Gallery 3.2: Bigeye and Yellowfin Tuna
Yellowfin and bigeye tuna can be hard to distinguish, especially when they are not yet mature. Of the nine juvenile tuna in this photo, three are yellowfin, and six are bigeye. The following photographs will illustrate the differences between the two species, and provide some comparative examples. (Photo: David Itano)
Yellowfin tuna shorter than 50 cm (top) and between 50 and 100 cm (bottom) (Photo: K. Schaefer)
At the same length, juvenile yellowfin (top) and bigeye (bottom) look similar but not identical. (Photo: David Itano)
Examples of yellowfin tuna under 50 cm. Note the closely spaced, alternating pattern of lines and dots that extends from the tail to beneath the pectoral fin and to above the midlateral body line, with clear demarcation between the marked and unmarked regions. (Photo: David Itano)
Bigeye tuna at shorter than 50 cm (top) and between 50 and 100 cm (bottom) (Photo: K. Schaefer)
Comparison of small yellowfin (top and bottommost fish) and small bigeye (second and third fish from the top). Though there are characteristics of coloration that can distinguish bigeye from yellowfin, the colors fade very quickly after death and therefore are not a reliable means when identifying species during a purse seine fishing operation. Note that the pink color is a reflection in the photo and not a color of the fish. (Photo: David Itano)
Examples of larger yellowfin (top and bottommost fish) and bigeye (second and third fish from the top). In the top photo, note the yellowfin’s smaller head and eye size. When undamaged, the pectoral fin for fish under 150 cm can be a useful secondary diagnostic, since the bigeye’s is longer, more flexible, and tapers to a thin point. (Photo: David Itano)
Examples of bigeye tuna under 50 cm. Note that the white lines that are both irregularly spaced and sized, with few to no rows of dots present, and the demarcation between the marked and unmarked region is generally not clear. (Photo: David Itano)
Large adult yellowfin have a streamlined, elongated body shape (above left), while large bigeye have a more rounded and deep body shape (lower right). (Photo: K. Schaefer)
Yellowfin pectoral fins (top left) are straight and stiff, while bigeye pectoral fins (top right) make a smooth arc and have flexible tips. The caudal fin in a yellowfin has two ridges that make a distinctive V-shaped notch at the center of the fin, while the same area on the bigeye’s caudal fin is relatively flat. (Photo: David Itano)