Why use electronic tags?
The traditional way to assess populations of large marine vertebrates was to observe their distribution from a ship or plane. While a lot can be learned using this method, it is limited in that animals can only be observed at the ocean’s surface and in regions reachable by ships or planes. This method is also time consuming and, quite often, very cost- and resource-intensive. In contrast, electronic tags allow us to follow the animals wherever they go, independent of where the tags were originally deployed, providing more accurate information over a larger geographic range. For example, prior to the deployment of electronic tags, northern elephant seals were thought to range just offshore along the west coast of North America (Figure 1A), but data obtained using electronic tags show they utilize most of the northeastern Pacific Ocean (Figure 1B).
Figure 1. Distribution of Northern elephant seals as determined using boat and plane based surveys(from Riedman 1990)18. B. A kernel density plot showing the distribution of northern elephant seals determined using satellite telemetry. In addition to a more realistic distribution, the figure shows areas of relative use.
Electronic tags generate a sequence of data points taken at regular intervals (a time series) that can last from months to years and provide researchers with information used to identify animal behaviors and associated habitats. Depending on the type of tag deployed, scientists can acquire a variety of data including simple surface tracks (Figure 2A), tracks with diving behavior (Figure 2B), or a record of the physical characteristics of the animals’ environment (Figure 2C), such as temperature, salinity, chlorophyll, and light level.
Figure 2. Tracks of Southern elephant seals in the western Antarctic Peninsula. A. Only the surface track. B. Surface track along with diving behavior. C. Temperature and salinity profiles that can be obtained to provide data on the physical environment the animals are moving through. Figure from Costa et al. 2010a.9
Such behavioral data are important in identifying differences in the movement patterns and habitat utilization of different species. For example, some species may travel over considerable distances (e.g., Southern elephant seals), while others may remain within a smaller home range (e.g., Weddell seals; Figure 3). Several international efforts have deployed electronic tags across ocean basins and on multiple species, making possible unprecedented observations of marine organisms on a global scale.
Figure 3. Differences in the movement patterns of Southern elephant seals (yellow), crabeater seals (red), and Weddell seals in (green) are shown along the Antarctic Peninsula. The tracks cover the same time period during 2007. Figure from Costa et al. 2010a.9.
Scientists use a variety of different tags depending on what data are needed and how the data are gathered and transmitted. There are three primary methods for tracking marine organisms:
- Archival (data storage) tags. Archival tags record data as a time series from sensors that measure depth (pressure), water temperature, salinity, light level, and the animal’s body temperature. The major limitation of archival tags is that, to obtain the data, the tags must be physically recovered. However, careful selection of animals, such as use on exploited species where a reward is offered, has provided a wealth of information on the foraging behavior and habitat utilization of a large group of marine organisms.1,2 Movement patterns can be derived from archival tag data by examining changes in light level to determine local sunrise and sunset times which, along with an on board clock, can be used to calculate approximate longitude and latitude. These locations can then be further corrected using sea surface temperatures.1 These tags collect large quantities of detailed data, allowing scientists to study both fine- and large-scale behavioral patterns, migratory routes, and physiology, all in relation to the surrounding environment. One of the most sophisticated tags is the D-tag, which has been used to monitor the movement patterns of beaked whales in three dimensions and can indicate when the animals are echo-locating and even capturing prey (Figure 4).3
Figure 4. Representative sections of dive profiles from Ziphius cavirostris (A) and Mesoplodon densirostris (B). Intervals with regular click vocalizations are indicated by a thicker trace and times of buzzes are indicated by small open circles. The depth of occurrence of buzzes heard in the audio recording during the two dives is shown in the histogram on the right-hand side of each panel. Figure from Tyack et al 2006.3.
- Satellite tags, or satellite-linked data recorders. Satellite-linked data recorders have expanded our understanding of the fine-scale movements of marine birds,4 sea turtles,5 sharks,6,7 and marine mammals.8,9 These tags are fisheries independent, meaning they do not require recapture of the animal to acquire the data. However, because the antenna on the satellite transmitter must be out of the water to communicate with an orbiting satellite, the technology has been used primarily on air-breathing vertebrates that surface regularly. For large fish and other marine animals that remain continuously submerged, a pop-up satellite archival tag has been developed.1 Pop-up satellite tags combine data storage tags with satellite transmitters that communicate with Argos satellites. The Argos system records the transmissions from satellite tags and downloads these data back to Earth providing locations of varying precision, with the most accurate being within 150 meters (m). However, locations of such quality are quite rare in marine species, and most locations are only accurate to within a kilometer or worse.10 A new tag has recently been developed by the Sea Mammal Research Unit of St Andrews University, Scotland, that uses cell phone technology to transmit data that have been stored on the tag.11
- GPS tags. The recent development of tags that use the global positioning system (GPS) has significantly improved the precision of animal movement data to 30 m. While navigational GPS tags have been widely available for terrestrial animals, their application to marine animals was limited because the time required to obtain position information was greater than the time a marine animal typically spends at the surface. The development of Fastloc™ (Wildtrack Telemetry Systems Ltd., Leeds, England), a system that obtains GPS satellite information in less than a second and then compresses the information so that it can be transmitted using ARGOS, solved this problem. This system captures the GPS satellite signals, identifies the observed satellites, calculates their approximate ranges, and produces a location estimate that can be transmitted quickly. Final locations are processed after the data are received using archived GPS satellite position data accessed through the Internet. Along with significantly greater location resolution, Fastloc™ GPS provides many more locations per unit time, thus allowing unprecedented observations of fine-scale animal movements (Figure 5).
