Differences in acoustic target strength pattern between fish with one- and two-chambered swimbladder during rotation in the horizontal plane

Pattern of target strength (TS) during 360° rotation in the horizontal plane was studied for six common European fish species: brown trout (Salmo trutta), perch (Perca fluviatilis) bream (Abramis brama), roach (Rutilus rutilus), carp (Cyprinus carpio), and bleak (Alburnus alburnus). Trout and perch have one-chambered swimbladders, and the other species have two-chambered swimbladders. In general, the lowest TS was recorded when the head or tail faced the sonar beam and the highest values when the side was perpendicular to the sonar beam. The TS distribution differed between fish with one- and two-chambered swimbladders. For two-chambered fish, the TS transition between minima and maxima was linear, and the median of the TS value was similar to the mean. For one-chambered fish, TS distribution was skewed and the median TS was considerably smaller than the mean. The relationship between TS and fish aspect relative to the sonar beam, can be described by a linear model for fish with two-chambered swimbladders and by a quadratic model for fish with one-chambered swimbladders. This study provides new general equations for modeling the dependence of TS on any body aspect in the horizontal plane as well as equations for common European fish species.

Research highlights
► Pattern of TS during horizontal rotation differs between one- and two-chambered swimbladder fish.
► For two-chambered fish, the median of the TS distribution is equal to the mean.
► For one-chambered fish, TS distribution is skewed the median is considerably smaller than the mean.
► General equations for the dependence of TS on any body aspect in the horizontal plane is presented.