In situ target strength studies on Atlantic redfish (Sebastes spp.)

Abstract

In situ acoustic target strength (TS) experiments were conducted on Atlantic redfish (Sebastes spp.) in Newfoundland waters (1996–1998) using deep-tow dual beam and hull-mounted split beam echosounders (38 kHz). The dual and split beam mean TSs did not differ. The deep-tow system was deployed at various depths over several aggregations. Calibration corrections were made for depths from 5–70 m (<1 dB). The TS declined at ranges <50 m from the top of the fish shoal suggesting avoidance behaviour. It was biased upward at ranges >200 m and a number of fish per sampled volume >0.04. After being controlled for variations related to range, reverberation volume and fish density the TS did not differ with respect to depth, distance from bottom, fish sex ratio, condition factor or weight. The mean length was the dominant influence on the mean TS. Pooled ex situ experimental data and controlled in situ data – which did not differ – indicated a length-based regression (weighted by s.e.−1) in standard format: TS=20 log [length (cm)] −68.7 (r2=0.49).