A CTD is an instrument cluster that measures conductivity (a measure of the salinity of ocean water), temperature, and depth. These are important physical characteristics to consider when studying many different ocean processes, including marine animal distributions. Scientists often attach CTDs to other devices that collect or count organisms, such as Video Plankton Recorders or Trawls, thus allowing measurements of some non-biological characteristics that may be associated with animal distribution.
A CTD and water sampling Rosette, showing the series of instruments that can be used to take multiple measurements and collect water samples from multiple depths on a single launch. (NOAA Ocean Explorer)
A CTD being recovered after use during a research cruise. (NOAA Ocean Explorer)
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Conductivity-Temperature-Depth rosette being recovered on a rare calm day in western Alaska waters. (Allen M. Shimada, NMFS)