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McQuinn, I. H., and Winger, P. D. 2003. Tilt angle and target strength: target tracking of Atlantic cod (Gadus morhua) during trawling. - ICES Journal of Marine Science, 60: 575-583. Vertical orientation (tilt angle) is known to affect the target strength (TS) of ensonified fish and is a large component of the variability inherent in acoustic-biomass estimates. To measure the effects of changes in tilt angle on TS during diel vertical migrations, a concentration of migrating Atlantic cod (Gadus morhua) was observed acoustically from a research vessel over several days. Single-target data were collected from a split-beam echosounder and were subsequently tracked, corrected for vessel orientation and move- ment, and analysed for 3-dimensional displacement (speed and direction). The results revealed a large variability in TS and several patterns of swimming behaviour from random to directed orientation and movement, with changes in both vertical and horizontal dis- placements and inferred orientation. These behavioural patterns and their affects on TS were analysed as a function of ''time-since-sunset''. Regular diel orientation patterns were ob- served as cod rose from the ocean bottom in the evening, increasing their tilt angle, and descended at sunrise to regain the ocean floor. Standardized TS (B20) was found to be highly correlated with tilt angle. This relationship can be used to correct for the diel changes in the TS of these migrating cod as a function of the in situ-measured tilt angle and thus to improve the accuracy of acoustic-biomass estimation. Crown Copyright 2003 Published by Elsevier Science Ltd on behalf of International Council for the