Manta rays range among the largest planktivorous fishes and are classified as Vulnerable to Extinction on the IUCN Red List of Threatened Species. Their gentle disposition has made them a major tourism asset in many parts of the world. However, despite the increased public and scientific interest in manta rays, major knowledge gaps remain that currently hamper our understanding of their general ecology. Little is known about their movements, habitat selection process, dispersal abilities and the drivers for their distributions. Addressing such knowledge gaps is becoming urgent as sub-populations face increasing fishing pressure in several regions of the world, including Australia’s nearest neighbour, Indonesia.
This thesis presents the first detailed examination of the movement ecology of reef manta rays Manta alfredi. Using a combination of satellite tracking technology, Geographical Information Systems (GIS), biophysical oceanography and ecological modelling, I investigated several aspects of the movement ecology of a population of reef manta rays in eastern Australia.
A comparative synthesis of the movement ecology of large plankton-feeding elasmobranchs, including whale sharks (Rhincodon typus), basking sharks (Cetorhinus maximus), megamouth sharks (Megachasma pelagios), manta rays (Manta spp.) and mobula rays (Mobula spp.), was conducted in terms of their life history, distribution, aggregative behaviour, movement patterns and underlying reasons, and bioenergetics. A meta-analysis of the peer-reviewed literature indicated an exponential increase in the number of published studies on the movements of these planktivores over the past decade, highlighting their great movement potential, and showing that movements recorded by these species are primarily related to the search for food. Future research directions were identified, such as clarifying the links between movements and the surrounding biophysical environment, that should help gain further insights into the movement ecology of Earth’s largest fishes and refine conservation strategies.
An examination of multi-year observational records from a popular manta ray aggregation site, Lady Elliot Island (LEI) in the southern Great Barrier Reef (GBR), led to the identification of a set of environmental parameters as key influences on the occurrence and relative abundance of individuals visiting the site. Chlorophyll-a concentration, sea temperature, wind speed and tidal status were found the main environmental drivers, with a distinct peak in manta ray numbers at the site during austral autumn and winter. A spatial analysis of behavioural observations highlighted several sites around the LEI reef as ‘multi-purpose’ areas where cleaning and foraging activities commonly occur, while the southern end of the reef is primarily a foraging area. The regular supply of plankton enriched water to the reef appears to be related to the dynamics of the mesoscale Capricorn Eddy forming off the continental shelf edge, just off LEI.
Satellite-tracking investigations provided the first evidence that reef manta rays commonly use offshore waters to exploit productive oceanographic features or travel long distances. Pop-up archival satellite transmitting (PSAT) tags were attached to ten reef manta rays to examine their movement patterns, dispersal abilities and potential migratory movements after they departed LEI. Tagged individuals undertook extensive movements, travelling up to 2,440.6 km in 118 days and dispersing as far 520 km from the tagging site. Random walk simulations indicated that tracked manta rays spent a significantly higher amount of time in the Capricorn Eddy than models. Further state-space analysis of the tracking data suggested that the region where the cyclonic Capricorn Eddy forms is an important offshore foraging ground for reef manta rays in eastern Australia. One tagged ray was also observed to move to waters near another known aggregation site (North Stradbroke Island) and back in 120 days, using offshore waters to travel up and down the coast.
Physical retrieval of six PSAT tags allowed for a detailed examination of the collected high resolution depth data, to gain insight into vertical movement patterns and habitat for the species. Reconstruction of individual dive profiles revealed that tagged reef manta rays spent most time within the first 75 m of the water column, with occasional deep dives down to 294.5 m. During dives, ascents were typically faster than descents. A Fast Fourier Transform analysis highlighted diel and circatidal periodicities in the depth data, likely related to zooplankton dynamics. Individuals exhibited different types of dives ranging from repeated small-scale vertical oscillations through to U-shaped dives marked by long periods spent at near constant depth, and deeper, V-shaped dives. High-resolution temperature data recorded by the tags were compared to sea temperature output from the BlueLink Ocean Model, Analysis and Prediction System (OceanMAPS) to examine properties of the waters traversed by tagged rays and gain further insights into their vertical habitat use.
By examining the largest horizontal and vertical movement datasets ever analysed for M. alfredi, this thesis has improved our understanding of the movement ecology and behavioural decision process in manta rays. The tracking data presented has demonstrated that reef manta rays are highly mobile marine vertebrates that commonly exploit productive shelf-edge and offshore environments. Upwelling processes and subsequent enhanced food availability appear as a key drivers for the distributions of reef manta rays along the east Australian seaboard, with sea temperatures, wind stress, lunar and tidal cycles some important additional influences. One individual is recorded to move between two popular aggregation sites off the east Australian seaboard using offshore, deep waters, and venturing as far as 155 km off the shelf. Although threats to manta rays are comparatively low in Australia, the distances recorded in this study suggest that some degree of exchange between distinct sub-populations throughout the Coral Triangle is possible. There is a clear need to consider the movement ecology of manta rays when implementing regional management strategies, as their dispersal abilities could lead them to face increased threats in countries where unsustainable targeted fisheries occur.