Archival tags, as their name suggests, archive (or store) data. These small devices are deployed on a fish where they log a variety of information. If the fish is recaptured and the tag recovered, the logged data can be downloaded. Tags may log a variety of variables depending on the system including, temperature (sometimes both body temperature and water temperature), depth, light intensity and/or compass heading, among others.
The most important variable for measuring movements is light intensity. Similar to the ancient sailors before the advent of GPS, light levels are used to estimate latitude and longitude. Longitude (the number of degrees from Greenwich, England) is determined from local noon and is relatively straightforward to calculate. Latitude is calculated by measuring day length, the time from sunrise to sunset. As we move from the equator to the poles, day length changes, depending on the day of the year. Thus, from day length we can calculate the latitude. Latitude is a little more complicated to calculate than longitude and its accuracy changes depending on the location and time of the year.
Archival tags provide phenomenal data. With temperature, depth, and light intensity recorded every two minutes it is possible to examine otherwise subtle behaviors. In addition, the fish is acting as a sampling platform collecting data along its path. Thus, information on the fish's physical environment is obtained in accordance with variations in the fish's behavior. For example, how temperature changes with depth will indicate the location of the thermocline (the depth at which the temperature decreases dramatically), a feature that greatly influences the behavior of many fish.
The advantages of archival tags are that they collect high intensity data potentially for years. Using this data we can learn about both fine and large-scale movements in relation to environmental conditions. The disadvantage is that these devices are relatively expensive and must be recovered to obtain the data. Projecting a 10% recovery rate and a $1,500 price tag, each recovered tag costs $15,000.
PIER uses Archival tags in tracking: