SUMMARY
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 in the terrestrial and freshwater environment, radio telemetry is not an option in the ocean as radio waves do not propagate well through seawater. The use of acoustic telemetry tags to track the large-scale movement of animals is also impractical as these tags use lower sound frequencies that have only a one nautical mile range and thus limited to studying animals over short temporal or spatial scales. To overcome these limitations to real-time telemetry, researchers have teamed up with engineers to devise ingenious means to follow their subjects with out getting wet. Two solutions employed to overcome these constraints are archival and satellite tags. The application of these technologies to bluefin tuna in the Atlantic has provided unparalleled information on movements and behaviors in relation to environmental conditions.
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 the archival tags the data is downloaded directly from the tag when the animal is recaptured. For satellite tags the data is transmitted to the scientists via the Argos satellite system. This is either accomplished when the tags come to the surface at the end of the deployment or when the animal is at the surface through out the deployment.
ARCHIVAL TAGS
Archival tags have been used extensively to study marine fish, reptiles, birds, and mammals. In the past 5 years, rapid advances in archival tag technology (smaller size, increased memory capacity and lower cost) have made it possible to gather information on an expanding range of pelagic animals from seabirds to elephant seals. Archival tags are implanted or secured externally on the animal where they record a range of parameters. For fish, recorded variables generally include depth, environmental light, internal body temperature and external water temperature. Light levels are used to estimate latitude and longitude (see below). For mammals, this same data can be collected plus heart rate and swimming velocity among others. These microprocessor-controlled devices have the capacity to record these variables at a programmed rate over periods ranging from a few days to years. Because these tags record depth and temperature, important environmental data is also obtained along the track of the tagged animal. This level of data intensity 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. This limitation restricts their use to organisms (animals, such as fish and sharks) that have a sufficiently large fishery associated with them or animals, such as seabirds, marine turtles and mammals that return to a predictable location when the tags can be recovered.
SATELLITE TAGS
Unlike archival tags, satellite tags are equipped with powerful radio transmitters that transmit the data stored on the tag to the polar orbiting Argos satellites. Successful transmission requires that the antenna be above the surface of the water. The first satellite tags were very large and could only be carried by the large animals such as sharks and marine mammals. In the last few years however this technology has advanced substantially with the development of a range of smaller, more complex tags. These tags fall into two general categories: pop-up satellite archival tags (PAT) that transmit at the end of the deployment and 2) and satellite tags that transmit in real-time through out the deployment. The use of satellite tags have provided the opportunity to collect long-term, fisheries independent, data sets on species from which archival tags could not be recovered, dramatically expanding the range of marine organisms that can be studied.
For studies of bluefin tuna, the TRCC team uses PAT tags. PAT tags are designed to track the large-scale movements and behavior of pelagic fish and other animals that don't spend enough time at the surface to allow the use of a traditional satellite tag. These small devices are secured to the fish were they log data on ambient temperature, light and depth. At a user defined time the tag releases from the animal through the use of a burn wire, floats to the surface and up loads the logged data to the Argos satellites. Because of the limitation of data transmission imposed by the satellites, 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 on board where as light and depth values at sunrise and sunset are transmitted for later calculation of latitude (see below). In addition, a full archival record is maintained in non-volatile memory (types of memory that retain their contents when power is turned off). Thus, should the PAT be recovered, researches have the same detailed data that are collected by archival tags. PAT tags have been deployed with a high level of success on a variety of animals including tuna, marlin, sharks, swordfish, ocean sunfish (mola mola), halibut, eels and sea turtles.
LIGHTBASED GEOLOCATION
The most innovative feature of the archival and some of the satellite tags is the use of ambient light levels and the tag’s internal clock to estimate longitude and latitude. 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 time shift between local noon and noon GMT you can calculate longitude very accurately. Latitude is estimated from measures of day length, day length changes predictable along the earth's meridians. While longitude can be calculated very accurately, latitude is more complicated and errors are considerably larger. Thus, to improve upon latitude estimates made based on daylength, the satellite observed sea surface temperatures along the calculated longitude is compared to the sea surface temperatures recorded by the tag. The algorithms necessary to use sea surface temperature to improve latitude estimates were recently developed in the Block lab. These algorithms reduce the error in estimates substantially, from 3 - 5 to approximately 1 degree.
