Pop-up satellite archival tag

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SeaTag-MOD Pop-Up Satellite Tags ready for deployment
Thomas Gray, Desert Star Systems, is holding a SeaTag-MOD pop-up satellite tag which is ready for deployment on a whale shark (Isla Mujeres, MX).

Pop-up satellite archival tags (PSATs) are used to track movements of (usually large, migratory) marine animals. A PSAT is an archival tag (or data logger) that is equipped with a means to transmit the data via satellite. Though the data are physically stored on the PSAT, its major advantage is that it does not have to be physically retrieved like an archival tag for the data to be available. They have been used to track movements of ocean sunfish,[1] marlin, blue sharks, bluefin tuna, swordfish and sea turtles. Location, depth, and temperature data are used to answer questions about migratory patterns, seasonal feeding movements, daily habits, and survival after catch and release, for examples.[2]

A PSAT is generally constructed of several components: a data-logging section, a release section, a float, and an antenna. The release sections includes an energetically popped off release section or a pin that is actively corroded on a preset date or after a specified period of time. Some disadvantages of using PSAT are their depth limitations (2000m), their costs ($1500–4000), and their vulnerability to loss by environmental issues (biofouling) or ingestion by a predator. For the PSAT that functions by measuring ambient light, they have a functional depth limitation of about 1000 meters. Additionally these tags cost between $3500–4000 per unit is the main reason many studies employ a dozen or fewer PSATs. However, newer tag technologies employed in the SeaTag product line (SeaTag-MOD, etc.) use Earth's magnetic field to replacement light measurements for latitude estimations. Therefore, depth is no longer a cause for location uncertainty. Additionally these newer tags cost about half that of existing tagging technologies, on the order of $1500–2200 per PSAT.

General information[edit]

PSATs are usually used on large marine animals and have been used in several studies of the bluefin tuna.
Deployment of a PSAT on a bluefin tuna.

PSATS range in length from about 125–175 mm (4.9–6.9 in) and weigh 65–75 grams in air. A tag must be small compared to the size of the animal so that it does not interfere with normal behavior. Smaller versions of about half that length and weight have been recently[when?] introduced and are more suitable for use on smaller species or younger animals. Furthermore, the SeaTag-GEO/PSAT is the lightest PSAT manufactured at under 30 grams including its float, release section, and antenna.[citation needed]

PSATS record such information as temperature, magnetics, acceleration, light level, and pressure at set intervals of a few seconds to several hours.[3] Data are often collected for several weeks, and memory capacity may allow data to be taken for up to a year with older tags. Newer tags that use microSD as memory storage allow data to be collected and stored for centuries. PSATs record data in non-volatile memory so that data are retained even if battery or solar power runs out.

When the PSAT releases from the animal on which it was attached, it floats to the surface, and begins to transmit data to one of the manufacturer’s satellites at a frequency of about 400 MHz. Therefore, the PSAT does not have to be physically recovered for the data to be obtained. Depending on the number and coverage of the satellites, it can take 7 to 10 days or longer for the data to be completely transmitted.

Major disadvantages of the PSAT are that it is subject to loss by malfunction of any key component, environmental effects such as biofouling, ingestion by a predator, its depth limitation and cost. Most PSATS have internal software designed to detect damaging or sub-optimal conditions that will trigger an early release and transmission of data. For example, PSATs can withstand pressures to depths of 2,000 to 2,500 metres (6,600 to 8,200 ft) depending on the model. If data indicate no change in pressure (depth) for a period of time, this could indicate an early release due to malfunction or death of the animal to which it was attached. Such internal checks can alert researchers to unexpected or undesirable events. Ingestion by a predator is more difficult to detect; loss of ambient light might trigger release, but the device may not be free to float to the surface as designed.[4]

Previously the cost of $3500–$4000 per PSAT was the main impediment to researchers using more than a dozen units during a project. With small sample sizes, the loss of a single tag can significantly affect the results. Also, there is greater risk that the animals tagged could display unusual behavior or only one of several types of behavior. Thus an observer could come to an incorrect or incomplete generalization about the entire species. However, with recent technologies and manufacturing companies, the PSAT costs have significantly dropped to nearly $1500 per PSAT.

