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



203 



15 I i m i ni 20 I i 



70 75 80 85 90 95 70 75 80 85 90 95 



35 



30 



Age 2 



35 



30 -- 



Age 3 



25 I I I I I I I I I I I I I I I I I I II I I I I I I I I I 25 I I I I I I I I I I I I II I I I I I I I I I I I I I I I 

 70 75 80 85 90 95 70 75 80 85 90 95 



40 



35 



Age 4 



.. Age 5 



40 -- 



35 



30 I I I II I I I I I I I I I I II II I I I I I I II I I 30 I I I I I I I I I I I I I I III I I I I I I I I I I I I 

 70 75 80 85 90 95 70 75 80 85 90 95 



Year 



Figure 6 



Time series of observed (open circles) and modeled (solid line) values of mean fork 

 length (FL) at ages 0-5 during 1970-97 for chub mackerel iScomber japonieus). 



There was a positive correlation between FL at age and 

 l°xl° block SST in the waters of 32-34°N and 144-149°E, 

 located south of the Kuroshio Extension flowing eastward 

 at the latitude of 35-37°N from April to June (Figs. 1 and 

 5A). But the correlation coefficient was not significant, and 

 this area was not considered to be inhabited by juvenile 

 mackerel (Watanabe, 1970). Thus, we considered that the 

 SST in the waters of 32-34°N and 144-149°E was not a 

 significant factor on the variation of FL at age 0. 



The low SST in the waters bounded by 38-40°N and 

 141-143°E is indicative of a large inflow of Oyashio Cur- 

 rent waters (Hirai and Yasuda, 1988), which is a cold 

 water current and has high productivity (Odate, 1994), 

 into the Kuroshio-Oyashio transition zone, where is one 

 of the main feeding grounds of mackerels (Odate, 1961; 

 Watanabe, 1970; Watanabe and Nishida, 2002; Fig. 1). 

 Thus, we hypothesized that the large inflow of Oyashio 

 current waters into the Kuroshio-Oyashio transition zone 

 improved the feeding condition and accelerated the growth 

 of juvenile mackerel. Jobling ( 1988) suggested a parabolic 

 relationship between water temperature and fish growth. 

 The range of SST in this area, which was negatively cor- 



related with FL at age of mackerel, was 9-13°C (Table 1 ). 

 This temperature range is near the lowest nonstressful 

 temperatures for mackerel ( 10-12°C, Schaefer, 1986). Thus, 

 we do not consider that the negative relationship between 

 growth and SST was the result of suppressed growth by 

 the high ambient temperature. 



In mackerel, maximum egg production appears to have 

 shifted to later in spring during the 1990s, as compared to 

 the late 1970s and 1980s, resulting in a shorter period of 

 growth and thus smaller fish (Fig. 8, Mori et al. 4 ; Kikuchi 

 and Konishi 5 ; Ishida and Kikuchi 6 ; Zenitani et al. 7 ; Kubota 

 et al. 8 ). In the early 1970s, the main spawning period was 



4 Mori, K., K. Kuroda, and Y. Konishi. 1988. Monthly egg 

 production of the Japanese sardine, anchovy, and mackerels off 

 the southern coast of Japan by egg censuses. Datum Collect. 

 Tokai Reg. Fish. Res. Lab. 12:1-321. [In Japanese. Available 

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

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



5 See next page. 



6 See next page. 



7 See next page. 



8 See next page. 



