196 



Abstract— Annual mean fork length 

 (FL) of the Pacific stock of chub mack- 

 erel {Scomber japonicus ) was examined 

 for the period of 1970-97. Fork length at 

 age (6 months old) was negatively cor- 

 related with year-class strength which 

 fluctuated between 0.2 and 14 billion in 

 number for age-0 fish. Total stock bio- 

 mass was correlated with FL at age but 

 was not a significant factor. Sea surface 

 temperature (SST) between 38-40°N 

 and 141-143°E during April-June 

 was also negatively correlated with FL 

 at age 0. A modified von Bertalanffy 

 growth model that incorporated the 

 effects of population density and SST on 

 growth was well fitted to the observed 

 FL at ages. The relative FL at age for 

 any given year class was maintained 

 throughout the life span. The variabil- 

 ity in size at age in the Pacific stock of 

 chub mackerel is largely attributable to 

 growth during the first six months after 

 hatching. 



Effects of density-dependence and 



sea surface temperature on interannual variation 



in length-at-age of chub mackerel 



(Scomber japonicus) in the 



Kuroshio-Oyashio area during 1970-1997 



Chikako Watanabe 

 Akihiko Yatsu 



National Research Institute of Fisheries Science 



Fisheries Research Agency 



2-12-4 Fukuura, Kanazawa 



Yokohama 236-8648. Japan 



E-mail address (for C Watanabe): falconer a affrc go ip 



Manuscript approved for publication 

 22 September 2003 by Scientific Editor. 



Manuscript received 20 October 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:196-206(2004). 



Variability in growth of marine fishes 

 has been attributed to the effects of 

 density-dependence or environmental 

 factors such as water temperature, or 

 to the effects of both factors (e.g. Moyle 

 and Cech, 2002). Size-at-age data are 

 crucial because they are necessary 

 for stock assessment methods such 

 as virtual population analysis, yield 

 per recruit, and spawning-per-recruit 

 analyses (Pauly, 1987; Mace and Sissen- 

 wine, 1993; Haddon, 2001) and are pos- 

 sibly useful for detecting regime shifts 

 as well (Yatsu and Kidoroko, 2002). 

 Around Japan, the effects of population 

 density and sea water temperature on 

 fish growth have been shown for the 

 Pacific stock of chub mackerel (Scomber 

 japonicus) (Iizuka, 1974), Japanese 

 Spanish mackerel {Scomberomorus 

 niphonius) (Kishida, 1990), the Pacific 

 and Tsushima Current stocks of Japa- 

 nese sardine (Sardinops melanostietus) 

 (Hiyama et al, 1995; Wada et al., 1995), 

 and Japanese common squid ( Todarodes 

 pacificus) (Kidokoro, 2001). 



The Pacific stock of chub mackerel is 

 one of the most important commercially 

 exploited fish populations in Japan and 

 has been managed by the total al- 

 lowable catch (TAC) system in Japan 

 since 1997. Chub mackerel seasonally 

 migrate along the Pacific coast of Japan 

 from Kyushu to Hokkaido. They spawn 

 in the coastal waters around Izu Islands 

 and off southwestern Japan between 

 February and June (Fig. 1, Watanabe, 

 1970; Usami, 1973; Murayama et al., 

 1995; Watanabe et al., 1999). Adult fish 

 (after spawning) and their offspring 

 migrate eastward along the Pacific 



coast with the Kuroshio Current. Ju- 

 venile mackerel of about 6 months old 

 usually recruit to the purse-seine and 

 set-net fisheries off the coast of north- 

 eastern Japan at the end of summer 

 (Fig. 1, Odate, 1961; Kawasaki, 1966; 

 Watanabe, 1970; Iizuka, 1974). The 

 total catch of the Pacific stock of chub 

 mackerel increased during the 1960s 

 and 1970s, peaked at 1.5 million metric 

 tons in 1978, and then declined to 2.3 

 thousand tons in 1990 (Fig. 2). The es- 

 timated total biomass increased in the 

 1970s from 2.8 million tons in 1970 to 

 5.9 million tons in 1977, and the consec- 

 utive occurrences of large year classes 

 exceeded 7 billion age-0 (6-month-old) 

 fish in the early and mid 1970s. In 1990, 

 the biomass was reduced to a minimum 

 of 0.2 million tons in 1990 (Table 1, Fig. 

 2; Yatsu et al. 1 ). Relatively large year 

 classes occurred in 1992 (2.8 billion 

 fish) and 1996 (4.5 billion fish), and 

 the total biomass increased in the mid 

 1990s, but it remained at about 10% 

 of the level attained in the mid 1970s 

 (Yatsu et al. 1 ). 



On the basis of year-class strength 

 and variations in fork length (FL) at 

 ages 0-2 for the 11 year classes present 

 from 1963 to 1973, Iizuka (1974) sug- 

 gested an effect of density-dependent 

 growth on young chub mackerels. In 



1 Yatsu, A., C. Watanabe, and H. Nishida. 

 2001. Stock assessment of the Pacific 

 stock of chub mackerel in fiscal 2000 

 year. /;; Stock assessment report, p. 

 64-87. |In Japanese. Available from Fish- 

 eries Research Agency, 2-12-4 Fukuura, 

 Kanazawa. Yokohama 236-8648. Japan.] 



