We evaluated the condition factor (K), an index of weight per unit length, in a sample of 49 761 longline-caught specimens of Antarctic toothfish, Dissostichus mawsoni, from the Ross Sea. Our sample consisted of specimens from northern (60–70°S) and southern (70–78°S) regions. Fifty percent of the northern sample and 4.9% of the southern sample were large individuals with a low condition factor (Kah ≤ 1.0182) and displaying an “axe handle (ah)” morphology with a notably thinner trunk. In the northern sample 55.4% of the males and 43.2% of the females were less than Kah. In the southern region 5.3% of the males and 4.5% of the females were less than Kah. The axe handle morphology is attributable to the metabolic loss of muscular and subcutaneous lipid stores, and probably proteins, from white muscle. Our discussion considers energy metabolism as related to migration, feeding and reproduction. We suggest that migration, a life history trait of the phyletically basal notothenioid clades Pseudaphritis and Eleginops, persists in D. mawsoni. The spawning migration from the southern shelf to ridges and seamounts in the north may reflect fidelity to historic spawning grounds. As evidenced by the axe handle specimens neutral buoyancy of large D. mawsoni may be ephemeral, fluctuating over the course of adult life.