FISHERY BULLETIN: VOL. 86, NO. 3 



tant diet component of sei whales taken by the 

 Japanese commercial fleet. This species normally 

 feeds on zooplankton and micronekton (Kawamura 

 1982), but in this year, 195 whales were recorded 

 to have fed extensively on armorhead. Although 

 data records for whale stomach contents extend 

 from 1952 to 1979, 1969 was the only year when 

 large numbers fed on armorhead, with the excep- 

 tion of two occurrences each in 1971 and 1972. 

 Based on the known feeding mode of sei whales, 

 Chikuni (1970) suggested that the armorhead ap- 

 parently schools in surface waters. In the 1970's, 

 catches were relatively small, but in the 1980's, 

 specimens have been captured in each year, with the 

 greatest catches in 1985. In that year, fish occurred 

 in more easterly areas as compared to other years. 



Relationship of Distribution Patterns 

 to North Pacific Oceanography 



The SE-NHR seamounts are located in the mid- 

 Pacific transition zone, between subarctic and sub- 

 tropical water masses (Roden 1970). The spawning 

 season of armorhead typically lasts from late 

 November through March (Sasaki 1974; Bilim et al. 

 1978), and larvae are neustonic, at least through the 



first few months of life (G. W. Boehlert unpubl. 

 data). Near surface drift in this region is largely 

 wind driven (McNally 1981); as shown by Lagran- 

 gian drifters, mean surface flow in the SE-NHR sea- 

 mount region during winter months is typically 

 eastward (Kirwan et al. 1978; Emery et al. 1985) 

 or southeastward (McNally et al. 1983), and long- 

 term mean Ekman transport has a southeastward 

 component (Favorite et al. 1976). Based on mean 

 ship-drift data, progressive vector diagrams over a 

 3-mo period, with starting points at four SE-NHR 

 seamounts (Bakun^), show this typical eastward flow 

 with a southward component (Fig. 3). If pelagic 

 armorhead followed such a pattern, individuals 

 would remain in the subtropical gyre, continuing 

 eastward and then turning south, entering the 

 California Current system. The scarcity of speci- 

 mens in this region (Figs. 1, 2) suggests instead that 

 most fish move northeastward in some manner, 

 entering subarctic waters and residing within the 

 Alaska gyre; surface drift in winter is northeastward 

 in anomalous years (McNally 1981), and average 



'Compilation by the NMFS Pacific Fisheries Environmental 

 Group from historic ship drift files assembled by the U.S. Naval 

 Oceanographic Office (A. Bakun, Pacific Fisheries Environmen- 

 tal Group, National Marine Fisheries Service, NOAA, P.O. Box 

 831, Monterey, CA 93942, pers. commun. April 1987. 



36°N 



35°N 



30*N 



25°N 



17rE 



175°E 



180' 



178°W 



Figure 3.— Progressive vector diagrams of mean surface currents (starting date, 15 

 December; duration, 90 days), from long-term mean ship drift data from 1° squares, 

 with distances calculated on a daily basis. Triangles indicate starting locations at four 

 seamounts with armorhead spawning populations. Each mark on the vector between 

 the origin and arrow represents a 5-d period. See text footnote 6. 



458 



