Fish orientation is consistently identified as a major influence on fish target strength (TS). Generally, orientation is defined as the tilt angle of the fish with respect to the acoustic transducer, whereas a more accurate definition includes tilt, roll, and yaw. Thus far, the influences of roll and yaw on fish TS have only been examined cursorily. We used in situ single-target data to create fish tracks, to estimate fish tilt and yaw, and correlated these estimates with TS. The results show that tilt, yaw, and beam position have a significant influence on fish TS. To investigate further how yaw and beam position affect TS, we calculated the expected backscatter from each fish within simulated fish aggregations using a backscatter model. The TS of individual fish at 38 and 120 kHz varied by as much as 11 and 19 dB with changes in yaw and beam position. Altering the fish�s tilt, yaw, and beam position resulted in TS differences of 14 and 26 dB at 38 and 120 kHz, respectively. Orientation had a minimal influence on an aggregation�s average TS if the aggregation had a variable tilt-angle distribution and was dispersed throughout the acoustic beam.
Changes in the swimming direction of a fish resulted in TS differences, regardless of the tilt angle. Figure 1 is a polar plot that summarizes the effect of yaw and beam position on TS at three different tilt angles (58 head down, horizontal, and 58 head up). Predicted values of TS were contoured at increasing distance from the acoustic axis, but still within the main lobe of the transducer (Figure 2).
The largest TS differences were found when a fish was orientated perpendicular to the vessel (yaw 908 or 2708). In contrast, when a fish was aligned in the same as or the opposite direction to the vessel (yaw 08 or 1808), the TS was equal at all beam positions (Figure 1). A fish directly on the acoustic axis always had the same TS regardless of yaw. As a fish moves farther from the acoustic axis, the greater the influence yaw will have on TS. This effect is generally more pronounced at 120 kHz than at 38 kHz. At 38 kHz, a fish with a tilt angle of 58 �head down�, 08, or 58 �head up� had maximum TS differences of 7, 11, and 5 dB with changes in yaw and beam position (Figure 2a�c), respectively. At the same tilt angles, a fish measured at 120 kHz had maximum TS differences of 10, 19, and 5 dB with changes in yaw and beam position (Figure 2d and e).
The swimming direction of a fish can have a great influence on TS if the fish is not directly on the acoustic axis. This effect is most pronounced when a fish enters or exits the acoustic beam on a trajectory that passes through the acoustic axis. In such a situation, the incident acoustic beam strikes the fish on the head or tail, effectively changing the tilt angle of that fish. Although TS differences were large for an individual fish, comparison of TS distributions from simulated aggregations with a fixed (schooling) or random yaw (shoaling) showed no significant differences if fish were dispersed within the acoustic beam.