Species identification of pelagic fish schools on the South African continental shelf using acoustic descriptors and ancillary information

Stock assessment of exploited pelagic species often relies on hydroacoustic surveys, but a major limitation of this approach stems from an inability to distinguish between the echoes of co-occurring species. We make acoustic measurements of morphometric, energetic, and bathymetric features of anchovy (Engraulis capensis), sardine (Sardinops sagax), and round herring (Etrumeus whiteheadi) schools, to test the hypothesis that schools of each species can be distinguished on the basis of such acoustic descriptors. School measurements were extracted using commercially available software from acoustic data collected by a vertical echosounder during trawling operations of pelagic stock surveys of the South African continental shelf, November 1997, 1998 and 1999. Principal components analysis of 18 school descriptors of the 214 schools for which the species composition was satisfactorily determined by trawl samples suggested that schools of the three species differed in energetic features and position in the water column, but not in morphometrics. Discriminant function analysis indicated that schools could be correctly identified to species in 88.3% of all cases (94.9% for anchovy, 82.6% sardine, 82.6% round herring). Sardine were distinguished from anchovy and round herring primarily on the basis of school depth and energy, and anchovy from the two other species based on an index of school altitude. Correct classification was not improved by including the ratios of measurements of school features made at a −65 dB processing threshold relative to measurements made at −60 dB. Inclusion of ancillary information (latitude, longitude, sea surface temperature, bottom depth, and time of day) improved the accuracy of school identification to 94.9%, since such variables allowed discrimination on the basis of inter-specific differences in habitat use, as well as in schooling behaviour.