Variations in the target strength of Atlantic cod during vertical migration

Experiments conducted at sea in June 1999 and January 2000 indicated that the mean target strength (TS) of Atlantic cod (Gadus morhua) varies not just with length, but also with pressure (depth) and fish orientation, particularly during vertical migration. In June, when fish were migrating and spawning, vertical migration was pronounced, extending up to 150 m off the seabed, and the associated TS declined by as much as 5 dB. In January, when the fish were located nearer the seabed, mean TS was more stable and matched a conventional model of TS vs. length [L, cm; i.e. TS = 20 log(L) - 66] based on measurements of ex situ fish orientated horizontally and positioned at close range. This paper demonstrates that mean TS is inversely related (p < 0.05) to the range off the seabed (r, m), which includes 90% of fish. Based on this finding, a new multivariate TS model is proposed: TS = 20 log(L) - 65 - 0.05 r. In this model, r is a proxy for swimbladder volume and fish orientation. A survey in May 2007 found that cod (mean L = 63 cm) dispersed in such a way that single targets could be resolved up to 100 m from the seabed. Measurements of TS of in situ individual fish (TS(individual)) and mean TS inferred from a comparison of area-backscattering coefficients (s(a)) and count-based densities (TS(indirect)) were positively correlated with a slope not different from unity. Means of these TS estimates were -32.4 and -32.2 dB, respectively, or approximate to 2.3 dB less than that predicted by the conventional model. In contrast, the new multivariate model predicts TS = -32.5 dB, which is nearly identical with the means of TS(individual) and TS(indirect).