Acoustic telemetry uses acoustic signals (also called ‘pings’) which propagate into the water, and are detected by acoustic receivers. Each tag has a specific set of ‘pings’ which are associated with a specific ID, allowing us to identify which animal was detected. The tags come in different sizes depending on the size of the animal you wish to tag, though smaller tags have a shorter battery life. This approach is by far the most widely used telemetry method.
- Used in both freshwater and saltwater
- Fine scale behaviour
- Can be combined with other sensors like depth and temperature sensors
- Tag life limited in smaller tags
- Receiver deployments and maintenance can be expensive
PIT (Passive Integrated Transponder) telemetry is used to identify and follow individuals, typically as they migrate in and out of rivers, or over artificial barriers such as dams. The tag is detected once an animal is in close proximity to an antenna. The antenna passively charges the tag, causing the tag to emit a signal specific to an ID. This means that the tag has no battery, and can last for several decades.
- Tag lasts for decades
- Inexpensive, so studies are not financially hindered
- Can be used to tag small fish where other tags are still too large
- Can only be used in specific locations where for example the river is shallow and narrow enough
- Only tells you that the fish has passed the antenna, no information on specific behaviour life depth use
Radio telemetry uses radio signals, which propagate both into the water and air, and are detected by radio receivers. Each tag emits a signal with a unique ID, enabling recognition of individual fish. The tags come in different sizes, depending on the size of the animal you wish to tag. The smaller tags have a shorter battery life and often a shorter range.
- Antennas in air can receive the signals, which enable surveying large areas for tagged fish by use of airplane and cars
- Fine scale behaviour
- Shape of tags well-suited for both internal and external attachments
- Can only be used in freshwater
- Does not work well when the fish is in deep water
Archival tags (data storage tags, DST), are tags with sensors that store the recorded information in the tag. There is usually no transmission of signals in the field, and the tag needs to be retrieved to be able to downloaded stored data, usually by recapturing the fish. Alternatively, DSTs with floats have been developed. When the fish dies, the tag will rise to the surface and may drift to shore and be found and returned to the labeled address.
- Useful when tracking is expensive, impractical or impossible due to environmental conditions or habitat size
- Tags available with different sensors recording different parameters, including water depth, temperature, light, salinity, three-dimensional tilt angle, and earth’s magnetic field strength (compass)
- Long-term studies
- Need to retrieve the tag
- Mostly data from surviving individuals
- No direct info on horizontal location (but can roughly be estimated for tags with light sensors)
POP-UP SATELLITE ARCHIVAL TAGS (PSAT)
Pop-up satellite archival tags (PSAT) enable large scale ocean migration studies of large fishes. The tags collect information on light, pressure (depth) and water temperature. After a pre-programmed period, or if the tag measures constant depth for a pre-defined period indicating mortality or tag shedding, it will release, float to the surface and start transmitting stored data and present position to ARGOS satellites. The satellites transfer the data to base stations.
- Can monitor long-distance fish migrations in the ocean
- The fish do not need to be recovered to retrieve data
- Data are also retrieved from fish that dies, which is often as important as getting data from only those surviving
- Large tags, can only tag large fish
- Expensive tags
- Most of these tags have an eroding release mechanism that depends on salt water, and do no release if a fish enters fresh water