CTD SRDL tags are small computers designed and built by Sea Mammal Research Unit (SMRU), St Andrews, Scotland with sensors which allow them to take measurements of key environmental parameters. They have memory in which they store data, a transmitter, a clock, and batteries, all moulded into a resin block. Tags are fixed harmlessly to seals' fur, typically for periods of several months. The tags are not buoyant, and are lost when they drop off. The satellite tags are 10.5x7x4 cm, weigh 545g, 254 mm3 and last for up to a year.
They do five key things:
- Take measurements of key environmental parameters: time of measurement, status (wet or dry), water conductivity, water temperature, depth.
- Analyse measurements to create profiles of seal behaviour, and ocean temperature and salinity changes with depth. These profiles are used to decide what information to store in the tag's journal.
- Store journal data until transmission opportunities occur.
- Transmit stored data to the Advanced Research and Global Observation Satellite (ARGOS) system.
- Monitor and manage the tag's own limited resources of power and memory.
Valeport CTD sensor head:
- Temperature: range -5 º to 35º C, accuracy +/- 0.005 º C, resolution 0.001 º C
- Conductivity: range 0 to 80mS/cm, accuracy +/- 0.01mS/cm, resolution 0.002mS/cm
- Pressure: range 0 to 2000dBar, accuracy 2dBar, +/-(0.3 + 0.035%*reading)/ºK, resolution 0.05dBar
Archival Global Location Sensing (GLS) tags are small computer data loggers that can record and store information on date, time, swim depth, water temperature, body temperature and light levels. The tags provide delayed mode data as they must be recovered before the data can be downloaded. The trackers are typically leg-mounted on the snow petrels and the shearwaters. MK-19 and MK3005 GLS tags (Biotrack Ltd., Wareham, UK) are used in short-tailed shearwaters and MK14 in snow petrels as they are well suited for tracking small Procellariiform seabirds due to their miniature size and inexpensive cost. Prior to the pre-laying exodus a threshold weight of 540 g was used when choosing individuals for tag deployment to ensure that no birds in poor condition were included in the sample. Individual tags were placed in an open space for three days at the deployment location to produce a sequence of light recordings at a known location from which solar elevation estimates could be calculated. After retrieval, the raw archived tag data was downloaded, and adjustments for internal clock drift were made. Wildlife Computer SPLASH tags were used in Emperor penguins, this tags transmit to the Argos system and are used to get both horizontal movement and additional information such as vertical behavior (depth).
Current archival tags can store data from up to four sensors (e.g. internal and external temperature, pressure (depth) and light intensity) taken at one minute intervals over one year. Battery power and memory storage are limiting factors. This type of logger can run for up to eight to ten years depending on battery power and memory storage.
Light levels are used to calculate the time of local dawn and dusk and hence day length. This information can be used to estimate an approximate daily position of the animal using standard astronomical formulas and comparing the offset of local dawn/dusk to Greenwich meantime.
No new deployments have been done since the end of 2014.