Does the seasonal variation in fat content of blue whiting affect the acoustic conversion factor (TS)

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Blue whiting is a physoclist gadoid distributed in the Northeast Atlantic. The catches in recent years have ranged between 1-1.7 mill tonnes. The stock is currently being assessed from VPA and acoustic surveys in the spawning season. The latter usually yielding estimates twice the size of the former. Acoustic estimates in late summer in the Norwegian Sea in the early 80's, however, yielded lower acoustic estimates compared to those obtained during the spawning season the same year. The total fat content of blue whiting varies significantly during the year. Being at a minimum in April/Mai after spawning and at a maximum in August at the height of the feeding season. The gas filled swim bladder represents most of the acoustic back scattering energy (echo) and any changes in the volume of the swim bladder would affect the echo. If it is assumed that blue whiting is at neutral buoyancy in seawater, then changes in fat content, which has a lower specific density than seawater, may affect the amount of gas needed to attain neutral buoyancy. By simple modelling of the swimbladder we find an inverse relationship between the acoustic target strength and fat content, and seasonal variations in fat content of 7% might lead to a bias of up to 12% in acoustic biomass estimates, if they were compared between the low-fat and high-fat period. New in situ target strength measurements of blue whiting are asked for, also covering the natural variation in fat content of the fish.