Role of wax esters in determining buoyancy inHoplostethus atlanticus (Beryciformes: Trachichthyidae)

Specimens of the orange roughyHoplostethus atlanticus were caught at the edge of the Chatham Rise, east of the South Island of New Zealand, in June 1988, during a commercial fishing voyage. Several tissues of the fish were found to contain large quantities of lipid almost entirely as wax esters. These tissues include a fat-invested swimbladder, the skin, skull and spine, and a fat-infiltrated tissue found in the neurocranial cavity. The physical properties of the wax esters have been examined relative to the temperatures that exist where the fish are found. Differential scanning calorimetry and NMR spectroscopy both suggest that at 6°C, the temperature at which the fish are located in the water column, the lipid is partially solid. However, at 14°C, the temperature at the surface where the fish are positively buoyant, the lipid is completely liquid. It is proposed that the increased density of the lipid at 6°C would make the fish neutrally buoyant in its normal habitat. The location of the lipidrich tissues has been examined relative to their possible buoyancy contribution, and a histological examination of the swimbladder has been performed.