Target strength studies on Antarctic Silverfish (Pleuragramma antarcticum) in the Ross Sea

Target strength (TS) of preserved (frozen and defrosted) Antarctic silverfish (Pleuragrammaantarcticum) of total length (TL) from 110 to 202.5 mm were measured ex situ (AnconaBay) at 38, 120 and 200 kHz in May 2007 and in February 2009. Overall ex situ TS-TLrelationships 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 trawlexperiments targeted at juveniles and post-larvae of P. antarcticum with a total lengthfrom 13 to 70 mm, were conducted in the southeast of the continental slope and shelfof 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 adultsof P. antarcticum differ consistently from those of post-larvae and juveniles. However, asimple model derived from the geometrical and acoustic characteristics of different partsof adult P. antarcticum and designed to fit fish life stages from adults to post-larvae andfrequencies from 38 to 200 kHz, can predict the experimental TS measured both ex situ andin situ with a mean difference <1.5 dB, although the differences between the predicted andmeasured TS at some fish lengths were high (from 7.16 to -3.14 dB). The model shows thatthe main contribution to the scattering for small fish is from soft tissues and for larger sizesfrom hard tissues (e.g. skeletal structures). This may explain why small and large fish havedifferent TS-TL relationships. In the authors' opinion the model could be used to give afirst indication of the absolute abundance of P. antarcticum in the Ross Sea. A critical pointis the decision rule to be used to differentiate P. antarcticum from Euphausia superba andE. crystallorophias that are present in the same region of the Ross Sea. Following the threefrequencydecision criterion described in Azzali et al. (2004), some sizes of P. antarcticumcould be confused acoustically with E. crystallorophias, but not with E. superba.