It is well known that the skipjack shoals, Katsuwonus pelamis, are often accompanied by birds, a whale, Physeter catodon, or a shark, Rhincodon typus, a timber or a floating object, and are often seen near to a bank in the ocean.
In this paper, the author has attempted to examine the frequency of the same fish size group and variations in the condition factor by a study of the body size composition of the fishes in each age group in the shoals.
The following results were obtained;
1) The shoals with birds ranged from 25cm to 80cm in body size and are mostly composed of the same age groups.
2) The fish sizes of the shoals with timber and a shark or a whale ranged from 25cm to 60cm and age groups of the shoals are constituted of more than two.
3) Each shoal of fish near banks is composed of only one age group.
4) The relationship between the body length (x) and the condition factor (y) of skipjack is as follows;
Shoal with birds y=13.20+0.14x
Shoal with a timber y=27.03-0.20x
Shoal with a shark or a whale y=24.31-0.10x
