High-resolution target strength measurements in deep water

Abstract

High-resolution target strength data can only be claimed valid in situations where it can be safely shown to be much less than one target per pulse volume. Many fish species occur in densities and at depths where this demand hardly can be met with standard, hull-mounted, survey transducers. This paper describes a new and simple method for obtaining such data with some examples of target strength and target tracking data obtained in deep water at three different cruises. The system used is the Simrad EK-500 split beam echo sounder, connected to an oil-filled ES38D, pressure-resistant transducer, on cable lengths from 400-800 m. The transducer was lowered as a probe the desired depth, often inside or close to the fish layer, with the vessel stationary, or slowly drifting. For maximizing the number of detections per fish, the echo sounder was operated at maximum pulse repetition frequency. Recordings shown are from three different species, small myctophid fishes at 200–400 m, hake at 200–400 m, and herring at 50–400 m. High-quality target strength distributions were frequently obtained in less than 1 h at a typical target strength station.