NOTE Hattorl et a\ Hatching date, nursery grounds, and early growth of juvenile Theragra chalcogramma 



473 



the low-growth group that probably spawned in Funka 

 Bay mixed with the high-growth group that was prob- 

 ably produced locally in the Tohoku area. This mixing 

 of the two groups could have occurred primarily be- 

 cause of either advection or migration. Assuming that 

 this interpretation is correct, it would be important 

 for local fisheries managers to take account of these 

 variations especially if the years of extra production 

 from the Tohoku area expand the exploitable stock. 



Length-frequency distribution and hatching date 



The length-frequency distribution and hatching date 

 composition for juvenile pollock among regions were 

 similar in 1999 and 2001 but were different in 2000 

 (Figs. 7 and 8). In 2000, large fish over 40 mm TL, 

 designated as forming part of the low-growth group, 

 constituted the majority of the samples from Funka 

 Bay, and much smaller fish of the same group (<40 

 mm TL) were widely distributed in waters from east 

 of Tsugaru Strait to southern Tohoku. All juveniles 

 from Funka Bay were categorized as low-growth group 

 fish that hatched between late December and late 

 February, and juveniles of the low-growth group that 

 hatched after March 1 were caught widely from east 

 of Tsugaru Strait to southern Tohoku in 2000. These 

 results appear to indicate that late hatching fish origi- 

 nating from Funka Bay were transported to the Tohoku 

 area by the strong coastal branch of the Oyashio Cur- 

 rent after March 2000. 



Nakatani and Sugimoto'^ showed that only fish hatch- 

 ing in January and February survived in Funka Bay 

 in summer, whereas pollock usually spawn between 

 December and March. Nakatani (1998) 

 thus assumed that late hatching fish (af- 

 ter early March) would not be able to con- 

 sume enough food during their settlement 

 stage because the abundance of copepo- 

 dids of Pseudocalanus spp., the main food 

 for juveniles below 30 mm TL, decreases 

 in late May and because late hatching 

 fish cannot eat Neocalanus plumchrus 

 due to its large size. Therefore, he sug- 

 gested that there are two critical periods 

 for walleye pollock during their early lar- 

 val and settlement stages in Funka Bay. 

 Considering these results, we suggest that 

 when late hatching fish were transported 

 to the Tohoku area, their survival rates 

 were higher than those of juveniles that 

 remained in Funka Bay. 



Nakatani. T., and K. Sugimoto. 1998. Re- 

 productive strategy of walleye pollock and 

 the environment in the south region of Hok- 

 kaido, Pacific. In Kaiyo Monthly special 

 vol., p. 182-186. Graduate School of Fish- 

 eries Sciences, Hokkaido University, 3-1-1 

 Minato-cho, Hakodate, Hokkaido 041-8611, 

 Japan. |In Japanese, the title was translated 

 by the authors.) 



50 



10 



A Low-growth group 

 A High-growtti group 



High-growlh group 



Low-growth group 



120 



60 80 100 



Number of otolith increments 



Figure 6 



Size-at-age relationship for two growth groups of walleye pollock (Ther- 

 agra chalcogramma) collected in 2000. Low- and high-growth groups 

 were divided according to the SOR 30 distribution (see text and Fig. 5). 

 The regression lines for each growth group are as follows: low-growth 

 group: rL = 0.017+0.429xQge [r2=0.941, n = 594, P<0.0001]; high-growth 

 group: rL = 4. 694 + 0. 486xa^e [r2=0.715, n = 152, P<0.0001]. 



