UC San Diego Library – Scripps Digital Collection

The Bureau of Marine Fisheries has been fortunate in obtaining specimens of tuna from the Atlantic Coast and from Australian waters. The fish in both samples belong to the genus Thunnus. The Atlantic form is generally accredited to the species Thunnus thynnus, and the Australian southern bluefin is Thunnus maccoyii. The Australian northern bluefin, Kishinoella tonggol, does not enter into this discussion. The bluefin tuna of the Pacific Coast of the United States is also included in the species Thunnus thynnus. The Pacific Coast bluefin was extensively described by Godsil and Byers (1944), and the acquisition of the Atlantic and Australian specimens afforded an exceptional opportunity to make a thorough comparison of the three varieties. The present study was therefore modelled on the procedure outlined by Godsil and Byers, and the same characters were investigated. The measurements used, the technique and the nomenclature adopted throughout are identical.

The present study was undertaken to clarify the systematic relationship of the various tunas. The classification of the tunas is confused, and has been based largely upon the examination of variable external characters in relatively few specimens. Such characters have led in some cases to unwarranted specific separations, while in others they have obscured more significant differences. Our particular purpose in this study was to determine whether positive, diagnostic differences exist between the Atlantic bluefin and our California bluefin, on the one hand, and between the Australian southern bluefin and the California bluefin on the other hand. Accordingly, we looked for characters, constant within the sample, which would enable one to distinguish fish from these three distinct geographical regions.

Unfortunately the dissections were not made at the same time. However the Atlantic and Australian specimens were examined within a relatively short interval, when the recollection of detail was still vivid. Because the dissections of the California bluefin had been made about seven years earlier, the authors repeated the entire routine examination upon three local specimens in the interval elapsing between the study of the Atlantic and Australian samples. Available data from the measurement and dissection of all California bluefin have been used in this report, and for this reason the number of fish and the size range shown in Table 1, and discussed in the text, varies in the different proportions. Complete measurements were not made upon all these fish.

The scope of the study was as extensive as time and circumstances permitted. Most of the Atlantic specimens had been damaged in one region or another and injections proved difficult. This is invariably true of commercially caught fish. The Australian specimens were in generally good condition, but each had been bled by cutting through the isthmus and severing the ventral aorta or the heart. This was a severe handicap in the visceral injections. All the material was frozen and stored for a period, and in consequence color notes and external markings were of little value. Each fish was necessarily thawed completely before examination. The amount of detail that could be observed was limited by the gradual softening and deterioration in the tissues.

Those characters were selected for examination which had been found in the earlier studies to differ most in the several species investigated. Where differences between the samples were discovered or suspected, particular care was thenceforth devoted to such characters. In the study of the bones comparisons were made of the entire skeleton: but only those elements are described which showed or suggested differences.

Because the three varieties were so similar, a complete description of each character in each sample would entail endless and unnecessary repetition. To avoid this, each character in the Atlantic bluefin is described in detail and the Australian southern bluefin is compared with this description. If it is identical, this fact is so stated. If differences exist, the features differing are described in detail. Finally both the foregoing descriptions are compared with earlier findings in the California bluefin. Similarities with either or both are noted, and differences emphasized.

Throughout the descriptive part we have arbitrarily selected a common name to designate each variety. Because two of them are generally known as bluefin, we have applied that name also to the Atlantic variety. A discussion of the specific relationship of the three varieties is given in the final section.