Table 5 Dive records

Dive records resulting from most telemetry tags using the CLS-Argos system will almost always be incomplete, i.e. have some missing dives, locations and CTD upcasts. This is exacerbated by the lack of acknowledgement when transmitted information is received, or “handshake” between the platform and the satellite. This lack of handshake creates the need for a stochastic transmission strategy of the information accumulated in the transmission buffer.

The proportion of dive data that is lost will depend on a number of factors, including the geographic region in which the instrumented animal operates (e.g. polar regions have better coverage by Argos satellites than equatorial regions because they are polar orbiting), the animal’s diving ecology, how long the animal spends at the surface where transmission is possible, and the frequency with which that transmission window occurs.

For elephant seals, there are two main reasons why data are lost. Firstly, as for all species, there will be periods during deployment when conditions for satellite transmission are poor, and secondly, there may be periods when data accumulate at a higher rate than the tag can transmit them, for example periods of high diving activity that exceed the size (in terms of time) of the transmission buffer. Southern elephant seals are a species that spend very little time at the surface (mean surface time 2.18 min ± 2.74 standard deviation, range 0.05-9.48 min, n = 187 832 dives, SMRU-IG unpublished data), providing a narrow window for transmission. The consequences of elephant seal surfacing behaviour on the number of messages sent and the quality of the locations are shown in Fig. 3 of [22]. The transmission buffer in CTD-SRDLs designed for deployment on southern elephant seals is made large enough to hold many days worth of dives, to minimize the proportion of information lost during low Argos coverage, and to avoid data return being biased in favour of data collected during high Argos coverage (Table 2). In addition, as mentioned above, stored data are selected randomly from the transmission buffer. Information remains in the transmission buffer either until its “sell-by” date expires or it becomes displaced by newer information being added to the buffer.