A probabilistic approach to calculate the Sv fractions for small pelagic fishes and zooplankton with a bi-frequency system

A common practice in acoustic data post-processing is to set, a priori, the intervals of volume backscattering strength for small pelagic fishes zooplankton, which may induce a bias in biomass estimations. To reduce this, a method based in the joint probability event to find one of these groups was evaluated. In a frequency distribution the when joint probability reaches 0.25 corresponds with the maximum overlap zone between acoustic signals. Fish shoals and zooplankton scattering layers were identified in 35 echograms recorded with a SIMRAD EK60 split beam echosounder (38,120 kHz) in the Gulf of California, México. Acoustic prospections and confirmatory trawls were performed, obtaining in some cases a correspondence of 90% between backscattered energy values and trawl species composition (Engraulismordax). A total of 839 and 843 backscattering strength values were obtained from the echograms. From both frequencies, the "0.25" scenario is reached for a -62 to -56 dB interval. The maximum likelihood estimators (L), calculated from a binomial model were, for fishes L(f)= 0.14 and for zooplankton L(zp)=0.11 respectively, and corresponds to a probability value of=0.48, wich is consistent with the joint probability values previously estimated. The 95% upper (-44 dB, fishes) and 5% lower (-70dB, zooplankton) limits were defined from their probability distribution. Thus, for a bifrequency system of 38-120 kHz, we propose for small pelagic fishes a volume backscattering strength interval from -56 to -44 dB, and -70 to -62 dB for zooplankton in the Gulf of California.