A TDR was attached to the belt to position it as close to the dugong as possible. The length of the tether and the buoyancy of the cylinder allowed the satellite antenna to be exposed above the selleck compound water when a dugong was in water <3 m or swimming near the surface very slowly, thereby maximizing uplink opportunities. The whole tracking apparatus was retrieved as described in Sheppard et al. (2006). Data from TDRs were decoded using software provided by the manufacturer.
We used custom software to preprocess the data by identifying the level of the water surface (zero-offset) and removing dugong spikes, i.e., biologically implausible rapid changes in depth (Appendix S1A, Hagihara et al. 2011). Dive data collected within 5 min of a GPS and QFP fix were then subsampled (10 min in total, 5 min before and Selleckchem 3-Methyladenine after each fix, Appendix S1B). To avoid any potential postrelease behavioral responses, we only used data recorded ≥3 d after the day of tag deployment. However, no apparent changes in diving patterns were observed in the
3 d after deployment; capture and handling did not appear to trigger a flight response and dugongs stayed in the vicinity of the capture area (Sheppard et al. 2006; RH, unpublished data). Bathymetric models and tidal records (Maritime Safety Queensland, Department of Transport and Main Roads) were used to estimate the water depth at the time and geographic location for each fix. The bathymetric models were of 100 m resolution and generated by Sheppard (2008) in Hervey Bay and by Beaman (2010) in Moreton Bay. The depth at the location of each fix was identified by importing the bathymetric models and location fixes into ArcGIS 9.3.1 (Environmental MCE公司 Systems Research Institute 2009). Tidal heights were added or subtracted to the depth on the bathymetric charts to calculate the water depth experienced by the dugong at the time of each fix. Tidal range in Hervey Bay was ca. 4 m and in Moreton Bay 2.6 m during the deployment periods. We assumed
that estimated water depths remained constant for the 10 min around each fix. Previous experiments using dugong replicas found that the availability of dugongs varies with levels of turbidity and sea state (Pollock et al. 2006). Following Pollock et al. (2006), we examined the proportion of time dugongs spent in two detection zones: 0–1.5 m of the surface for turbid water and Beaufort sea state 3 (rougher conditions with very few whitecaps); and 0–2.5 m of the surface for clear water and sea state ≤2 (calm conditions with no whitecaps). We assigned “1” when a depth measurement was recorded within each of the detection zones and “0” when a depth measurement was recorded outside of the detection zone. The proportion of time dugongs spent in each detection zone was calculated by the sum of these numbers divided by the number of depth records.