Types[edit]

Using light level[edit]

The SeaTag GEO is available in data logger or PSAT form. It uses the earth's magnetic fields to compute latitude and noontime estimates for longitude.

The most common type of PSAT is a tag that is battery powered and uses a galvanic release. This type of tags relies on light levels to determine latitude and longitude. These tags also record temperature and depth. The temperature can also be coupled with the light level readings to improve accuracy in depth and location through different software packages available to researchers. The tags are attached to the animal in various ways including adhesives, and are released to "pop up" and begin transmitting data via the galvanic release. Manufacturers of this type include Wildlife Computers, Microwave Telemetry, and soon-to-be Lotek Wireless.

Using Earth’s magnetic field and light level[edit]

The alternative approach to light level measurements is to use the earth’s magnetic fields for latitude. These tags still use noon time estimates based on light measurements for longitude. Desert Star Systems LLC takes this approach with their PSAT tag. The tag acquires magnetic values using an on-board magnetometer (for latitude) and uses a solar cell to acquire light (for longitude). The advantage is two-fold. First the solar cell acts as a charging mechanism for a capacitor. Therefore the tag itself is not battery powered, and can store and transmit more data (in theory centuries of data when coupled with the tag's internal microSD card). The tag's stored solar power system utilizes new capacitor technologies allowing it to power its internal components for up to two weeks of total darkness; the tag recharges when it is at two times water visibility depth. Location estimates using this tag are more accurate than those using only light, especially in areas where it is difficult to determine length of day (North and South Poles) and during equinoxes. Additionally, this tag uses an energetic release rather than a galvanic release; this different release design allows a greater breaking strength to reduce the risk of accidental release. Finally, the per tag costs are half that of the other companies using light level technologies allowing for significantly increased sample sizes.

See also[edit]

References[edit]

  1. ^ Thys, Tierney (2003-11-30). "Tracking Ocean Sunfish, Mola mola with Pop-Up Satellite Archival Tags in California Waters". OceanSunfish.org. Retrieved 2007-06-14. 
  2. ^ Some examples include: Block, B, Dewar, H, Farwell, C, Prince, ED. A new satellite technology for tracking the movements of Atlantic bluefin tuna. Proc. Natl. Acad. Sci. USA Vol. 95, pp. 9384–9389, August 1998; Hoolihan, John P. Horizontal and vertical movements of sailfish (Istiophorus platypterus) in the Persian Gulf, determined by ultrasonic and pop-up satellite tagging. Marine Biology (2005) 146: 1015–1029 DOI 10.1007/s00227-004-1488-2;Stokesbury, MJW, Harvey-Clark, C, Gallant, J, Block, BA, Myres, RA. Movement and environmental preferences of Greenland sharks (Somniosus microcephalus) electronically tagged in the St. Lawrence Estuary, Canada. Marine Biology (2005) 148: 159–165 DOI 10.1007/s00227-005-0061-y
  3. ^ Block, Ba, Dewar, H, Farwell, C, Prince, ED. A new satellite technology for tracking the movements of Atlantic bluefin tuna. Proc. Natl. Acad. Sci. USA Vol. 95, pp. 9384–9389, August 1998; Nielson, A, Bigelow, KA, Musyl, MK, Sibert, JR. Improving light-based geolocation by including sea surface temperature. Fish. Oceanogr. 15:4, 314–325, 2006.
  4. ^ Kerstetter, DW, Polovina, JJ, Graves JE. Evidence of shark predadation and scavenging on fishes equipped with pop-up satellite archival tags. Fish. Bull. 102:750–756 (2004).

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