The coded transmitters emit a train of “pings” at 69
kHz randomly every 40–114 seconds that contains a
specific ID code allowing users to identify individuals.
Most of the coded transmitters had a life span of 1429
days. The continuous transmitters emit a signal every
two seconds at a specific frequency ranging between
51–84 kHz and had a life span of 401 days. The continuous
transmitters were used to facilitate active tracking, while the coded transmitters provided depth information
and were detected by passive acoustic receivers
(listening at 69 kHz) deployed throughout Puget Sound.
1.2 Daily movement
Active tracking During the summer and autumn of
2006 and 2007, and a few other opportunistic times (Table
1), we used Vemco© VR100 and VR28 acoustic
receivers to collect GPS-based positions of tagged sixgills
while following individuals for 24-hour periods.
The VR100 hydrophone was deployed 1 m below the
water’s surface, while the VR28 hydrophone was towed
simultaneously behind a slow-moving 21’ boat using a
haired-fairing cable at a depth of ~5 meters. The detection
ability of each receiver varied with weather and site
conditions, but average max range of detection was 300
– 500 m (K. Andrews, unpublished data). Most tracking
events occurred in Elliott Bay or on the south end of
Bainbridge Island (Table 1; Fig. 1) in central Puget
Sound. We used data from individual paths of sharks in
analyses if we had a minimum of 5 hours of tracking in
both day and night hours, which resulted in using 21
tracks from 13 different sharks. Tracking efforts typically
began at ca. 0800 hrs. For each shark track, we reduced
the data to a single GPS-position every 30-minutes, thus
reducing autocorrelation of the data (Turchin, 1998) as
sharks were detected nearly every minute