Pergamon Lkrp-Scn Rrmwrh I. Vol 42. No. 4. pp. 45.547375. AY95 Elsevier Science Ltd 0967-0637(94)E0005-4 Prmted in Great Britam ll967N37/94 $Y.5(lf 1).00 Bias in acoustic biomass estimates of Euphausia superba due to die1 vertical migration DAVID A. DEMERIT and ROGER P. HEWITT* (Received 17 December 1993; in revisedform 30 June 1994; uccepted 6 July 1994) Abstract-The die1 vertical migration (DVM) of Antarctic krill (Euphausia superba) can greatly bias the results of qualitative and quantitative hydroacoustic surveys which are conducted with a down-looking sonar and irrespective of the time of day. To demonstrate and quantify these negative biases on both the estimates of biomass distribution and abundance, a time-depth-density analysis was performed. Data were collected, as part of the United States Antarctic Marine Living Resources Program (AMLR). in the vicinities of Elephant Island, Antarctica, during the austral summers of 1992 and 1993. Five surveys were conducted in 1992: two covered a 105 by 105 n.mi. area centered on Elephant Island, two encompassed a 60 by 35 n.mi. arca immediately to the north of the Island, and one covered a 1 n.mi.’ area centered on a large krill swarm to the west of Seal Island. The 1993 data include repetitions of the two small-area and two large-area surveys. Average krill volume dcnsitics were calculated for each hour as well as for three daily periods: day, twilight and night. These data were normalized and presented as a probability of daily average density. With spectral analysis to identify the frequencies of migration, a four-term periodic function was fitted to the probability density function of average daily biomass versus local apparent time. This function was transformed to create a temporal compensation function (TCF) for upwardly adjusting acoustic biomass estimates. The TCF was then applied to the original 19Y2 survey data; the resulting biomass estimates are an average of 49.5% higher than those calculated disregarding biases due to die1 vertical migration. The effect of DVM on the estimates of krill distribution are illustrated by a comparison of compensated and uncompensated density maps of two 1992 surveys. Through this technique, high density krill areas are revealed where uncompen- sated maps indicated low densities. INTRODUCTION EFFECTIVE management of the Antarctic krill fishery requires accurate information about biomass distribution and abundance. To acquire this information over large survey areas, echo integration instruments are used due to their low cost, survey speed and apparent ease of data analysis. However, an application of echo integration theory requires a meticulous and cautious interpretation of the data with respect to many theoretical assumptions. Some major concerns of using echo integration for biomass estimation include calibration inaccuracies (DEMER and HEWITT, 1993); target strength determination and variation (FOOTE et al., 1993); species identification and separation (MADUREIRA et al., 1993a,b; DEMER and HEWITT, submitted), size classification (HOLLIDAY et al., 1989), echo coherence, extinction, shadowing, and multiple scattering (MACLENNAN and SIMMONDS, ‘Supported by the Hertz Foundation. +University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA 92093-0203, U.S.A. tsouthwest Fisheries Science Center, La Jolla, CA 92038. U.S.A. 355