Somerton and Kikkawa Population dynamics of Pseudopentaceros wheelen 



Ibl 



to changing catchability, then such an increase should 

 not be evident in the Soviet catch-and-effort data, 

 because the Soviet fishery had developed earlier and 

 was likely beyond its "fishing-up" phase. Soviet data 

 do display an increase from 1971 to 1972 (73-104 

 million fish/vessel day; Borets 1975), but this is con- 

 siderably less than that experienced by the Japanese 

 fishery. Thus, it is not entirely clear whether the ap- 

 parent increase in 1972 was real and due to recruit- 

 ment or an artifact due to changing catchability. 



The dependance of recruitment on spawning biomass 

 was examined by Wetherall and Yong (1986) and found 

 to be essentially nonexistent, at least at the high levels 

 of spawning biomass extant during 1969-77. Recruit- 

 ment, however, must ultimately be limited by spawn- 

 ing biomass at low population levels; therefore, we 

 reexamined the relationship over the period 1980-90, 

 when the spawning biomass was considerably lower. 

 This was done by plotting, on a log-log scale, the esti- 

 mated spawning biomass on SE Hancock Seamount 

 against the estimated recruitment 2 years later (Fig. 

 8). Since a clear relationship is not evident, it is pos- 

 sible that recruitment and spawning biomass are only 

 weakly related even at the low population levels ex- 

 amined. There are, however, at least two other pos- 

 sible reasons why no relationship was found. First, 

 since no apparent genetic difference exists among the 

 armorhead collected at the various seamounts (Borets 

 1979), recruits to SE Hancock Seamount are likely the 

 progeny of the entire North Pacific population. If the 

 SE Hancock Seamount population does not vary con- 

 cordantly with the entire North Pacific population, any 

 relationship between recruitment and spawning 

 biomass would be obscured. However, plots of the 

 estimated biomass on SE Hancock Seamount against 

 Japanese CPUE on all SE-NHR seamounts show a 

 strong concordance (Fig. 9). Second, if spawning 

 biomass does exert an influence on recruitment, it may 

 do so only by limiting the maximum level attained. 

 Thus, at higher levels of spawning biomass, higher 

 levels of recruitment are possible— but not assured— 

 because of environmental variability. One interpreta- 

 tion of Figure 8 could therefore be that recruitment 

 did increase with spawning biomass, but at the higher 

 levels of biomass there were several environmentally- 

 poor recruitment years. 



Management implications 



Since armorhead do not grow after they recruit to the 

 fishery and therefore cannot be growth-overfished, 

 management strategies could be designed solely to 

 achieve some optimum level of spawning stock biomass 

 (SSB). One approach is to define this optimum SSB by 



using a spawner-recruit relationship as is done for some 

 species of Pacific salmon (Ricker 1975). Another ap- 

 proach is to define it in terms of a fixed percentage 

 of the equilibrium biomass in the absence of a fishery 

 (Beddington and Cooke 1983). But in either case, the 

 spawning population must include the entire SE-NHR 

 population rather than the small component examined 

 here. In addition, some form of international agree- 

 ment controlling the armorhead catch will be required 

 before any management measures are effective. 



Acknowledgments 



We thank George Boehlert, Bob Humphreys, Bill 

 Lenarz, and Jeff Polovina for reviewing the manuscript 

 and offering helpful suggestions. 



Citations 



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