USE OF ACOUSTICS TO ASSESS A SMALL AGGREGATION OF ORANGE ROUGHY, HOPLOSTETHUS-ATLANTICUS (COLLETT), OFF THE EASTERN COAST OF TASMANIA

A relatively small aggregation of orange roughy (Hoplostethus atlancti cus) was located in April 1989 off the eastern coast of Tasmania. A Si mrad EK400 (38 kHz) scientific echo-sounder was used to survey the agg regation over a period of eight days, during which time the aggregatio n was commercially fished. The aggregation was confined to an area of approximately 4 km2, with the dimensions of the aggregation varying wi thin and between days. High densities of orange roughy were located ne ar the bottom on some days and more than 24 m off the bottom on others . Average fish densities during the survey and an estimate of the extr emes of densities (fish m-3) are presented. Estimates of the original biomass of this aggregation as obtained from acoustic data and commerc ial catch-and-effort data are compared, and the mean target strength o f the population is estimated.