Abstract.— Pelagic armorhead 

 Pseudopentaceros wheeleri are the 

 target of a directed trawl fishery on 

 many of the southern Emperor- 

 northern Hawaiian Ridge seamounts. 

 The population dynamics of armor- 

 head for the period 1970-90 were 

 reconstructed for Southeast Han- 

 cock seamount, the southernmost of 

 the seamounts commercially fished, 

 by using commercial catch-and-effort 

 statistics, various biological measure- 

 ments, and research stock-survey 

 data. The population declined almost 

 continuously from a 1972 high of 

 5500 metric tons (t) to a 1989 low of 

 25 1. In addition to the intense fish- 

 ery, this decline was due partly to the 

 sporadic pattern of armorhead re- 

 cruitment. Natural mortality rate 

 was estimated as 0.54/year; how- 

 ever, females had a higher mortality 

 rate than males. 



Population dynamics of pelagic 

 armorhead Pseudopentaceros wheeleri 

 on Southeast Hancock Seamount 



David A. Somerton 



Honolulu Laboratory, Southwest Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 2570 Dole Street, Honolulu, Hawaii 96822-2396 



Present address: Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA 

 7600 Sand Point Way NE, Seattle, Washington 981 15-0070 



Bert S. Kikkawa 



Honolulu Laboratory, Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



2570 Dole Street, Honolulu, Hawaii 96822-2396 



Manuscript accepted 5 July 1992. 

 Fishery Bulletin, U.S. 90:756-769 (1992). 



Pelagic armorhead Pseudopentaceros 

 wheeleri have an unusual life history 

 that includes two distinct postlarval 

 phases: a pelagic juvenile phase, and 

 a demersal adult phase. During the 

 first 1.5-2.5 years of their lives (Uchi- 

 yama and Sampaga 1990), juvenile 

 armorhead inhabit the epipelagic 

 zone over a broad area of the north- 

 east Pacific, where they acquire large 

 fat reserves before migrating west- 

 ward to the southern Emperor-north- 

 ern Hawaiian Ridge (SE-NHR) sea- 

 mounts (Boehlert and Sasaki 1988, 

 Humphreys et al. 1989). After arriv- 

 ing at the seamounts, armorhead 

 mature and assume demersal habits. 

 Because of the rigors of spawning or 

 the inability to obtain sufficient prey 

 (Seki and Somerton, In prep.), adult 

 armorhead subsequently lose weight 

 to such an extent that they eventual- 

 ly become emaciated and moribund 

 (Humphreys et al. 1989). 



Armorhead form dense nighttime 

 aggregations over the relatively flat 

 summits of the SE-NHR seamounts. 

 Soon after these aggregations were 

 discovered in 1967, they were sub- 

 jected to intense fishing effort first 

 by Soviet trawlers (Komrakov 1970) 

 and 2 years later by Japanese trawl- 

 ers (Sasaki 1986). The combined 

 annual catch of armorhead rapidly 

 increased and reached a high of 



164,000 metric tons (t) in 1973 

 (Borets 1975, Takahashi and Sasaki 

 1977) before plummeting to 875 1 in 

 1978. This decline in catch was evi- 

 dently due to a decline in armorhead 

 abundance, because the Japanese 

 catch-per-unit-effort (CPUE) showed 

 a corresponding drop from a high of 

 54.0t/hr in 1972 to 0.4t/hr in 1978 

 (Sasaki 1986). 



Although never a participant in 

 this fishery, the United States be- 

 came involved in 1976 when imple- 

 mentation of the Magnuson Fishery 

 Conservation and Management Act 

 extended its exclusive economic zone 

 (EEZ) to include the Hancock Sea- 

 mounts, the southernmost of the 

 SE-NHR seamounts supporting the 

 armorhead fishery. Although Soviet 

 trawlers ceased operations on the 

 Hancock seamounts after the juris- 

 dictional change, Japanese trawlers 

 continued to fish but were subject to 

 an annual harvest quota and were re- 

 quired to carry U.S. observers who 

 monitored the catch. Regardless of 

 these management efforts, catch 

 rates continued to decline and the 

 Japanese discontinued fishing on the 

 Hancock Seamounts in 1984. In re- 

 sponse to the apparent stressed con- 

 dition of the armorhead population, 

 the National Marine Fisheries Ser- 

 vice (NMFS) in 1985 initiated a stock 



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