Apparent Abundance, Distribution, and Migrations of 



Albacore, Thunnus alalunqa , on the 



North Pacific Longline Grounds 



By 



BRIAN J. ROTHSCHILD,' Fishery Biologist 



and 



MARIAN Y. Y. YONG, Mathematician 



Bureau of Commercial Fisheries Biological Laboratory 

 Honolulu. Hawaii 96812 



ABSTRACT 



This paper considers the dynamics of albacore, Thunnus alalunga, on the 

 Japanese North Pacific longline grounds. In addition to changes in apparent 

 abundance and distribution, the modes of immigration and emigration from the 

 longline grounds are considered in terms of the migratory route of the albacore 

 in theNorth Pacific Ocean. The data show a clear decline in apparent abundance 

 on the longline grounds during the 1949-61 study period. This decline could not 

 be related to changes in the average size of the fish or fishing effort. Spatial 

 statistics were computed to describe the distribution of the albacore on the 

 longline grounds. These show very clear cyclical changes each year. These 

 changes reflect a net southwest movement of the two-dimensional first-moment 

 at a velocity of about 6.5 miles (12 km.) day"'. The second order spatial sta- 

 tistics showed a maximum longitudinal expansion and latitudinal contraction 

 during the peak of the fishing season. The time-space coordinates of the two- 

 dimensional first-moment of the albacore distribution appear to be highly pre- 

 dictable. The migration route of the albacore among North American, pole- 

 and-Iine, and longline fisheries is considered. It appears that the dynamically 

 most significant movement of albacore is from the North American fishery, to 

 the pole-and-line fishery, to the longline fishery. 



INTRODUCTION 



The albacore, Thunnus alalunga . of the Pa- 

 cific Ocean apparently comprise two popula- 

 tions. One of these populations lives south of 

 the Equator and has been fished by the Japanese 

 longline fishery at a relatively low intensity 

 from the early 1950's until 1956. After 1956 

 the intensity of longline fishing for albacore in 

 the South Pacific Ocean increased substantially; 

 this increase has been documented by Otsu and 

 Sumida (1968). The other population--the sub- 

 ject of the present paper--lives in the North 



Pacific Ocean. It has been exploited by Japa- 

 nese since the inception of the Showa Era in 

 1926 (Nakamura, 1951: p. 14 in English trans- 

 lation). 



In the 1950's, there were essentially three 

 fisheries for albacore in the North Pacific 

 Ocean: (1) the summer troll and pole-and-line 

 fisheries by United States and Canadian fisher- 



^Present address: Fisheries Research Institute 

 and Center for Quantitative Sciences in Forestry, 

 Fisheries, and Wildlife, University of Washing- 

 ton, Seattle, Wash. 98105. 



