Bias in acoustic biomass estimates of Euphausia superba Dana to diel vertical migration

The diel vertical migration 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 independent 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 for this study was collected in the vicinity of Elephant Island, Antarctica during the Austral Summer of 1992. The data includes five surveys conducted from mid-January to mid March. The first and fourth surveys covered a 105 by 105 n.mile study area centered on Elephant Island; the second and third surveys covered a 60 by 35 n.mile area immediately north of Elephant Island; the fifth survey covered a 1 n.mi.2 area centered on a large krill swarm to the west of Seal Island. Average krill volume densities were calculated for each hour as well as for three daily time periods; day, twilight and night; these data were normalized and presented as a probability of daily average density. A function was fit to the probability of average daily biomass versus local apparent time. This function was used to create a temporal compensation function(TCF), for upwardly adjusting acoustic biomass estimates due to diel vertical migration. The TCF was then applied to the original survey data; the resulting biomass estimates are 2.3 to 99.6% higher than those calculated disregarding biases due to diel vertical migration.