PELAGIC BIOGEOGRAPHY OF THE ARMORHEAD, PSEUDOPENTACEROS 

 WHEELERI, AND RECRUITMENT TO ISOLATED SEAMOUNTS 



IN THE NORTH PACIFIC OCEAN 



George W. Boehlerti and Takashi Sasaki^ 



ABSTRACT 



The pelagic armorhead, Pseudopentaceros wheeleri, occurs widely in the North Pacific Ocean. Benthic 

 specimens have been taken from Japan, the Hawaiian Archipelago, and the west coast of North America, 

 but the main reproductive populations are located on southern Emperor-northern Hawaiian Ridge sea- 

 mounts between lat. 29° and 35°N. The period between spawning and recruitment to the seamounts 

 is apparently between 1.5 and 2.5 years, suggesting an extended pelagic existence. We describe the 

 distributional patterns in the North Pacific based upon over 30 years of published and unpublished records. 

 The majority of pelagic specimens are captured in the subarctic water mass in the northeast Pacific. 

 Based upon the distributional patterns and the oceanography of the North Pacific, we propose migratory 

 paths for both the main population and for the individuals that occur rarely in other locations. The long 

 pelagic period and variability in ocean conditions may play an important role in recruitment to seamounts 

 and the variability in year-class strength for this species. 



The pelagic armorhead, Pseudopentaceros wheeleri, 

 is a member of the boarfish family Pentacerotidae. 

 Until recently, considerable confusion existed about 

 its taxonomy and distribution. Originally described 

 as Pentaceros richardsoni Smith 1844, it was 

 thought to be distributed virtually worldwide, with 

 centers of abundance in the North Pacific and in the 

 South Atlantic near South Africa (Fujii 1986). Oc- 

 currences were typically sporadic; most records 

 were from pelagic captures, hence the name pelagic 

 armorhead. Several authors, including Welander et 

 al. (1957), Clemens and Wilby (1961), and Wagner 

 and Bond (1961), used the name Pseudopentaceros 

 richardsoni, but more recent studies used the 

 original binomial following Follett and Dempster 

 (1963). 



The 1967 discovery of large concentrations of 

 armorhead in the mid-Pacific, over southern 

 Emperor-northern Hawaiian Ridge (SE-NHR) 

 seamounts (from lat. 29° to 35°N) (Sasaki 1974; 

 Uchida and Tagami 1984) stimulated increased 

 interest in this species. Over the next 10 years, 

 nearly 1 million metric tons (t) were taken by Soviet 

 and Japanese trawlers (Borets 1975; Takahashi and 

 Sasaki 1977). The increased availability of specimens 

 allowed Hardy (1983) to revise the family. He first 



'Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, HI 

 96822-2396. 



^Far Seas Fisheries Research Laboratory, 5-7-1 Orido, Shimizu, 

 Shizuoka 424, Japan. 



separated the Pentaceros richardsoni complex 

 from other members of the genus by referring it to 

 Pseudopentaceros Bleeker, based on morphological 

 considerations. Second, he separated the complex 

 into three species: P. richardsoni confined to the 

 Southern Hemisphere, and P. wheeleri and P. 

 pectoralis in the North Pacific. The features used 

 to distinguish P. wheeleri and P. pectoralis were 

 largely morphological. Pseudopentaceros pectoralis 

 is more robust and deeper bodied and its dis- 

 tribution is typically pelagic; P. wheeleri was 

 known only from benthic specimens on sea- 

 mounts. Hardy (1983), however, lacked transitional 

 specimens; subsequent work based on morpho- 

 logical and electrophoretic grounds revealed that 

 P. wheeleri was a morphological derivative of 

 P. pectoralis, with changes occurring after settle- 

 ment. The deeper bodied pelagic form ceases growth 

 in length after recruiting to the seamounts and 

 instead transforms body shape to a leaner form. 

 "Fat" and "lean" specimens of the same length thus 

 differ significantly in appearance (Humphreys et al. 

 in press). 



Pseudopentaceros wheeleri has an unusual life 

 history. Spawning occurs at the benthic population 

 centers at the SE-NHR seamounts between Novem- 

 ber and March (Sasaki 1974; Borets 1976; Bilim et 

 al. 1978). Larvae and juveniles are pelagic (Honma 

 and Mizusawa 1969; Fedosova and Komrokov 1975; 

 Borets and Sokolovsky 1978) and widely distributed 

 in the North Pacific. Recruitment to the seamounts 



Manuscript accepted April 1988. 

 Fishery Bulletin; Vol. 86, No. 3, 1988. 



453 



