The Tags, They Are A-Changin'

The first dive-depth sensor for marine mammals was a kitchen timer covered with a piece of glass. Attached was a record-player needle that responded to water pressure by etching the glass. The length of the etchings translated to dive depth.

Now, everything is electronic. Sensors have grown in sophistication, while shrinking in size.

Marine biologist Daniel Costa's elephant seals carry a multitude of small sensors, bundled into plastic-cased packages the size of flip-top cell phones or iPods. Time-depth recorders measure how long and how far down seals dive for food. They also carry whirring turbines that calculate swim speed, and salinity meters to create ocean profiles.

To make sense of the data, the researchers need the animal's location at sea. Labeled with satellite receivers and global positioning system (GPS) units, the animals ping a slew of space satellites. The Costa team matches these locations with the behavioral and oceanographic data, creating a complete profile of the life of a hungry seal.

Albatrosses carry smaller versions of the location tags. Sunlight sensors read light levels every minute. When the levels change markedly minute-to-minute, it means the sun is rising or setting. Those times allow researchers to calculate the local hour of noon. Comparing that to Greenwich Mean Time yields the bird's longitude, and the length of the day reveals its latitude.

The next generation of tags is about size and versatility. Companies create custom-made combinations of sensors based on the user's needs. New tags correlate food levels in the ocean to levels of chlorophyll, the phytoplankton engine. From this, oceanographers and marine biologists match the nutrition of an ocean spot to its resident wildlife.

The bottom line is quiet observation. The lighter the sensor, the less the animal reacts to it, says Samantha Simmons, a six-year elephant seal veteran in the Costa lab. Her favorite times are when she is alone on the beach in a sea of seals. "I like when I can be an observer. There's that element of discovery," she says. To quietly observe a half-ton animal, it's best to be a fly in the sand.