Occupancy of bottlenose dolphins (Tursiops aduncus) in relation to vessel traffic, dredging, and environmental variables within a highly urbanised estuary
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Coastal areas, and thus coastal species, are at increasing risk from human activities. Sections of the coastline of Western Australia are undergoing intense coastal development to fulfil commercial, industrial, and recreational requirements. Multiple populations of bottlenose dolphins (Tursiops aduncus) occur around this coastline; however, small community sizes and limited genetic exchange rates make them susceptible to anthropogenic pressure. This study investigated the occupancy of dolphins within the Swan–Canning Rivers, an urbanised estuary, with regard to (1) presence/absence, (2) abundance, and (3) duration in terms of time spent in the area. These response variables were related back to environmental conditions (tidal state, tidal height, salinity, temperature), vessel traffic, and dredging activities using generalised additive modelling. Theodolite tracking data revealed high levels of boat traffic at the two sites considered; however, dolphin occurrence was only negatively affected by vessel density at one of these sites. Dolphin occupancy was also significantly influenced by temperature, with possible seasonal effects. No dolphins were sighted on days when backhoe dredging was present; however, low sample sizes limited statistical interpretation. These results highlight the need to consider context in behavioural response studies, in terms of habitat type studied, explanatory variables considered, and response variables selected.
KeywordsAnimal occupancy Theodolite tracking Anthropogenic activities GAMs
The authors would like to thank the Department of Spatial Sciences at Curtin University for generous use of their theodolite, and Eric Kniest (University of Newcastle) for support during theodolite and Vadar software set-up. Fremantle Council and the Botanic Gardens and Parks Authority (BGPA) of Kings Park kindly allowed access to their sites for data collection, with Jake Tanner going above-and-beyond to provide assistance in accessing Cantonment Hill. This study had extraordinary support from over 60 volunteer observers, without whom data collection would not have been possible. Of these, special thanks go to Phillippa Adamson, Mariana Barbosa, Jordan Claytor, Ana Costa, Shannon Dee, Caroline Delaisse, David Grose, Robin Hare, Adam Koziol, Jacob Noonan, Tamar Orly, Lucy Rudd, Charlotte Tecknenoff, Alessandra Wasserman, and Katie Werndly. Thanks also go to Angela Recalde Salas and Phil Bouchet for assistance in field site set-up and helpful statistical discussions. Finally, we would like to thank the Swan River Trust for continuing support of dolphin research in the Swan-Canning Rivers.
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