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Why did animal X stop transmitting?
The most common reason for a lack of transmissions for one or two days is a change in the tracked animal's behavior. For example, as water temperatures cool, sea turtles may spend large amounts of time on the seabed effectively 'hibernating'; only coming up for air a few times a day. Because transmissions can only be picked up during short windows of time when Argos-receiving satellites are overhead, this means that there may be periods of one or more days with no transmissions received.
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 animals we are tracking:
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. Most sea turtle 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.
Attachment Failure: The material used to attach the transmitter to the animal may have failed. For example, 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 behaviours may 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.
Antenna Failure: The animal may damage the transmitter antenna through repeated impact with another object. For example, the back scratching behavior mentioned above for sea turtle may result in damage to the antenna. Many turtle biologists feel that this is the number one reason for transmitter failure despite efforts to design more sturdy antennae.
Saltwater Switch Failure: To extend battery life most transmitters applied to sea turtles have a saltwater switch which tells the transmitter when it is at the surface of the water, allowing economic use of battery power. 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.
Mortality: Many of the animals tracked by satellite belong to species of conservation concern. This is one of the reasons that justifies the use of relatively expensive satellite tracking. These species are of conservation concern for a reason and can often be found on national and international endangered species lists because their populations have become reduced, purposefully or accidentally, through human activities. For example, many sea 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.[an error occurred while processing this directive] [an error occurred while processing this directive]