468 



Hatching date, nursery grounds, and 

 early growth of juvenile walleye pollock 

 (Theragra chalcogramma) off northern Japan* 



Tsutomu HattorP 



Akira Nishimura^ 



Yoji Narimatsu' 



Daiji Kitagawa' 



' Hachinohe Branch 

 Tohoku National Fisheries Research Institute 

 25-259 Shimomekurakubo, Same, Hachinohe 

 Aomori 031-0841, Japan 

 E-mail address (lor T Hattori) hmadara afra affrc go ip 



2 Hokkaido National Fisheries Research Institute 

 116 Katsurakoi, Kushiro 

 Hokkaido 085-0802, Japan 



Walleye pollock (Theragra chalco- 

 gramma) is widely distributed in 

 the North Pacific Ocean and plays 

 an important role in coastal subarc- 

 tic ecosystems. The Japanese Pacific 

 population of this species is one of 

 the most important demersal fishes 

 for commercial fisheries in northern 

 Japan. The population is distributed 

 along the Pacific coast of Hokkaido 

 and the Tohoku area <Fig. 1), which 

 is the southern limit of distribution 

 of the species in the western North 

 Pacific. In Funka Bay, the main 

 spawning ground for this popula- 

 tion, pollock spawn from December to 

 March (Kendall and Nakatani, 1992). 

 Planktonic eggs and larvae are trans- 

 ported into the bay, where juveniles 

 usually remain until late July when 

 they reach 60-85 mm in total length 

 (Hayashi et al., 1968; Nakatani and 

 Maeda, 19871. These juvenile pollock 

 then migrate from Funka Bay east- 

 ward to the Doto area off southeast- 

 ern Hokkaido (Honda et al., 2004). 

 Many studies on eggs, larvae, and 

 juveniles of the species have been 

 conducted in or near Funka Bay, but 

 little information is available on the 

 ecology of the early life stages in the 

 Tohoku area. Hashimoto and Ishito 

 (1991) suggested that eggs are trans- 

 ported from Funka Bay southward 

 to the Tohoku area by the coastal 



branch of the Oyashio Current, but 

 there has been no study to verify this 

 hypothesis. 



The aim of the present study was 

 to examine, by using otolith daily 

 growth ring analysis, regional dif- 

 ferences in hatching date, body size, 

 and early growth of juvenile pollock 

 in Funka Bay and the Tohoku area 

 and to elucidate the importance of 

 Tohoku area as a nursery ground 

 for the Japanese Pacific population 

 of pollock. 



Materials and methods 



The present study was carried out 

 from Funka Bay to the Tohoku area. 

 We divided the study area into four 

 regions, Funka Bay, east of Tsugaru 

 Strait, northern Tohoku, and south- 

 ern Tohoku (Fig. 1). East of Tsugaru 

 Strait region is located between 

 Funka Bay and the Tohoku area. 

 Sampling was done at a total of 54 

 stations mostly in late May during 

 three years, from 1999 to 2001: 10 

 for 1999, 23 for 2000, and 21 for 2001 

 (Table 1). In Funka Bay, sampling 

 stations were set up only at the bay 

 mouth area because there was a lot 

 of fishing gear inside the bay. Pol- 

 lock juveniles were collected with a 

 midwater trawl (NST-92-K1, Nichimo 



Co., Ltd., Shinagawa, Tokyo) onboard 

 the RV Wakataka Maru of the Tohoku 

 National Fisheries Research Institute 

 (TNFRI). 



The midwater trawl, which had a 

 round mouth opening with a diameter 

 of about 20 m, was towed only during 

 the day. The overall length of the net 

 was 98.3 m and it had a variable-size 

 mesh of 14 m (stretched) in the for- 

 ward section to 3 mm (square mesh) 

 at the codend. All tows were made 

 at an average ship speed of 4 knots. 

 The horizontal tows were carried out 

 in 1999, whereas the double oblique 

 tows were made from the surface to 

 near the bottom (10-20 m above the 

 bottom) in 2000-2001. The sampling 

 biases of the net were not investigat- 

 ed, but biases may have been neglible 

 for two reasons. First, our net had a 

 larger mouth opening than that of 

 the Methot trawl (5 m-) used for col- 

 lecting late larval and early juvenile 

 stages of walleye pollock (Brodeur et 

 al.. 1995). Second, we could sample 

 juveniles that hatched from mid-De- 

 cember to mid-April in our survey 

 and the juveniles were thought to be 

 from eggs that spawned throughout 

 the whole spawning season (Decem- 

 ber to March) in Funka Bay. 



Pollock juveniles were sorted from 

 the codend samples and preserved in 

 95% ethanol diluted with seawater 

 for otolith daily-growth analysis. To- 

 tal length (TL) of some pollock was 

 measured to the nearest 0.1 mm on 

 board the vessel, whereas the same 

 length of preserved fish was similarly 

 measured in the laboratory. The fol- 

 lowing formula was used to change 

 TL of preserved fish to TL of fresh 

 fish: 



' Contribution number B63 from Tolioku 

 National Fisheries Research Institute, 

 Fisheries Research Agency, Shinhama, 

 Shiogama, Miyagi, Japan. 



Manuscript submitted 29 July 2004 

 to the Scientific Editor's Office. 



Manuscript approved for pubhcation 



27 September 2005 by the Scientific Editor. 



Fish. Bull. 104:468-475 (2006). 



