Transmissions can only be picked up during short windows of time when certain satellites are overhead and the sea turtle comes up for a breath at the surface. Mote’s tags are programmed to transmit each time the turtle surfaces, though transmissions aren’t always successfully received by satellites. In other cases, tags may be programmed to transmit less often to save battery life. Also, transmissions vary in accuracy (see point 2 above), and Mote’s map does not show the least accurate transmissions. These factors can result in a few days with no received transmissions.
Eventually, however, all transmitters stop sending information, and that can happen for several reasons:
- Attachment or antenna failure: Sea turtles are known to hide under rocks, and loggerhead sea turtles have even been observed “scratching their backs” on rocks and reefs. These behaviors could dislodge the transmitter or break or damage its antenna.
- Biofouling: Most transmitters have a “saltwater switch” which tells the transmitter it’s at the surface of the water (when its sensors are dry), where it can send data. However, the saltwater switch could be compromised by algae, or even coral, mussels or barnacles growing over the sensor, making it seem wet all the time. Mote scientists put anti-fouling paint on satellite tags to prevent this for as long as possible.
- Mortality: All species of sea turtles are threatened or endangered, which is why we are interested in tracking their behaviors. Scientists can sometimes guess at a turtle’s cause of death based on transmitter data: For example, if a turtle has been caught as by-catch by a commercial fishery, frequent transmissions in a line towards shore could indicate the turtle is deceased aboard a fishing vessel headed towards shore.
- Dead battery: Most batteries on transmitters can last up to a year. To save energy, transmitters only actively try to transmit when the sea turtle is at the water’s surface.