Ecological Characteristics of Skipjack Tuna Schools Associated with Subpopulation Distinction in the Western Pacific Ocean

Fujino Kazuo

doi:10.2331/fishsci.62.866

On the basis of the clear distinction between fish schools from the two subpopulations of skipjack tuna Katsuwonus pelamis in the western Pacific Ocean reported already, an attempt was made to clarify the ecological characteristics of fish schools of the central-west Pacific subpopulation. Comparisons of body length frequency distribution as well as the relationship between body length and weight between fish schools of the western Pacific subpopulation and those of the central-west Pacific subpopulation, which both migrated to the east coast region of themainland of Japan, showed the following: 1) marked differences were observed between the two subpopulations on the modal sizes of fish schools sampled in the latter half of the fishing season, August through November, 2) analyses on the seasonal progression of body length frequency distribution strongly suggested the probable occurrence of three spawning seasons a year in the western Pacific subpopulation, but it is necessary to examine where the anticipated fish schools of the central-west Pacific subpopulation originating from one of the three spawning seasons migrate, and 3) fish of the central-west Pacific subpopulation were slightly fatter than those of the western Pacific subpopulation on average.

Ecological Characteristics of Skipjack Tuna Schools Associated with Subpopulation Distinction in the Western Pacific Ocean

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