Gasterochisma fallai (Serventy, 1948) The slender tuna, Allothunnus fallai, is a species of tuna, the only species in the genus Allothunnus, found around the world in the southern oceans between latitudes 20° and 50° South, although there are two records of probable vagrants, one in Los Angeles Harbour and the other from the North Pacific subarctic gyre.

[1] It has a more elongated body than other species of tuna with which it is symaptric such as the albacore The colour is blue-black on the back with silvery greyish-white sides, however some individuals have a coppery sheen soon after capture.

It has a small second dorsal and anal fins resembling a small albacore, but the slender tuna lacks the long sweeping pectoral fins characteristic of albacores.

[4] Its length is up to 1 metre (3.3 ft)[2] and it can weigh up to 12 kilograms (26 lb).

The high concentration of omega-3 polyunsaturated fatty acids in the flesh of this species caused the CSIRO to declare that the slender tuna was Australia's healthiest seafood dish.