Radio
(VHF)Radio telemetry uses VHF transmitters (tag) to emit signals through fresh water and air to receivers that detect and record them. Transmitted signals can be detected and recorded with antenna receivers however the number of distinct codes that can be transmitted per frequency is limited. Tagged fish are tracked by periodic detection by fixed remote antenna/receivers or more commonly by boat, road, air or foot. Like acoustic tags the size of the tag is dependent upon the battery size. Tag attachment may be esophageal insertion, surgically implanted into the fish body cavity or externally attached. Combined acoustic radio tags (CART) allow for both fresh and marine water detection. Combined telemetry and data storage tags permit both tracking fish and recovering environmental data.
Advantages:
- The fish only needs to be handled once for tag attachment
Disadvantages:
- Radio tags are typically only detected within the first 10m of the surface
- Signal transmission is reduced in salt water and at depth