Hydrophones are used to record both impulsive and continuous underwater sound. MARECO uses different types of hydrophones to perform field measurements, e.g. French RTSys recorders, a Canadian Aural recorder (today retired), American Ocean Sonic recorders and hydrophones. Low cost recorders from New-Zealand are also available in our instrumentation pool. For real time measurements, a Danish Bruel&Kjar complete chain is used. Hydrophones from Brüel&Kjar, Ocean instrument and HTI can be connected to the various recorders. Short term underwater sound measurements are performed on board of research vessels by lowering a hydrophone in the water column. Longer term surveys are performed by deploying a moored hydrophone at the seabed.