Sea Technology

MAY 2018

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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10 ST | May 2018 W hen University of Miami Professor Neil Hammerschlag tagged a tiger shark in the Bahamas with the first of a new generation of pop-up archiving satellite tags called SeaTag-MOD in late 2011, the primary purpose of the exper- iment was narrow: to track the migration of the shark. This would be done by tag transmissions to the Argos satellite constellation during brief opportunities when the fin-mounted tag broke the surface. In fact, only a few such oppor- tunistic transmissions were received, but internally the tag scanned and archived sensor readings every 4 min. Timing dawn and dusk via light measurements served as the proxy of the shark's longitude, and the strengthening or weakening magnetic field would indicate transitions of latitude. Spurts of acceleration might signal hunting activity, and a pressure sensor along with a temperature sensor would reveal the vertical habitat and temperature preference of the animal. A year onward, the tag separated from the shark, floated to the surface and, powered by its solar panels, provided a continuous stream of position fixes and basic status reports via Argos. Satellite localizing indicated a pop-up position in the northern Bahamas not far from the tagging location, with a drift north in the Gulf Stream. The following fall, as the tiny explorer drifted past Newfoundland, a growing fuzz of marine fouling dimmed the sunlight reaching its so- lar panel. Messages and location fixes became sparse and eventually stopped. This would have been the end of the story, except that two years on Hammer- schlag received an email from South Wales, reporting the tag had been found on the beach. Back at the lab, the returned tag proved to be in good shape. All systems worked, and the tag resumed satellite reporting once cleaned and floated. More significantly, the tag's sensor scans had continued throughout the journey, the dimmed sunlight being sufficient for scanning the environment if not for transmission. The track esti- mation from the light and magnetic sensors revealed that the shark had resided in the Bahamas until spring. It then headed northeast, described by a loop north of Bermu- da while probing depths to 800 m in brief dives. By summer, it took up residence at the continental shelf offshore Connecticut for the season, staying close to the surface and only once venturing deeper than 200 m. In the fall, it started its return journey south, tracing the continental shelf, again probing the depths, and ultimately return- ing with great site fidelity to the Bahamas, where the device popped off just tens of miles from the tagging position. The following drift across the Atlantic was documented as well, and stormy winter seas and summer periods of glass calm were apparent in the accelerometer readings Ocean Dashboard Inexpensive Smart Sensors Could Revolutionize Ocean Sensing By Marco Flagg

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