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Table 1 Summary of the various technologies used for tracking animals in freshwater with a brief summary of strengths, limitations, and common applications

From: Tracking animals in freshwater with electronic tags: past, present and future

Technology Summary of technology and techniques Strengths Limitations Applications
Acoustic telemetry; manual tracking Uses a transducer to convert electrical energy to acoustic energy that is detected by an underwater hydrophone Deep water (>20 m) Hydrophone must be submerged in water Some applications in FW
Animals usually tracked by boat using bearings and triangulation Can provide detailed movement data depending on tracking method and conditions Ineffective in shallow or turbulent water Mostly for fish, some use with alligator, FW mammals and elasmobranch
Not suitable for transmitters with long pulse interval (time between pulses)   Interference from macrophytes and noise (for example, boats, entrained air)  
Acoustic telemetry; fixed stations As above Can be deployed as gates, grids, or arrays to monitor animal movements for long periods, including under ice Generates large datasets Widespread use in FW
Autonomous or cabled hydrophones and associated loggers store time-stamped data when tagged animals enter reception zone Multiple stations can provide precise two or three-dimensional tracks of animals Requires significant post-processing and analytical efforts (can be challenging to recover data with some systems) Mostly on fish, some use with mammals that move between marine and FW environments
  Some systems provide real-time data transmission Interference from macrophytes and noise (for example, boats, entrained air)  
Radio telemetry; manual tracking Emit electromagnetic energy in the radio frequency range (usually in the VHF band between 30 and 300 MHz) Shallow water (<10 m) Deep water (>15 m) Widespread in FW
Signals detected by antennas (aerial or underwater) and a receiver (some have logging capability) Low-conductivity (<500 μS/cm) High conductivity Sensitive to localized interference Fish and other taxa, especially amphibious species (for example, basking turtles, amphibians, some mammals)
Tracking can occur from boat, vehicle, air, foot Relatively inexpensive   
Functions in moving water and through ice as well as on land and in air
Radio telemetry; fixed stations As above As above Antennas visible, thus can attract vandals Widespread use in FW
Fixed stations with multiple antennas detect and log tags when in the vicinity of an antenna Suitable for long-term deployments Not possible to obtain precise two-dimensional positions (mostly presence or absence in a given location) Mostly used with fish
Most often deployed in riverine systems to detect migration   Sensitive to interference  
   Effectiveness dependent on local geomorphology and station placement [19]  
Passive integrated transponder telemetry; manual tracking Integrated circuit chip and coil antenna that transmits a unique identity code when energized by a low-frequency radio signal (generally 125 to 400 kHz) No battery (therefore long life) Largely limited to shallow and restricted lotic systems owing to small detection range (usually <1 m) Widespread use in FW
Detected using hand-held (usually in a backpack with wand waved in or above water surface while wading or walking on ice) or boat-mounted readers Small Multiple tags in the same location can prevent signal transmission Mostly in small fishes, reptiles, amphibians and invertebrates
  Inexpensive Detection range depends on tag orientation relative to the reader  
Passive integrated transponder telemetry; fixed stations As above No battery (therefore long life) As above Widespread use in FW
Antennas deployed in systems that record the time and unique ID of a given tag detection Small Remote stations require power source to function Mostly for fish
A variety of antenna designs, mostly for small lotic systems, fishways or other areas where animals congregate Inexpensive   
Low-frequency electromagnetic telemetry Similar principle as PIT tags, but tags are continually powered Effective in areas with high noise levels where acoustic and radio may be problematic Site-specific detection only Limited use in FW
Greater detection range on the order of several tens of metres (for example, [20, 21]) Limited tag life The few examples use fish in lowland rivers [20]
Combined acoustic radio transmitters Both acoustic and radio output modules can either transmit simultaneously or, if dynamic, can switch between the two Works in marine and FW Large devices Some use in large diadromous fish species and marine mammals that move between marine and fresh waters
Switching driven by environmental cues (depth or conductivity) [22] Works for animals that use a wide range of depths or move into higher conductivity waters More expensive than either radio or acoustic tags
Archival tags (no transmitting capability) Biologging tags (for example, data storage tags, time-depth recorders, archival geolocation tags) Continuous monitoring of desired parameter (for example, temperature, depth) Require tag recovery to download data from onboard memory Some use in FW, mostly for logging temperature, depth, geolocation, and other environmental parameters (especially for ectothermic animals)
Do not transmit data remotely Occasionally log physiological information (for example, acceleration, heart rate)
Always equipped with some form of sensor  
Communicating histogram acoustic transponders and business card tags Hybrid loggers and transmitters, first logging data to memory and later transmitting data when interrogated by a receiver Useful for wide-ranging fishes that have low recapture probabilities but return to download sites [23] Reasonably large Not applied in FW to our knowledge (mostly marine applications)
Business card tags are a variant where the logging function actually serves as a mini acoustic receiver and can log other tagged animals that it encounters [24] Expensive
  Some versions require that tags are retrieved to download data
Smart position-only tag with real-time GPS Satellite platform transmitter which identifies tag location by real-time uplinks to orbiting Argos satellites whenever tag breaches the water surface (some variants may include archival capability rather than only transmitting in real time) Real-time positioning Relatively expensive Mostly marine
Provides information on broad-scale movement of animals (for example, ocean basin scale) Individual must breach water Some use in large FW animals that surface (for example, alligators, manatees)
Detailed records of environmental conditions experienced Limited to large animals  
Archival pop-up satellite archival transmitter tags Tags programmed to jettison at a given time or when tags experience a specific event (for example, fail to change depth for a specified period of time) at which time they float to surface and transmit summary data or packets to satellites Provides information on broad-scale movement of animals (for example, ocean basin scale) Most release links based on corrosive link, switched on via applied voltage and require sea-water to function Mostly marine
Geopositioning commonly based on light levels Detailed records of environmental conditions experienced Limited to large animals Most FW applications involve diadromous animals tagged during FW phase with pop-off scheduled for when they are in marine systems
Can also log depth and temperature   Relatively expensive Exception: studies on FW porpoises [25]
   Tags and downloading technology not always reliable, needs further refinement  
  1. FW, freshwater; GPS; global positioning system; VHF, very high frequency.