The marine environment imposes severe constraints on the types of tags that can be used to monitor animal behavior. Unlike the traditional tracking methods used on land and in freshwater, radio telemetry is not an option in the ocean as radio waves do not propagate well through seawater. The use of acoustic telemetry tags is also impractical for highly migratory species like bluefin tuna as these tags have only a one nautical mile range. To overcome these limitations, researchers teamed up with engineers to devise ingenious means to follow bluefin tuna...without getting wet. The solution? Archival and satellite tags.
Satellite and archival tags are similar in a number of important ways. Both log data on a range of environmental and behavioral parameters. The main difference is in how the data are retrieved. For archival tags, the data are downloaded directly from the tag so the animal must be recaptured. For satellite tags, the data are transmitted to scientists via satellite when the tag releases from the fish at a pre-programmed time and floats to the surface.
The TAG scientific team has used several iterations of archival tag as the technology has developed rapidly over the last decade. TAG "surgeons" implant the tags in the abdominal cavity of bluefin tuna, leaving the tag's sensor stalk emerging from the body. The sensors on the main body of the tag record depth and internal body temperature, while the sensors on the stalk record external water temperature and ambient light level. Light levels are used to estimate latitude and longitude (see below).
These mini-computers have the capacity to record data at a programmed rate (approx. every 2 minutes) over periods ranging from a few days to several years. The vast amount of data collected allows determination of an animal's fine and large-scale behavioral patterns, migratory routes and physiology, all in relation to the environment. The main limitation of archival tags is that they must be recovered to obtain the recorded data, meaning that the animal has to be caught AND the fisher has to return the tag. A $250-$1,000 reward is offered as an incentive to return the tag.
The video below shows archival tagging off Cape Hatteras, North Carolina.
The TAG team also uses pop-up satellite archival tags (PAT) to study bluefin tuna. These devices are secured to the back of the fish with a dart and log data on water temperature, light and depth. Unlike archival tags, satellite tags are equipped with powerful radio transmitters that transmit the data stored on the tag to the Argos satellite system, providing data even if the fish is not caught. Successful transmission requires that the antenna be above the surface of the water. At a pre-programmed time, an electrical impulse burns the wire that attaches the dart to the tag, and the tag releases from the fish. It then floats to the surface and uploads the logged data to the Argos satellites.
Because of the limitation of data transmission by satellite, the logged data is compressed prior to transmission. The temperature and depth data are synthesized in the form of histograms and temperature depth profiles. For geolocation, longitude is calculated onboard, whereas light and depth values at sunrise and sunset are transmitted for later calculation of latitude (see below). In addition, the full archival record is saved onboard the tag so if the PAT is recovered, researchers have the same detailed data that are collected by archival tags. A $500 reward is offered to fishermen, beachgoers and anyone else who finds a PAT as an incentive to return it.
The following cartoon depicts how data are retrieved from the tags via satellite.
The most innovative feature of the tags is the use of ambient light levels and the tag’s internal clock to estimate longitude and latitude, and therefore the precise position of the fish. By analyzing the light levels detected over a 24-hour time period, the tag's processors can determine the time of sunrise and sunset and subsequently the time at which high noon occurs. From there, the process works just like a ship's chronometer: By looking at the difference between local noon and noon Greenwich Mean Time (GMT), one can calculate longitude very accurately. Latitude is estimated from measures of day length, since day length changes predictably as you move away from the equator.
While longitude can be calculated very accurately, latitude is more complicated, and errors are considerably larger. Thus, to improve upon latitude estimates based on day length, TAG scientists developed algorithms that also take water temperature into account. Satellite-observed sea surface temperatures along the calculated longitude are compared to the sea surface temperatures recorded by the tag, reducing the error substantially, from 3 - 5 degrees of latitude to 1 degree.
A light level curve collected by a tagged bluefin tuna showing sunrise and sunset for calculation of geoposition (i.e., longitude and latitude).