Somerton and Kikkawa Population dynamics of Pseudopentaceros wheelen 



757 



35°N- 



30°- 



25° 



20° 



% 

 \ 



■■■■\ 



c 



SE Hancock Seamount 



Colahan Smt 



Kure Atoll 



^ ^ Midway Island 



^T-?e,„^, 



atv, 



9iia, 



'^h 



^la, 



PACIFIC OCEAN 



'''as 



Main Hawaiian Islands 



175°E 



180° 



175°W 170° 



165" 



160° 



155° 



Figure 1 



Location and bathymetry of Southeast Hancock Seamount. Depth contours are in meters. 



assessment pro-am to monitor armorhead abundance, 

 and in 1986 enacted a 6-yr moratorium prohibiting 

 trawl fishing on the Hancock Seamounts. 



The population dynamics of pelagic armorhead have 

 been previously examined (Borets 1975, Wetherall and 

 Yong 1986); however, the results of these studies are 

 questionable because they were based on either Soviet 

 or Japanese catch-and-effort statistics but not both. 

 The present paper attempts to rectify the problem of 

 incomplete data by focusing solely on the stock of 

 armorhead inhabiting the Southeast (SE) Hancock Sea- 

 mount, where catch-and-effort statistics were supple- 

 mented with various biological and vessel performance 

 data after the initiation of the U.S. observer program 

 in 1977 and were completely replaced by research 

 stock-survey data when the commercial fishery ended. 

 With these additional data, it is possible to obtain ab- 

 solute estimates of armorhead abundance, rather than 

 relative estimates, and to extend the time-series of such 

 estimates beyond the termination of the commercial 

 fishery. In addition to developing a continuous record 

 of armorhead abundance since the initiation of the 



fishery, this paper also includes estimates of the natural 

 mortality rate and annual recruitment of pelagic juve- 

 niles to the seamounts. 



Materials and methods 



SE Hancock Seamount 



The Hancock seamounts consist of two peaks, North- 

 west (NW) and SE Hancock Seamount, separated by 

 61km and situated on the NHR ~293km northwest of 

 Kure Atoll, the northernmost of the Hawaiian Islands, 

 and ~287km southeast of Colahan Seamount, the 

 closest seamount supporting an armorhead fishery 

 (Fig. 1). The SE Hancock Seamount is shaped some- 

 what like a truncated cone with a relatively flat, smooth 

 summit and steep, rugged flanks (Fig. 1). This topog- 

 raphy, combined with the tendency of armorhead to 

 nocturnally migrate from the flanks to the summit 

 (Humphreys and Tagami 1986), constrained the com- 

 mercial trawl fishery to operate primarily on the sum- 

 mit (<300m) at night (Sasaki 1986). 



