Tunnies and bonitoes are closely related to the mackerels, differing in the presence of enlarged scales on the pectoral region, forming a corselet, and of a longitudinal keel on each side of the caudal peduncle. They live chiefly in the open ocean, wandering in large schools, preying upon other pelagic fish, and approaching land only when attracted by the abundance of some special food. Most of the species have consequently a very extensive distribution, and the true tunny is almost cosmopolitan, although of rare occurrence in the north or at corresponding latitudes in the southern hemisphere. They are among the most beautiful and powerful of fishes, and admirably adapted to rapid motion. In connection with their extremely active life, allusion should be made to the fact, first ascertained by John Davy in 1839, that the temperature of the blood of a tunny may be considerably higher than that of the surrounding water, a discovery which disposed of the time-honoured division of vertebrate animals into warm-blooded and coldblooded.
The Tunny, which is of such enormous commercial importance in Spain and Portugal and on the south and west coasts of France, and reaches a length of 9 or 10 feet, has 13 or 14 spines in the anterior dorsal fin, 7 to 9 finlets above and as many below the tail, and the pectoral fin not longer, usually shorter, than the head. Its appearances on our coasts are very irregular, and never in great numbers, but it is regularly fished for off the northwest coast of Brittany.
Although of no great reputation as a game-fish in the Atlantic, the tunny is much sought for by anglers on the coast of Southern California. Professor Holder, whom we have quoted above a propos of the black sea-bass, says : " The most sensational fish of these waters is the leaping tuna, which well compares with the tarpon, and personally I prefer it to its Florida and Texas rival, and in my experience the average large tuna is a match for two tarpons of the same size. The tuna is the tiger of the Californian seas, a living meteor which strikes like a whirlwind, and when played with a rod that is not a billiard cue or a club in stiffness, will give the average man the contest of his life."
A second species of tunny is known as the Alba-core, or Germon (T. alalongd), distinguished from the preceding by the longer, sickle-shaped pectoral fin which, in the adult, measures nearly one-third or two-fifths of the total length. Its distribution is nearly as extensive as that of the typical species, but as a food-fish it is of much less value, the flesh being coarse and oily, and it only grows to about three feet. It is common in the south of Europe, but only a few stragglers have occurred on our coasts as far north as the Orkneys. Large quantities are captured in the southern parts of the Bay of Biscay in July and August.
Both species of tunny are the object of important fisheries on the coast of Portugal, and their variations and movements have been studied with special care by the King of Portugal, who has published a large illustrated memoir, entitled "A Pesca do Atum no Algarve im 1898, por Don Carlos de Braganca" (Lisbon, 1899), with a French r6sum£. The excellent figures and the charts accompanying this memoir should prove of great utility to tuna fishers.
Other less important Scombrids of exceptional occurrence on our coasts are the bonito (Euthynnus pelamys), the pelamid (Sarda mediterraned), and the plain bonito (Auxis rochet).