204 



Fishery Bulletin 102(1) 



0.015 T 



-0.015 



0.015 



i i i i i i i i i i i ii n i i 



70 



75 



80 



85 



90 



95 



-0.015 I I I I I I I I I I I I I I 

 70 75 80 

 Year 



Figure 7 



The total effect of I A) mean SST for the area of 38-40°N and 141-143"E from April to 

 June, and (Bl population density on k for each year class of chub mackerel i Scomber 

 japonicus). 



also in April (Kuroda 9 ). Delayed spawning in the 1990s 

 should have resulted in a reduction in the mean FL at ages 

 during September-December in the 1990s compared to the 

 1970s and 1980s; however the present study showed the op- 

 posite result (Table 1 ). We hypothesize that the effect of the 

 shift of spawning period on the FL at ages may have been 

 overwhelmed by the effect of population density (Fig.7). 



■"' Kikuchi, H.,andY. Konishi. 1990. Monthly egg production of 

 the Japanese sardine, anchovy, and mackerels off the southern 

 coast of Japan by egg censuses: January, 1987 through December, 

 1988, 72 p. National Research Institute of Fisheries Science. 

 Tokyo. [In Japanese. Available from National Research Insti- 

 tute of Fisheries Science, 2-12-4 Fukuura, Kanazawa. Yokohama 

 236-8648, Japan.] 



6 Ishida, M. and H Kikuchi. 1992. Monthly egg production of 

 the Japanese sardine, anchovy, and mackerels off the southern 

 coast of Japan by egg censuses: January, 1989 through December, 

 1990, 86 p. National Research Institute of Fisheries Science, 

 Tokyo. [In Japanese. Available from National Research Insti- 

 tute of Fisheries Science, 2-12-4 Fukuura, Kanazawa, Yokohama 

 236-8648, Japan.]. 



7 Zenitani, H., M. Ishida, Y Konishi, T Goto, Y. Watanabe, and R. 

 Kimura. 1995. Distributions of eggs and larvae of Japanese 

 sardine, Japanese anchovy, mackerels, round herring, jack mack- 

 erel and Japanese common squid in the waters around Japan. 

 1991 through 1993. Resources Management Research Report 

 Series A-2, 368 p. National Research Institute. Japan Fisheries 

 Agency, Tokyo. [In Japanese. Available from National Research 

 Institute of Fisheries Science, 2-12-4 Fukuura, Kanazawa, Yoko- 

 hama, 236-8648 Japan] 



s Kubota, H.,Y Oozeki, M. Ishida, Y Konishi, T Goto, H. Zenitani, 

 and R. Kimura. 1999. Distributions of eggs and larvae of 

 Japanese sardine, Japanese anchovy, mackerels, round her- 

 ring, jack mackerel and Japanese common squid in the waters 

 around Japan, 1994 through 1996, 352 p. Resources Manage- 

 ment Research Report Series A-2., National Research Institute, 

 Japan Fisheries Agency, Tokyo. [In Japanese. Available from 

 National Research Institute of Fisheries Science, 2-12-4 Fuku- 

 ura, Kanazawa, Yokohama 236-8648, Japan.] 



9 Kuroda, K. 2002. Personal commun. 1-1-3-406. Kasumi. 

 Narashino. Chiba 275-0022, Japan. 



Jun 6 



May 5 - - 



Apr 4 



Mar 3 



H — l — I — I — I — I— 

 78 80 82 84 



86 88 

 Year 



—l — l — l — l — l — i — i 

 90 92 94 96 



Figure 8 



Interannual variation in the peak period (weighted 

 monthly means) of egg production for the Pacific stock 

 of chub mackerel (Scomber japonicus), which includes a 

 small portion from the eggs of spotted mackerel (Scomber 

 australasicus) (Mori et al. 4 : Kikuchi and Konishi''; Ishida 

 and Kikuchi 1 '; Zenitani et al. 7 ; Kubota et al. 8 ). 



The estimated FL at age from our growth model, with 

 the use of AIC, fitted well to the observed FL at age 

 (Fig. 6). Mean growth increments / of each year class 

 from age (6 months old) to ages 1-5 (/„_,) were signifi- 

 cant and negatively correlated with FL at age (Table 2), 

 indicating that the growth rate of mackerel had changed 

 from year to year for a given year class. This negative 

 correlation indicated that the effects of population density 

 and SST was temporal, and influenced k rather than L r . 

 The negative correlation between FL at age and growth 

 increments also suggested that the FL at age of mack- 

 erel approximated the asymptotic length. Thus, mackerel 

 growth was best fitted to the modified von Bertalanffy 

 growth model with the temporal environmental effect on 

 k (Table 5). 



