Watanabe and Yatsu: Interannual variation in length at age of Scomber /aponicus 



205 



The effect of population density on growth of mackerel 

 was higher than the effect of SST (Fig. 7, Table 6). Our 

 result agreed with the results for Japanese sardine ( Wada 

 et al., 1995 ) and Atlantic cod ( Sinclair et al., 2002 ). Particu- 

 larly, the effect of population density was significant in the 

 late 1980s, which resulted in a remarkable increase in FL 

 at age (Figs. 3 and 7). 



The relative size at age was carried over to older ages 

 (Fig. 4), indicating that the cohorts that were small at age 

 could not compensate for this early small size. Iizuka 

 (1974) reported that the trend of growth established at 

 age for chub mackerel was maintained until age 2 for 

 the 1963-73 year classes. Toresen (1990) demonstrated 

 from length data that a trend in rate of growth for a given 

 year class of Norwegian herring was determined at the im- 

 mature stage and was consistent after maturation. Total 

 length of Hokkaido-Sakhalin herrings iClupea pallasii) at 

 age 5 and older was positively correlated with the length 

 at age 4 (Watanabe et al., 2002). Because fish first mature 

 at age 4. this implied that the trend in total length of each 

 year class was determined by the age at maturity. From 

 these results we hypothesize that the variability in size at 

 age in the Pacific stock of chub mackerel is largely attribut- 

 able to growth before maturity, especially during the first 

 6 months after hatching. 



Acknowledgments 



We would like to thank K. Meguro of Chiba Prefecture 

 Governmental Office and K. Kobayashi of Shizuoka Pre- 

 fecture Governmental Office for providing insights into 

 chub mackerel's growth and into age determination. We 

 also thank T. Akamine, M. Suda, and N. Yamashita of the 

 National Research Institute of Fisheries Science for advice 

 on the statistical analysis. We also thank Y. Watanabe and 

 C. B. Clarke of the Ocean Research Institute, University of 

 Tokyo, for their constructive comments on this manuscript. 



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