Global Positioning System (GPS) data are critical for acoustic estimates of population size. GPS data should be sent to the echosounder and stored with the acoustic data string. All units currently used in the Great Lakes have that capability. GPS data are required for measurements of species spatial distribution and determining vessel location relative to physical oceanographic conditions and topographic features. Proper practice requires monitoring GPS output during data collection, documenting the type of GPS data used, and documenting data storage and retrieval procedures.