Figure 5. A track from a female Cape fur seal foraging off the western coast of South Africa obtained using both Argos (black) and Fastloc™ GPS (yellow) locations. Figure from Costa et al. 2010b.10.
Animals as oceanographers
In addition to increasing our ability to study the movements and foraging behaviors of animals that were not otherwise possible, these new electronic tags provide oceanographic data in geographic areas where traditional conventional methods are limited or absent (Figure 6). Such data are particularly lacking in the polar oceans where ship time is limited (especially in the winter), cloud cover limits the use of satellite remote sensing, and currents or ice limit or prohibit use of floats.
Figure 6. A Weddell seal, Leptonychotes weddelli, is shown wearing a Sea Mammal Research Unit Conductivity Temperature Depth tag in McMurdo Sound Antarctic. Photograph by Dan Costa.
The absence of traditional sources for oceanographic data, in combination with an abundance of marine mammals that are relatively easy to handle and that cover large areas of the ocean throughout the year, make the polar oceans ideal places to use marine animals to collect oceanographic data. Additionally, different water masses have different temperature and salinity signatures that can be used to trace the origin of each water mass. Using sensors that measure the temperature and salinity of the water the animals swim through, these tags can be highly cost-effective platforms for collecting detailed oceanographic data on a scale not possible with conventional methods.10,12,13
What questions can animals help answer?
A prime example of how this work can address oceanographic questions, as well as provide insight into animal behavior, can be seen in the results of SEaOS. This program deployed conductivity-temperature-depth tags on 85 elephant seals simultaneously at Kerguelen, South Georgia, Macquarie, and the South Shetlands between January 2004 and April 2006.14 Nine times as many temperature and salinity profiles were collected south of 60°S as part of this effort than during previous efforts using traditional methods.12 Combined with conventional data, the seal data extended maps deep into ice-covered areas (Figure 7) and offered a circumpolar view of high latitude fronts within the Southern Ocean.12 Animal-collected oceanic data are thus complementing more traditional methodologies and are contributing to our knowledge of the global ocean.
Figure 7. Temperature field at 500 m from 2004 to 2005 from the Coriolis database and from the merged Coriolis and elephant seal databases. Mean front positions during the same period derived from Coriolis (A) or Coriolis and seal temperature field at 500 m (B, thick lines), and from altimetry (thin lines in A and B). Figure from Charrassin et al. 2008.12.
An equally important component of the SEaOS effort was the ability to examine the foraging behavior of elephant seals throughout much of the Southern Ocean and to define their foraging habitat in terms of physical oceanographic characteristics.14 Using changes in animal drift rate measured during periods when the seal was not swimming as an indicator of body condition (e.g., fatter seals tend to be more buoyant while leaner seals tend to sink), combined with salinity and temperature data obtained from the seals’ tags, researchers were able to identify characteristics of water masses where elephant seals had the greatest foraging success. These areas were associated with warm, deep, nutrient-rich water called circumpolar deep water, which travels in the Antarctic Circumpolar Current and upwells along the continental shelf.
Tracking tags can improve marine conservation and management
The importance of tracking data to the conservation and management of marine living resources is becoming increasingly clear. As more data become available on a wider range of species across the ocean, marine resource managers, as well as governmental and intergovernmental regulating authorities, can use the data to better inform their decisions about the following:
- Military and commercial operations. As described above, electronic tags can be used to identify patterns of habitat utilization by marine animals. Such information is critical in establishing times and locations where oil and gas development, naval activities, or shipping and research activities can be undertaken when marine mammals are not present. Electronic tags have also been used to monitor the response of marine mammals to underwater sounds generated as a result of these activities.15
- Management of fisheries and bycatch. Electronic tags can provide critical information for the management of marine species that are directly (e.g., tuna and sharks) or indirectly (e.g., leatherback sea turtles, shearwaters, and albatross) harvested by commercial fishing operations. Tracking data clearly demonstrate that many species of marine animals require multinational protection.2 For example, Laysan albatrosses tagged at Guadalupe Island, Mexico, have been found within the California Current System along the western coast of North America, in addition to the exclusive economic zones of at least three different countries in the eastern Pacific region. Tagging of Pacific bluefin tuna revealed that fish swimming to the eastern Pacific Ocean from Japan are so overexploited that few live long enough to make the trans-Pacific migrations back to their spawning grounds. And by tagging and tracking leatherback sea turtles, researchers discovered the animals using predictable travel corridors shaped by oceanographic features,16 leading the International Union for the Conservation of Nature to pass a resolution calling for the conservation of leatherback sea turtles in the open seas.
- The protection of endangered species. An important product of the Tagging of Pacific Predators (TOPP) program has been the identification of key geographic areas where the establishment of marine protected areas (MPAs) could protect critically endangered species. A specific example of the success of the TOPP program in this respect is the creation of an MPA off the coast of Baja California to protect loggerhead sea turtles.17 Similarly, tracking data helped lead to the listing of the black-footed albatross as an endangered species by the United States Fish and Wildlife Service, and BirdLife International relied on them for deliberations within the international Agreement for the Conservation of Albatrosses and Petrels.
Developments in sensors and data storage, transmission, and recovery methods are increasing our capability to monitor and study animals in nature. The development of new tags is also being accompanied by better analytical methods to process the complex data that are now being collected. As a result, our ability to understand the ocean and its life forms has been significantly enhanced by, and is increasingly dependent on, advances in instrumentation. These advances are leading to an increased awareness of the ocean and the need to wisely manage and protect this last frontier on Earth.
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