Hoki (Macruronus novaezelandiae) have been commercially exploited in New Zealand waters since the early 1970s. During the 1980s there was a considerable expansion of the fishery which in turn created a need for better information on the size of hoki stocks. The fishery is based on large spawning aggregations which form off the west coast of South Island during winter. The size of this spawning stock has been estimated using hydroacoustic techniques. The acoustic equipment and the survey design have evolved during the period 1985–91. Several practical and theoretical problems were overcome: those of acoustically surveying deepwater fish during winter, and achieving an adequate signal‐to‐noise ratio; and the development of a method for estimating the biomass of a transient fish population. The relative spawning biomass indices derived from west coast South Island surveys are now central to the assessment of the western hoki stock.