Information on natural mortality rates of the mackerel icefish, Champsocephalus gunnari, from the period prior to large scale commercial harvesting is discussed. Methods based on population age structure and growth parameters are used. It is concluded that only in the three earliest years, 195 1, 1964 and 1966, do the data reflect an unexploited population. The best estimate of M from analyses of the data from these seasons is 0.22. It is suggested that large scale fishing was in progress prior to the reported commencement in 1970.