SCOUT-Bathygraph tags (Wildlife Computers Inc., Redmond, WA) are data archiving tags that transmit data through the Argos satellite system. Unlike previous generations of tags (e.g., SPOT tags [Wildlife Computers Inc. Redmond]) that simply indicate an animal’s position through Argos Doppler estimation, the bathygraph tags acquire, archive and transmit depth and temperature data and have Fastloc® GPS capability. The latter is a geolocation estimation method that requires only a very brief exposure to take a “snapshot” of the visible GPS satellites. This snapshot is then used in post-processing to determine geographical position. In the case reported here, the tag was attached to a 2.9-m FL female tiger shark (Galeocerdo cuvier) caught and tagged on the windward coast of Oahu, Hawaii, on April 28, 2021, as part of an ongoing program of deploying bathygraph tags on tiger sharks in Hawaiian waters. The tag was attached using corrosible bolts passed through the dorsal fin .
The brevity of surfacing events and the limitations imposed by the comparatively low data throughput currently available via the Argos satellite system (~ 31 bytes/message, one message every 10–60 s and approximately only 20% satellite coverage time per day in Hawaii) require considerable onboard processing of the raw data and compromises must be made regarding the precision of the data that are uploaded and which types of ‘summary data’ are transmitted [5, 6]. Accordingly, even though the temperature and depth sensors have resolutions of 0.05 °C and 0.5 m, respectively, the transmitted resolutions are 0.1C and the depths are assigned to 8 m ‘bins’. Similarly, even though tag sensor data are sampled and stored once per second, for the purposes of satellite transmission, onboard processing performs an 11-point ‘broken stick’ analysis of the stored temperature/depth data (sensu [5, 6] 8]. Serendipitous recovery of a SCOUT Bathygraph tag (from a different shark) after a deployment of 6 months allowed post-deployment testing of the depth and temperature sensors. The results indicated that both sensors were still within manufacturer specification (Additional file 2: Table S1). Also, application of broken stick profiles to archived data from tags from the same manufacturer and using the same algorithm show that the technique accurately captures the raw temperature profiles (Additional file 1: Figure S1).
The tags have several user-definable functions. These include the maximum age of the temperature and depth profiles that will be transmitted (4 d in our case, after which they are removed from the queue), the minimum depth of dive required to trigger creation of a profile (in our case, 90 m) and the time elapsed between adjacent inflection points of the broken stick profile (see below) before a ‘discontinuity’ is declared (in our case, 1 h).
The tag generates broken stick profiles when the animal goes deeper than the user-defined threshold depth. Depth and temperature readings are sampled and stored at 1-s intervals. Once the animal reaches the surface, the tag creates an 11-point depth–temperature profile using a broken stick algorithm that captures temperature inflection points and includes the deepest and surface points. The profile is time-stamped with the time of the surface point. The tag also attempts to take a Fastloc® GPS snapshot after the profile is created. The Fastloc® snapshot is also time-stamped. The profiles and Fastloc® snapshots are stored as messages in a buffer for transmission, dropping out of the buffer once they reach the maximum age. During animal surfacing events, the tag cycles through the messages in the buffer and transmits them using an algorithm that prioritizes transmitting archived profiles with the fewest attempted transmissions—which typically means the profile that has just been created goes first. Depending upon the animal behavior, number of profiles collected and the Argos satellite coverage, some collected profiles and snapshots may not be received before they expire. Land-based receivers (Wildlife Computers Motes) tuned to the Argos frequency can significantly increase the number of received messages .
Because the SCOUT-Bathygraph tag is designed for non-airbreathers, an essential component of the onboard processing is the creation of the ‘virtual upcast’, whereby the temperature value for any given depth bin is refreshed (overwritten) every time the animal passes through that depth—either in ascent or descent. Thus, the values used to construct the ‘broken stick’ profile when the tag breaks the surface are the most recent temperatures associated with any given depth (Additional file 1: Figure S2). Because of data transmission constraints, time stamps are only available for the surface point (± 5 min) and the deepest point of each profile (± 15 min). Nevertheless, this allows for reasonable estimation of the time over which each profile is generated. Further, a ‘discontinuity flag’ is attached to a depth–temperature point if more than one hour elapsed between that point and the next shallower inflection point (Additional file 1: Figure S3). If there are no discontinuities in a transmitted broken stick curve, all depth–temperature points were collected less than an hour prior to the point immediately above it (i.e., shallower). It is possible that in the future these parameters could be adjusted to meet the specific needs of the end user. The profiles generated by the shark-borne tag were compared with profiles generated by the local Regional Ocean Model System which has been shown to effectively represent regional ocean structure  [see Additional Information]).