Pop-up satellite archival tag
Pop-up satellite archival tags are used to track movements of marine animals. A PSAT is an archival tag that is equipped with a means to transmit the collected data via the Argos satellite system. 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 and migration studies. They have been used to track movements of ocean sunfish, marlin, blue sharks, bluefin tuna, swordfish and sea turtles to name a few species. Location, depth, temperature, oxygen levels, 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 limitations of using satellite tags are their depth limitations, their costs, their vulnerability to loss by environmental issues, or premature release through ingestion by a predator.
There are two methods of underwater geolocation that PSATs employ. The first method is through light based geolocation which uses the length of the day and a noon time calculation to estimate the tags location while underwater. This method has a functional depth limitation of light penetration which can be as shallow as a few meters to upwards of hundreds of meters. Geolocation estimates based on light are usually coupled with additional satellite data like sea surface temperature or other available data input such as bathymetry, land avoidance, and physical limitations of the tagged animal. The other method available is through measuring ambient light and the Earth's magnetic field. This method has a functional depth limitation equivalent of the maximum depth limitation, generally 1800m. Magnetic based geolocation is generally not coupled with additional satellite data or other inputs, and relies on the Earth magnetic field for latitude estimations and light for longitude estimations.
General information
Pop-up satellite tags range in length from about 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.These tags record information such as temperature, magnetics, acceleration, light level, oxygen levels 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 the power source fails.
When the PSAT releases from the animal on which it was attached, it floats to the surface, and begins to transmit data to 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. Summarized data illustrating where the fish's migration started and ended is usually recovered from the tag within about seven days; however, tags can transmit significant amounts of oceanic data for months after they release from the fish.
Limitations of PSAT technology are that it is subject to loss by malfunction of the power source, 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 depending on the model. If data indicate no change in pressure for a period of time, this could trigger an early release due to premature release 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 in the sense of forcing a tag to report; however, in data processing it is indicated by an immediate loss of light and an increase in temperature that stabilizes while it is inside the predator.