Mobile hydrophone surveys have been used to record fish sounds and locate spawning areas (see section on How do fish communicate using sound?). 

    Scientists typically lower a hydrophone into the water and record acoustic data for a specified time period (usually a few minutes) then proceed to the next recording location. Mobile surveys are quite useful for covering a large geographic area, but require a great deal of effort and may be subject to weather conditions.

    Scientists desired a low cost recording device which could be deployed underwater for extended time periods to record and store sounds at programmed time intervals. In recent years acoustic datalogging devices have been developed and proven effective at collecting data over long time scales that are not possible with mobile hydrophone surveys. 

    Examples of these devices include the Long Term Acoustic Recording Systems (LARS-LF and LARS-HF) created by Dr. David Mann and the Autonomous Underwater Listening Station (AULS) created by Cliff Goudey.

    The LARS-LF records sounds within a frequency range of 0-3333Hz.This device can be programmed to record at specified time intervals and can be deployed for nearly one year without maintenance.Photos courtesy of James Locascio.

     LARS-HF is a long-term acoustic recorder and datalogger based on the

 PocketPC,supporting sampling rates up to 44.1 kHz. The LARS-HF allows 

recording either continuously or on a pre-programmed schedule for time periods 

up to two months. Photo courtesy of David Mann.
 

     The AULS is based on the Nomad Jukebox recording system. It can record sounds in an 11kHz to 44kHz range and be deployed for a maximum of 57 hours (at 11kHz sample rate). Photos courtesy of Cliff Goude.

    Passive acoustic monitoring data have revealed how sound production by spawning fishes varies over daily, seasonal, and inter-annual time periods and have shown differences in habitat use by different species. Much of what has been learned about spawning behavior and time frames from acoustic data corroborate well with knowledge learned from capturing fish and examining their gonads to determine the time and location of spawning. Thus, passive acoustic methods (which are non-destructive) may serve as a suitable substitute or complement to traditional methods of determining the timing of fish spawning.

    As advancements in technology continue to be made, remotely deployed acoustic dataloggers will become cheaper and more powerful (i.e. greater storage capacity, higher sample rates and data processing abilities). There are many exciting discoveries waiting to be made through the use of such devices.