In situ target strengths of Atlantic cod (Gadus morhua) and Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic

The accurate determination of target strength (TS) is essential in the conversion of area backscattering to fish biomass. We present the first detailed in situ TS measurements of Atlantic cod (Gadus morhua) and Atlantic mackerel (Scomber scombrus) in the Northwest Atlantic. Mean total lengths of cod and mackerel were 52 and 38 cm, respectively. The mean daytime and nighttime in situ TS distributions (split-beam method) of cod were -35.5 (b(20) = -69.8) and -35.2 decibels (dB) (b(20) = -69.5), respectively. There was a depth stratification of cod TS at night, as mean in situ TS was -31.7 dB between 140 and 220 m and -35.8 dB between 220 and 300 m. We argue that the effect of depth on swim-bladder inflation is primarily responsible for the lower cod TS near the bottom than higher up in the water column. Analysis of the same TS data in split-beam or single-beam mode yielded similar distributions, showing the reliability of the single-beam method. The mean in situ TS (single-beam method) of mackerel was -56.4 dB (b(20) = -88.0), with no day-night difference.