NOTE Hatlori et al Hatching date, nursery grounds, and early growth of juvenile Theragra chakogiamma 



469 



43"N 



19-23 May 1999 



20-29 May 2000 



20-27 May 2001 



140 E 



141 



142 



143 140 



141 



142 



143 140 



141 



142 



143 140 141 



142 



143 



Figure 1 



Sampling stations (open circles) for walleye pollock tTheragra chalcogramma) caught with a midwater trawl 

 net in four regions off the Pacific coast of northern Japan in May 1999-2001. Arabic numerals indicate station 

 numbers (see Table 1). 



TL of fresh fish = 1.0126(rL of preserved fish) + 1.7934 



[n = 97, r2=0.974]. 



To study the daily otolith increment of our sample, 

 we employed the methods established by Nishimura and 

 Yamada (1984), Bailey and Stehr (1988), Yoklavich and 

 Bailey (1990) and Bailey et al. (1996). Sagittal otoliths, 

 hereafter called "otoliths," from the juveniles were em- 

 bedded in epoxy resin on glass slides and ground with 

 carbon paper (no. 800-1200) perpendicularly to the oto- 

 lith flat plane along the long axis (frontal section). Pol- 

 ished otoliths were selected if they included the otolith 

 nucleus on the ground surface and were thin enough 

 to transmit light under a light microscope. Then 1171 

 specimens were polished with lapping film, cleaned, 

 and observed. Early narrow increments were counted 

 under 600x or lOOOx magnification. As the increments 

 increased in width, the magnification was changed to 

 400x. Hatching date was estimated by subtracting the 

 number of increments from the sampling date. 



Daily growth-ring diameter is the index of body 

 length at an age because a significant linear relation- 

 ship exists between log-transformed body length and 



otolith length data for pollock juveniles (Nishimura 

 and Yamada, 1988). In the present study, we measured 

 the short otolith radius (SOR) along the long axis ob- 

 tained from a frontal section and used the SOR from 

 the nucleus to the 30th ring (SOR 30) as the index of 

 early growth. SOR 30 was measured for the specimens 

 collected in 2000, by using the otolith daily growth ring 

 analyzing system (Ratoc System Engineering Co., Ltd., 

 Shinjuku, Tokyo). The Mann-Whitney t/-test was used 

 to identify differences in SOR 30 groups. We were not 

 able to use analysis of covariance (ANCOVA) because 

 an interaction was found by analysis of variance (ANO- 

 VA) in size-at-age relationships among regions in 2000 

 (P<0.0001) and 2001 (P=0.035). Comparisons of specific 

 growth rate (SGR) for the juveniles were made between 

 our regions by the Mann-Whitney {/-test or ANOVA. 

 Comparisons of means were made using Scheffe's test. 

 SGR values of individual fish were calculated by using 

 the following formula (Ricker, 1979): 



^^^_ (hiL2-hiLi)xl00 



Co t^ 



