reported a close association between swordfish longline
effort and oceanic features, such as shelf breaks
and thermal fronts. Herron et al. (1989) found that the
butterfish catch rate increased with an increase in SST
gradient. These studies show that frontal features
strongly influence the biological productivity of an
area in terms of the presence of secondary producers,
which attract higher trophic levels feeders, such as
Todarodes pacificus. Class 1 and Class 3 fishing areas
formed at the center of the Sea of Japan, around the
Yamato Rise. Isoda (1994) pointed out that the polar
front stretches across latitude 40°N, and Classes 1 and
3 formed to the north and south area of this front,
respectively.