Abstract
Manta rays forage for zooplankton in tropical and subtropical marine environments, which are generally nutrient-poor. Feeding often occurs at predictable locations where these large, mobile cartilaginous fishes congregate to exploit ephemeral productivity hotspots. Investigating the zooplankton dynamics that lead to such feeding aggregations remains a key question for understanding their movement ecology. The aim of this study is to investigate the feeding environment at the largest known aggregation for reef manta rays Mobula alfredi in the world. We sampled zooplankton throughout the tidal cycle, and recorded M. alfredi activity and behaviour, alongside environmental variables at Hanifaru Bay, Maldives. We constructed generalised linear models to investigate possible relationships between zooplankton dynamics, environmental parameters, and how they influenced M. alfredi abundance, behaviour, and foraging strategies. Zooplankton biomass changed rapidly throughout the tidal cycle, and M. alfredi feeding events were significantly related to high zooplankton biomass. Mobula alfredi switched from non-feeding to feeding behaviour at a prey density threshold of 53.7 mg dry mass m−3; more than double the calculated density estimates needed to theoretically meet their metabolic requirements. The highest numbers of M. alfredi observed in Hanifaru Bay corresponded to when they were engaged in feeding behaviour. The community composition of zooplankton was different when M. alfredi was feeding (dominated by copepods and crustaceans) compared to when present but not feeding (more gelatinous species present than in feeding samples). The dominant zooplankton species recorded was Undinula vulgaris. This is a large-bodied calanoid copepod species that blooms in oceanic waters, suggesting offshore influences at the site. Here, we have characterised aspects of the feeding environment for M. alfredi in Hanifaru Bay and identified some of the conditions that may result in large aggregations of this threatened planktivore, and this information can help inform management of this economically important marine protected area.
Highlights
Manta rays are large planktivores that inhabit tropical and subtropical waters globally, which are generally oligotrophic (Marshall, Compagno & Bennett, 2009)
Two studies have recently explored the diet of manta ray species based on stomach contents: one on oceanic manta rays Mobula birostris taken in a fishery in the Philippines (Rohner et al, 2017), and one on a historic stomach sample from a reef manta ray M. alfredi collected from eastern Australia (Bennett et al, 2017)
generalised linear models (GLMs) analyses showed that Zooplankton biomass in Hanifaru Bay was significantly related to Tide and Behaviour (Fig. 3)
Summary
Manta rays are large planktivores that inhabit tropical and subtropical waters globally, which are generally oligotrophic (Marshall, Compagno & Bennett, 2009). Similar to other tropical planktivores, such as leatherback turtles Dermochelys coriacea (Hays et al, 2006) and whale sharks Rhincodon typus (Rohner et al, 2015), manta rays aggregate where and when conditions result in elevated local productivity (Dewar et al, 2008; Anderson, Adam & Goes, 2011; Jaine et al, 2012) These productivity ‘hotspots’ are ephemeral in nature and often difficult for researchers to locate and characterise (Harris et al, 2020; Harris et al, 2021), which makes the direct study of planktivore feeding ecology challenging (Sims, 2008; Rohner et al, 2015). Biochemical analyses lack resolution, such as identifying and quantifying preferred prey species, whereas direct observation of animal feeding allows simultaneous sampling of the feeding environment (Sims & Merrett, 1997; Rohner et al, 2015; Fortune et al, 2020)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
