Why did turtle X stop transmitting?

When turtles reach their foraging locations, they spend an even greater proportion of their time underwater and thus will transmit less frequently. Given transmissions can only be picked up during a few short windows each day, by chance this means that there may be periods of several days or longer with no transmissions received. As water temperatures cool, turtles may spend large amounts of time on the seabed effectively 'hibernating'; coming up for air after extended intervals, further exacerbating the lack of data transmitted.

Eventually, however, all transmitters stop sending information. There are a whole suite of reasons why we might cease to receive transmissions from one of the turtles we are tracking:

1. Dead Battery: It appears from the results of most workers that few transmitters actually reach the end of their working battery life with some other factor being responsible for premature cessation of transmissions. In general transmitters will only actively try and send data to the satellite when the turtle is at the surface (see Saltwater Switch Failure below). Given that most turtles spend >97% of their time submerged and, even the smallest of transmitters have a battery life of some 20 days or more,most should last at least a year.
2. Attachment Failure: Sea turtles are known to like to hide under rocks and submerged reefs when resting and loggerhead turtles have even been observed 'scratching' their backs on these reefs, possibly to reduce often heavy barnacle loads. These behavioursmay dislodge the transmitter. Satellite transmitters typically do not float so when they are dislodged, they fall to the seabed and will not send anymore effective signals. Our attachment methods have improved greatly over the years and a transmitter can be expected to remain attached to a turtle for a year or morewhich is long enough for us to observe migratory routes and describe feeding locations.
3. Antenna Failure: Although turtles my not wholly dislodge their transmitter whilst undertaking back scratching and resting under rocky ledges, they may damage the antenna. Many turtle biologists feel that this is the number one reason for transmitter failure despite efforts to design more sturdy antennae.
4. Saltwater Switch Failure: To extend battery life all transmitters have a saltwater switch which is broken as the animal surfaces allowing economic use of battery life. A transmitter may become fouled with marine organisms, such as algae, which may temporarily inhibit the saltwater switch and cause the transmitter to be unable to recognize when the turtle is at the surface. If a more permanent attachment by a marine organism occurs, such as an encrusting coral species, mussels or barnacles, then transmission may be permanently inhibited, even though the batteries are not dead.
5. Turtle Mortality: Many turtles each year are captured in marine fisheries and a proportion are killed. Often these events can be identified as suddenly changing transmission frequency and location quality increase, often suggesting fast linear movements towards fishing harbours (i.e. movement of the fishing vessel). Transmitters with depth sensors or surface counters sometimes send data showing the transmitter at the surface. Undoubtedly, however, in a proportion of cases the capture event could occur when there are no satellites overhead and the transmitter is irretrievable damaged or discarded either with or separately from the turtle before a transmission indicating the capture can be received.