Istituto di Scienze Marine     
Azzali M., Leonori I., Biagiotti I., De Felice A., Angiolillo M., Bottaro M., Vacchi M. Target strength studies on Antarctic Silverfish (Pleuragramma antarcticum) in the Ross Sea. In: CCAMLR Science, vol. 17 pp. 75 - 104. CCAMLR, 2010.
Target strength (TS) of preserved (frozen and defrosted) Antarctic silverfish (Pleuragramma antarcticum) of total length (TL) from 110 to 202.5 mm were measured ex situ (Ancona Bay) at 38, 120 and 200 kHz in May 2007 and in February 2009. Overall ex situ TS-TL relationships were: TS38 = 36.83Log TL(cm) -103.62 (N = 18); TS120 = 34.75Log TL(cm) -84.20 (N = 19); TS200 = 26.71Log TL(cm) -73.74; (N = 16). Nine in situ acoustic trawl experiments targeted at juveniles and post-larvae of P. antarcticum with a total length from 13 to 70 mm, were conducted in the southeast of the continental slope and shelf of the Ross Sea at depths from 30 to 120 m, during the Italian expeditions of 1997/98, 2000 and 2004. The regression lines for estimating TS from nine experiments were: TS38 = 57.58Log TL(cm) -123.77; TS120 = 51.94Log TL(cm) -105.92; TS200 = 46.61Log TL(cm) -95.29. These results indicate that the TS-TL relationships at 38, 120 and 200 kHz of adults of P. antarcticum differ consistently from those of post-larvae and juveniles. However, a simple model derived from the geometrical and acoustic characteristics of different parts of adult P. antarcticum and designed to fit fish life stages from adults to post-larvae and frequencies from 38 to 200 kHz, can predict the experimental TS measured both ex situ and in situ with a mean difference <1.5 dB, although the differences between the predicted and measured TS at some fish lengths were high (from 7.16 to -3.14 dB). The model shows that the main contribution to the scattering for small fish is from soft tissues and for larger sizes from hard tissues (e.g. skeletal structures). This may explain why small and large fish have different TS-TL relationships. In the authors' opinion the model could be used to give a first indication of the absolute abundance of P. antarcticum in the Ross Sea. A critical point is the decision rule to be used to differentiate P. antarcticum from Euphausia superba and E. crystallorophias that are present in the same region of the Ross Sea. Following the threefrequency decision criterion described in Azzali et al. (2004), some sizes of P. antarcticum could be confused acoustically with E. crystallorophias, but not with E. superba.
Subject Antarctic silverfish
Pleuragramma antarcticum ex situ target strength
in situ target strength
target strength models

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