Location of VPS study areas. The three VPS arrays were located (a) in a coastal kelp forest on the continental shelf of southern California, (b) in the tidal freshwater portion of the San Francisco estuary, and (c) in the main channel of the Sacramento River in the Central Valley of northern California. Sites contained six to 10 independent hydrophone receivers (Vemco VR2W) and six to 13 coded transmitters (Vemco 69 kHz). Transmitters were moored in known, fixed locations either independently or co-located with a receiver. Number of recorded positions (N pos) of each transmitter are displayed at right by transmitter reference code (Trns). Base maps from Esri (DigitalGlobe, GeoEye, i-cubed, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community).