Deep towed vehicle for fish abundance estimation concept and testing

A research program was initiated in Norway in 1988 for measuring the abundance of fish resources at depth from 500 to 1000 metres. It was decided to develope a deep towed vehicle, to bring acoustic transducers down to 400 metres depth. The vehicle serves as a stable and quiet platform for the transducer. The attenuation due to air bubbles in the upper water layers is avoided, and the transducer is closer to the fish so that the resolution of single fish is improved. The vehicle has a weight of 500 kg and is designed as a streamlined torpedo with accommodation for the transducer, a depth sensor and inclinometers. A special feature is that the transducer face is in direct contact with the waterflow below the vehicle. This eliminates the need for an acoustic window, and all the well known problems of air bubbles attached to the inner surface of the window. This paper describes the concept of the vehicle and the transducer, and gives test results obtained during surveys in 1990 and 1991