An acoustic method to observe the distribution and behaviour of mesopelagic organisms in front of a trawl

We describe a method to allow acoustic sampling at depths not reachable by the higher frequencies of ship hull-mounted echosounders and observe the abundance and behaviour of individual organisms in front of trawls. A self-contained scientific echosounder with a 120 kHz transducer was mounted forward-facing on the headline of a macrozooplankton trawl that was obliquely towed from 0 to 1000 m depth, to investigate the mesopelagic fauna. With the use of a forward-facing echosounder, we were able to estimate organism densities in front of the trawl, the vertical profiles of organism target strength and the movement of organisms in front of the trawl. We demonstrate that a forward-facing trawl-mounted echosounder is a simple and useful method to investigate the distribution of mesopelagic fauna at depth.