throughout their range.
Highlights
Reef manta rays (Mobula alfredi) are large zooplanktivorous elasmobranchs of the family Mobulidae (Hosegood et al, 2020; Marshall et al, 2009; White et al, 2017)
Alfredi displayed similar residency at Egmont Atoll, which is in contrast to patterns observed in Seychelles, where the residency indices (RI) was lower for adults (Peel et al, 2019b), indicating that the M
Similar diel patterns have been recorded during studies of various M. alfredi subpopulations, where individuals frequented shallow coastal and island reef systems more often during daylight hours (Dewar et al, 2008; Jaine et al, 2012; Couturier et al, 2018; Setyawan et al, 2018; Peel et al, 2019b)
Summary
Reef manta rays (Mobula alfredi) are large zooplanktivorous elasmobranchs of the family Mobulidae (Hosegood et al, 2020; Marshall et al, 2009; White et al, 2017). Aggregation behaviour is characteristic of the species, whereby subpopulations will concentrate the majority of their activities at certain ‘hotspot’ locations (Couturier et al, 2018; Setyawan et al, 2018; Harris et al, 2020). These aggregations typically occur within particular discrete habitats (Stevens, 2016; Harris et al, 2020) such as cleaning stations (O’Shea et al, 2010), and locations where they engaged in social (Stevens, 2016; Perryman et al, 2019) or reproductive activities (Stevens et al, 2018). Large feeding aggregations 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)
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