Bigelow Age and growth of Onychoteuthis borealijaponica 



23 



atically assess the type of growth model which best 

 describes the data. A statistical comparison of sev- 

 eral growth models found that growth in O. 

 borealijaponica from the paralarval to subadult size 

 range could be sufficiently described with an expo- 

 nential model, though there was weak evidence that 

 a logistic model may be sufficient to describe growth 

 in males from the paralarval to adult size range. The 

 most appropriate growth model (exponential or lo- 

 gistic) for the entire life cycle of O. borealijaponica 

 will emerge when sexually mature males and fe- 

 males are aged. 



Estimated growth rates from this study were 

 higher than estimates derived from length-fre- 

 quency analysis of fisheries data (Murata and Ishii, 

 1977). Growth estimates based on length-frequency 

 analysis with time often provide evidence of de- 

 creased growth rate, which is usually described by 

 an asymptotic model (Patterson, 1988). Length-fre- 

 quency analysis may be inappropriate for estimat- 

 ing growth in cephalopods (Jackson and Choat, 

 1992), either because 1) cohorts are difficult to de- 

 tect because spawning occurs throughout the year, 

 2) variable individual growth rates produce Lee's 

 phenomenon (Ricker, 1975), or 3) samples of a mi- 

 grating population are taken at a point along the 

 migration route, which results in overestimating 

 growth in young squid and underestimating growth 

 in older squid. 



Growth data presented for O. borealijaponica from 

 the central North Pacific provide a useful compari- 

 son of growth between males and females. The ex- 

 ponential models predict that males and females 

 grow in length at similar rates (0.80% ML/day), but 

 females grow faster in weight (1.90% WT/day) than 

 do males (1.40% WT/day). These rates correspond 

 closely with the average growth rates of similar 

 sized squids from temperate waters (e.g., Illex 

 illecebrosus, O'Dor, 1983; /. argentinus, Rodhouse 

 and Hatfield, 1990b). 



The most significant advantage of using statolith 

 ageing techniques is the ability to produce indi- 

 vidual rather than population statistics. Using sta- 

 tolith analysis, spatial variations in size at age, 

 growth parameters, and maturity stage at age were 

 observed between O. borealijaponica individuals 

 from the western and central North Pacific. Little 

 is known concerning genetic variation and stock 

 structure of O. borealijaponica in the North Pacific; 

 however, female squid in the western North Pacific 

 were found to grow faster than both male and fe- 

 male squid in the central North Pacific and were 

 younger at maturity stages III and IV than central 

 North Pacific females. Apparent growth rate and 

 maturity stage differences may be related to water 



temperatures or food availability during the 

 paralarval stage. Forsythe and Hanlon (1989) 

 showed that temperature had a pronounced effect 

 on the increase in length and weight of the squid 

 Loligo forbesi. In their laboratory study, a tempera- 

 ture increase of 1°C increased the growth in length 

 and weight of paralarval squid 0.5% and 2.0% per 

 day, respectively. Subadults in the western Pacific 

 may have hatched in the warm Kuroshio Current 

 or in productive transition waters between the 

 Kuroshio and Oyashio fronts. Paralarvae hatched in 

 the western North Pacific may therefore experience 

 higher temperatures or a greater abundance of prey 

 species, or both, than paralarvae hatched in the 

 central North Pacific, which could explain the ob- 

 served spatial differences in growth. 



Acknowledgments 



I gratefully acknowledge the help of the officers and 

 crew of the research vessels Hai Kung, and Shoyu 

 Maru and the help of the officers, crew, and scien- 

 tific field party of the NOAA ship Townsend Crom- 

 well cruise 91-06. 1 would like to thank C. H. Fiscus 

 who kindly provided the statolith samples from the 

 eastern North Pacific and D. R. Kobayashi for assis- 

 tance in fitting the Schnute model. This paper 

 benefitted from comments by G. T. DiNardo, E. E. 

 DeMartini, C. H. Fiscus, and anonymous reviewers. 



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