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Pop-up satellite archival tags (PSATs) are used to track movements of (usually large, migratory) marine animals. A PSAT (also commonly referred to as a PAT tag) is an archival tag (or data logger) that is equipped with a means to transmit the collected data via satellite. Though the data are physically stored on the tag, its major advantage is that it does not have to be physically retrieved like an archival tag for the data to be available making it a viable, fishery independent tool for animal behavior studies. They have been used to track movements of ocean sunfish, marlin, blue sharks, bluefin tuna, swordfish and sea turtles. Location, depth, temperature, and body movement data are used to answer questions about migratory patterns, seasonal feeding movements, daily habits, and survival after catch and release, for examples.
A satellite tag is generally constructed of several components: a data-logging section, a release section, a float, and an antenna. The release sections include an energetically popped off release section or a corrosive pin that is actively corroded on a preset date or after a specified period of time. Some disadvantages of using satellite tags are their depth limitations (2000m), their costs ($499–4000+), and their vulnerability to loss by environmental issues (biofouling) or ingestion by a predator.
For the satellite tags that function by measuring ambient light, they have a functional depth limitation of light penetration which can be as shallow as a few meters to upwards of 1000 meters. Additionally these tags cost between $4000+ per unit which is the main reason many studies employ a dozen or fewer of these tags. Geolocation estimates based on light are generally less reliable, not available in dark or turbid waters, and of no use during times of the equinoxes.
For the satellite tags that function by measuring ambient light and the Earth's magnetic field, they have a functional depth limitation equivalent of the maximum depth limitation, generally 2000m. These tags are generally used in much higher quantities due to their advanced geolocation capabilities and lower cost ($499–2500). These tags offer sensory functions not available in light only tags such as body movement parameters, higher precision depth sensing, and re-usable release sections. However, there is only one manufacture of these tags, Desert Star Systems.
Pop-up satellite tags range in length from about 125–215 mm (4.9–8.5 in) and weigh 36-108 grams in air. A tag must be small compared to the size of the animal, anywhere from 3-5% of the total fish weight, so that it does not interfere with normal behavior. Smaller versions of about half that length and weight have been recently 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.
These tags record such information as temperature, magnetics, acceleration, light level, and pressure at set intervals of a few seconds to several hours. Data are often collected for several weeks or months but with new advances in memory technology microSD cards tags can store data 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 of the Argos satellites at a frequency of 401.65 MHz. Therefore, the tag 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.
Previously the costs of these tags $4000 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 and average reporting rate of 80% 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 capabilities, tag equipment costs have significantly dropped to $499 per tag.
One common method of determining an animal's location underwater requires the tag to acquire light levels throughout the day. Looking at the length of the day, from when the tag observed the first light until the last light, the tag can determine its latitudinal location (with accuracy exceeding 1 degree or 100 nautical miles). From the length of day the tag computes the noon time which is converted to a longitude location (with accuracy averaging about 0.5 degree or 30–50 nautical miles). This method of geolocation is suitable for animals that inhabit clear waters near the surface. At depths or in turbid waters, light based geolocation does not work. It also does not work during the equinoxes when the length of day is globally uniform. Manufacturers of this technology include Wildlife Computers, Microwave Telemetry, and Lotek Wireless.
Another approach to geolocation couples light and magnetics, using the Earth's magnetic fields for latitude estimations while still using noon time detection for longitude. These tags, SeaTag, measure the Earth's magnetic field on their built-in magnetometers during the day and averages the field value for a daily location. In addition, they record data during the day to get movements within the day as well as the average location (accuracy in the range of 10–30 nautical miles, both latitude and longitude). Manufacturers of this technology include Desert Star Systems.
The major difference between the two methods of geolocation is one relies on light and requires light penetration to function; whereas, magnetic based geolocation does not require light to function. However, magnetic based geolocation without light can only provide latitude movement estimations.
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