An Observational Assessment of the Feeding Ecology and Behaviours of Loggerhead Turtles Caretta caretta in Bateman Bay, Western Australia

Chris J O'Brien

Loggerhead turtles Caretta caretta are a globally distributed species of marine turtle. One of the largest populations of C. caretta occurs on the Western Australian coastline, however little is known about the in-water habits and ecology of this population, making effective management and protection of the species difficult. This study aimed to establish crucial baseline data and identify key aspects of the ecology of a population of C. caretta in Bateman Bay, Western Australia focusing on (i) seasonal habitat usage, (ii) seasonal feeding effort, and (iii) ecological relationships with other marine megafauna. It was hypothesised that feeding effort of the turtles would change between the end of April and the start of September of 2022 with seasonal changes in environmental conditions. Through three sampling periods in Mid-Autumn (MA), Mid-Winter (MW) and Late-Winter/Early Spring (LW/ES), a total of 29 loggerhead sightings were made (n = 10, n = 8 and n = 11 respectively) in the surveyed region north of Point Maud. A clear shift in habitat use was identified between MA and the latter sampling periods as loggerhead sightings shifted from a predominantly macroalgae habitat largely consisting of dense Sargassum spp. to a soft sediment/sandy habitat. This shift coincided with a higher incidence of green turtle Chelonia mydas sightings in the areas of higher macroalgae abundance and may be an indication of competition between the species for one or more resources. Despite this habitat shift, mean feeding and foraging effort did not fluctuate significantly between seasons. Observation tracks of loggerhead feeding were mapped and from the available data, at least three major feeding grounds can be identified over the sand flats along the Maud Sanctuary zone border. Mining behaviour was observed in recordings of four individual turtles and for all three sampling periods, a significantly higher mean number of feeding attempts was found in feeding areas where mining had occurred versus in feeding areas where it had not. It was further identified that all instances of mining in MW (n = 8) and LW/ES (n = 13) occurred in feeding depressions which were found within the three major feeding grounds identified and were likely made by benthic ray species over MW and LW/ES. The findings of this study have provided a baseline set of data for the feeding and behavioural ecology of C. caretta in the region and pose several avenues for further research into the species and this particular community’s in-water habits and the relationship between large benthic ray feeding grounds and links to the feeding grounds of C. caretta. There is also an avenue for citizen science through an identification catalogue of individual turtles in the region to establish more data on the population’s movements and habits.

An Observational Assessment of the Feeding Ecology and Behaviours of Loggerhead Turtles Caretta caretta in Bateman Bay, Western Australia

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