Target-strength studies on Atlantic cod (Gadus morhua) in Newfoundland waters

Acoustic target strengths (TS) of Atlantic cod (Gadus morhua L.) of lengths 12-65 cm were measured ex situ at 38 and 120 kHz (dual beam) under continuous video monitoring. Each fish swam within an acoustically inert monofilament mesh bag secured 10 m beneath fixed location transducers. Overall ex situ target strength-length (L, cm) relationships were TS=20 log L (cm)-66 at 38 kHz and TS=20 log L (cm)-65 at 120 kHz (n=16). Cod TS peaked at aspects of 5-10 degrees off horizontal (head-down). Full rotation directivities spanned 20dB. Stationary variations of low amplitude (<3 dB) correlated with swimming movements. A linear model was used to study in situ TS (38 kHz) from 49 acoustic-trawl experiments targeted at cod and associated species at depths from 200-375 m. Significant factors were species composition (40% of variance), depth distribution (20%), and time of day (5%; total r(2)=0.65). Modelled TS (standardized to cod) was 1.5 dB higher at night when cod were up to 140 m off bottom. Night-time TS did not differ from the ex situ TS-daytime TS was significantly lower by 1.5 dB. Our findings are consistent with cod being neutrally buoyant off bottom (night) and negatively buoyant nearer bottom in the day-time. Pooled neutrally buoyant in situ and ex situ TS (38 kHz) indicated the relationship (TS=20 log L-66, r(2)=0.80, n=39). (C) 1996 International Council for the Exploration of the Sea