Skimming the surface on plastic ingestion: a preliminarily assessment of plastics in feeding grounds of the reef manta ray Mobula alfredi along the north west coast of Australia

In the past decade, plastic pollution has been increasingly recognised as a threat to marine ecosystems and the species inhabiting them. As plastic-focussed studies increase there is a general lack of information of the direct impact of plastics on larger filter-feeding megafauna species, particularly within Australian waters, where plastics are relatively understudied. The waters of the Ningaloo Marine Park and Exmouth Gulf, situated along the North West Cape of Australia, support biologically diverse ecosystems frequented by charismatic filter-feeding megafauna. Reef manta rays (Mobula alfredi) using these waters as feeding areas may be exposed to both micro and macro plastics in the marine environment due to their broad range of feeding behaviours. The presence and concentration of micro and macro plastics in surface waters and sediment were assessed at six study sites across two separate locations (Bateman Bay and Exmouth Gulf) during April, June and August 2019 using surface net tows (n = 102), sediment samples (n = 33) and in-water tows from within the proximate feeding trail of manta rays (n = 11). Generalised linear models were constructed to identify spatial and temporal variations in presence and concentration of plastics, while the mean concentration of surface waters was used to estimate theoretical plastic ingestion rates (IRs). Plastics were present in 92.3 % and 45.0 % of surface water tows and feeding tows respectively, and 81.8 % of sediment samples. The concentration of plastics (Cp) in surface waters were significantly higher in Bateman Bay (mean Cp 0.17 pieces m-3), compared to Exmouth Gulf (mean Cp 0.07 pieces m-3) and varied between sampling months at both locations. Microplastics (<5 mm), and line fibres dominated the type of plastic pieces found in all samples, which may be ingested by manta rays due to their inability to exclude particles and filter feeding mechanisms. Calculated IRs suggested manta rays could be consuming 17.12 pieces hr-1, and 2.02 pieces hr-1 in Bateman Bay and Exmouth Gulf, respectively. This was comparatively lower than rates that have been previously estimated for manta rays utilizing feeding sites in Indonesia. The need for a better understanding on the fine-scale movement ecology of manta rays and localised oceanographic conditions are highlighted here, where many knowledge gaps still exist.