ACOUSTICS UNPACKED

A General Guide for Deriving Abundance Estimates from Hydroacoustic Data

 

 

 

 

 

 

 

 

 

Target strength

Backscattering cross section in situ σbs or target strength (TS) when expressed in dB is an important scalar for converting sv measurements to absolute numbers.  In the Great Lakes, in situ σbs values, taken from fish observed during the survey, are commonly used as the sv scaling factor.  For that reason, in situ σbs data must be collected with the goal of gaining a representative distribution.  Fish TS is primarily dependent on swimbladder size, but also on swimbladder shape and compression, state of maturity, and fat content (Ona 1990, Horne 2003).  The TS is also strongly affected by the orientation of the fish relative the sound beam (often referred to as aspect). The aspect depends on behavior, such as vertical migration, swimming, and feeding behavior. In addition, pressure changes during vertical migration can affect TS. In the case of fish with swimbladders, maximum TS occurs when the major axis of the swimbladder is aligned perpendicular to the transducer. TS decreases significantly as the major axis of the organism aligns parallel to the acoustic axis. TS of a fishcan vary over 30 dB due to different tilt and roll angles (Reeder et al. 2004, Frouzova et al 2005).  Note that all calculations of fish density should be done with σbs including calculations of averages. When referring to mean TS, this is the dB transformation of the mean σbs .