Fig. 6. CTD-SRDL manufactured by the Sea Mammal Research Unit and Valeport, Ltd.

Fig. 7. Histograms of (a) the maximum pressures of all T-S profiles and (b) the distance between consecutive T-S profiles obtained from the southern elephant seals during the Southern Elephant Seals as Oceanographic Samplers (SEaOS) program.

Fig. 8. The number of profiles collected per degree latitude in the Southern Ocean. The green line represents the Southern Ocean Database, containing 10,513 profiles collected until 2002. The blue line represents 19,463 profiles collected by 547 floats starting in 2001, and the red line represents 22,230 profiles collected by 65 seals during the SEaOS program (2003-2006).

The Sea Mammal Research Unit CTD-SRDL (SI Fig. 6) was developed based on the SMRU standard Series 9000 SRDL, and includes a CTD sensor head developed and built by Valeport Ltd. jointly with SMRU. It includes a Keller PA-7 pressure transducer (accuracy, ±5 dBar) along with a custom-made temperature probe containing a platinum resistance temperature detector (resolution, ±0.001°C; accuracy, 0.01°C) and an inductive coil for measuring conductivity (resolution, ±0.003 mS/cm). The instrument also incorporates a Hitachi H8/3048 microprocessor, internal clock, a lithium ion D-cell battery, and a 401 MHz RF unit and antenna for data transfer via the Argos system. The entire unit is potted in polyurethane and epoxy resin, measures 105 ´70 ´ 40 mm, and is pressure rated to 2,000 dbar. Behavioral and physical data are initially collected at high frequency, but because of the limited bandwidth of the Argos satellite relay system, they must be compressed to allow transmission of a limited number of values that represent the most salient features describing diving behavior and hydrographic properties. General data collection, compression, and transmission strategies have been described by Fedak et al. (1, 2). Briefly, time-depth profiles for characterization of dive shapes were summarized by four inflection points where the time-depth trajectory changed most rapidly (i.e., local maxima of the second derivative of the time-depth function). Temperature and conductivity measurements initially collected at 2 Hz (along with their associated pressures) were summarized by 20 points selected using a hybrid method including 10 fixed pressures adaptively selected based on the maximum pressure of the dive, and augmented by 10 points selected by a broken-stick algorithm (3) to capture rapid changes in seawater properties between the fixed pressure points. The mapping scheme used for surface plots of dive depths and change in drift rate was based on an optimal interpolation method modified from Boehme and Send (4).

As stated in the main text, instruments lasted on average 160.9 ± 83.3 days, or ≈5-6 months. Elephant seals are among the most extreme breath-hold divers known, with dives frequently lasting 30-45 min and in extreme cases >2 h. The depth ranges covered allowed us to collect temperature and conductivity profiles throughout the upper 2,000 m of the water column. SI Fig. 7a demonstrates the fact that the majority of these profiles exceeded 500 m. Elephant seals are also extreme in that these deep and long dives are interspersed with extremely short surface periods, usually <4 min. Whereas the disadvantage of these short surface periods is the limited time for data transmission via Argos, the advantage is that it allows virtually continuous vertical sampling of the water column. The main limitation is set by the power consumption of the conductivity cell, and the resolution in time and space can be specified by the user, depending on a preference for overall instrument longevity or sampling resolution. The instruments in this study were programmed to provide approximately two to three profiles per day, and because of the migration patterns of elephant seals (rarely exceeding 150 km/day horizontal displacement), this gave us a very high spatial resolution (SI Fig. 7b). These features of the dataset make it a very useful complement to other autonomous sampling approaches, such as the Argos program, and is especially useful because it managed to obtain T-S profiles from the Southern Ocean where such data remain extremely sparse, particularly south of 60°S. SI Fig. 8 provides a comparison between Southern Elephant Seals as Oceanographic Samplers (SEaOS) data and other datasets currently freely available from the Southern Ocean. Temperature and salinity profiles collected during the SEaOS program are available through GDACS and the World Ocean Database.

1. Fedak MA, Lovell P, Grant SM (2001) Mar Mamm Sci 17:94-110.

2. Fedak M, Lovell P, McConnell B, Hunter C (2002) Integr Comp Biol 42:3-10.

3. Rual P (1996) in Summary of Ship-of-Opportunity Programmes and Technical Reports (Intergovernmental Oceanographic Commission of UNESCO and World Meteorological Organization, Paris), pp 142-152.

4. Boehme L, Send U (2005) Deep Sea Res Part II Top Stud Oceanogr 52:651-664.