Integrated Ocean Observing System, U.S.

Bluefin tuna swim so fast, far, and deep that it has been difficult to learn
about their lives in the wild. Marine biologists use satellite transmitters to
track sharks, seals, and turtles, which spend time near the ocean surface, but
tuna live beyond the reach of satellites.* So scientists had to develop an
alternative solution. In the 1990s, they realized that they could take advantage
of the fact that tuna are commercially harvested and outfit the fish with tags
that store location information for later, rather than transmitting it in real time.
The idea was that when a fisherman landed a tuna equipped with one of these
“archival tags,” he could remove the device and return it to researchers. The
fisherman would get a financial reward for his service, and the biologists
would get weeks, months, or years of detailed data that would enable them to
reconstruct the tuna’s path.
Barbara Block, a Stanford marine biologist who directs the TRCC, helped
pioneer the archival tagging of tuna, and she has used hundreds of the devices
to follow the migrations of fish in the Atlantic and Pacific. To deploy the tags,
Block and her team head out to sea, where they often brave stormy weather as
they fish for tuna that can outweigh them by hundreds of pounds. Once
they’ve wrestled one of these giants into the boat, they lay it on the deck,
cover its eyes with a wet towel, and use a hose to irrigate its gills with
seawater. One team member makes a three-to-four-centimeter incision in the
tuna’s side and places an archival tag inside the abdominal cavity. The tag is a
marvel of miniature engineering. It crams a multitude of electronics into a
small stainless-steel cylinder approximately the size of a tube of lipstick. It
contains a suite of environmental sensors, a microprocessor, a tiny battery,
and enough memory to store years’ worth of data.* The whole thing weighs in
at one-tenth of a pound and can operate more than a mile below the ocean’s
surface, at temperatures below freezing. Tucked inside the tuna’s belly, the tag
will measure the fish’s depth and internal body temperature as it swims.