Electronic supplementary material
Quantifying movement patterns for shark conservation at remote coral atolls in the Indian Ocean
Receiver range testing and detection efficiency
To determine the maximum detection range of our receivers, we tested each receiver after deployment by producing transmissions using VEMCO V13-1H transmitters suspended from an inflatable rubber boat at known locations following the technique indicated by VEMCO (VEMCO 2010). To provide a known transmission source we attached three spare tags to a weighted rope that could be hung in mid-water from the boat. These tags were transmitting at the same frequency as the tags deployed on the sharks. To minimise transmission overlap between tags, we turned each one on one minute apart. The boat was then moved to a location away from the receiver location, tags placed in the water and outboard engine turned off. The boat was then allowed to drift toward or away from the receiver. The track of the boat was recorded every 5 seconds using a Garmin eTrex GPS device (Garmin, Southampton, Hampshire, UK). After a period of drifting to either within 50 or 750 m away from the receiver, the boat was moved to another location. This was repeated at least twice for each receiver. Once the receivers were retrieved at the end of the study, the detection data were downloaded and compared with the boat’s track. Range testing data were recorded by all receivers except by the VR2w receiver at location I1. The location of each transmission was estimated from the boat’s GPS track and the distance from each location to the receiver was calculated. The maximum distances at which transmissions were detected for each receiver are presented in ESM Table S1.
In the initial phases of data processing we worked with colleagues at VEMCO to check and confirm the validity of our data. From determining the ratio of the number of rejections versus the number of pings (also described as the rejection coefficient by Simpfendorfer et al. 2008) and the number of rejections versus the number of detections, it was clear that there were few ID collisions and false positives. Both metrics describe the number detections likely to be incorrect and both were low, although there was a slight increase with time. To date only one study (Simpfendorfer et al. 2008) in a riverine setting has examined the effect of receiver performance and comparison with our data shows similar code detection efficiency and lower rejection coefficients in our study (Table S2).
Our tags were transmitting around every 180 seconds. The total number of pings that could be detected by the receivers per day exceeds 300000 (VEMCO 2010) and our maximum number of pings received was 5000. In our study, detections were well below (highest mean number of detections = 8 hour-1 and maximum = 35 hour-1). The maximum ratio of the number of rejects/pings was 0.4 %, with a median value = ~ 0.1 % and the rejections/detections was generally < 5 %. Given the number of detections per hour never exceeded 35 (possible theoretical maximum with our transmission rate = 250), it is highly unlikely that collisions had influenced our data.
References
Simpfendorfer CA, Heupel MR, Collins AB (2008) Variation in the performance of acoustic receivers and its implications for positioning algorithms in a riverine setting. Can J Fish Aquat Sci 65:482-492
VEMCO (2010) VR2 Receiver user manual. AMIRIX systems Inc, Halifax, Nova Scotia, Canada. 20 p
ESM Figure Captions
ESM Fig. S1. The location of the Rowley Shoals of the northwest coast of Australia.
ESM Fig. S2. The size distribution of the tagged sharks at Imperieuse and Clerke Reefs. Grey and black bars indicate the number of male and female sharks, respectively, within each size bin.
ESM Fig. S3. Relationship between maximum period of detection and shark total length, at Imperieuse (o) and Clerke Reefs (×).
ESM Fig. S4. Examples of dominant peaks observed from the spectral analysis of long-term detection data for two grey reef sharks from the Rowley Shoals. Note different log10 scales on the y-axis and the relative strength of the 24- and 12-hour peaks, and minor peak around 8 hours.
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