Multiple cohorts of juvenile jack mackerel Trachurus japonicus in waters along the Tsushima Warm Current

Fisheries Research 95 (2009) 139–145

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Multiple cohorts of juvenile jack mackerel Trachurus japonicus in waters along the Tsushima Warm Current Yu Kanaji a,b,∗ , Yoshiro Watanabe b , Tomohiko Kawamura b , Songguang Xie c , Yoh Yamashita d , Chiyuki Sassa e , Youichi Tsukamoto e a

National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan State Key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Chinese Academy of Science, Wuhan, Hubei 430072, China d Maizuru Fisheries Research Station, Kyoto University, Nagahama, Maizuru, Kyoto 625-0086, Japan e Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Taira-machi, Nagasaki 851-2213, Japan b c

a r t i c l e

i n f o

Article history: Received 10 April 2008 Received in revised form 1 August 2008 Accepted 6 August 2008 Keywords: Cohort Early life history Population structure Tsushima Warm Current Trachurus japonicus

a b s t r a c t The jack mackerel, Trachurus japonicus, has a prolonged spawning season and widely spread spawning grounds. The population in the coastal waters of Japan seems to be composed of several cohorts spawned seasonally from different waters. To understand its population structure along the Tsushima Warm Current, we analysed hatchdates and growth histories of fish from Kunda Bay, the southern, central and northern East China Sea (ECS), the southern Sea of Japan, and Maizuru Bay. Seven cohorts were detected from fish collected between June 2005 and June 2006 in Kunda Bay. Comparing hatchdate distributions and growth trajectories of the seven cohorts with those of the other five regional samples, we did not find that cohorts collected in Kunda Bay originated in the southern ECS. Therefore, these coastal waters of Japan appear to be significant spawning grounds for juvenile jack mackerel. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The jack mackerel, Trachurus japonicus, is one of the most important fishery resources in Japan, distributed from the East China Sea (ECS) to the coastal waters of middle Honshu Island (Yamada, 1958; Ochiai and Tanaka, 1986). The annual catch has ranged from 100 to 300 thousand metric tons over the last 30 years (Fisheries Agency and Fisheries Research Agency, 2006). Koto (1990) hypothesized that much of the population comprises cohorts born in the ECS during years of high biomass, whereas in periods of low biomass numbers from the ECS decrease and the proportion of fish born in coastal waters increases. A high density of early larvae of jack mackerel occurs at the shelf break of the southern ECS in February (Sassa and Konishi, 2002; Sassa et al., 2006). These larvae recruit into the continental shelf region in the central and northern ECS and are supposed to be transported northeastward to the Pacific waters off Japan by the Kuroshio Current, and to the Sea of Japan by the Tsushima Warm

∗ Corresponding author at: National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan. Tel.: +81 45 788 7513; fax: +81 45 788 5004. E-mail address: [email protected] (Y. Kanaji). 0165-7836/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fishres.2008.08.004

Current, when they recruit to coastal fishing grounds (Fukataki, 1960; Kozasa, 1971). The distribution patterns of mature adults and larvae suggest that spawning also occurs along the coast of southern Japan (Kazihara and Yamada, 1958; Fukataki, 1960; Seino et al., 1968; Hotta and Nakajima, 1971; Kobata, 1972; Sawada, 1974; Sakamoto et al., 1986; Suda et al., 1987; Nishida and Hasegawa, 1994; Yoda et al., 2004). Population structure is essential information for understanding population dynamics and effective fisheries management. Two stocks of jack mackerel are recognized: the Tsushima Warm Current stock in the East China Sea and in the Sea of Japan, and the Pacific stock in the Pacific waters along the Kuroshio Current (Fisheries Agency and Fisheries Research Agency, 2006), although genetically distinct local populations were not detected (Kijima et al., 1985). As this species has different spawning seasons according to spawning grounds, the fishing stock seems to be composed of seasonally spawned cohorts from different waters. Larval growth rates are strongly affected by environmental factors such as water temperature and food availability (Ochiai et al., 1984), and therefore hatchdates and growth histories are key information for distinguishing cohorts of different origin. The microstructure of fish otoliths can be used in determining hatchdate, age and growth history. Daily increment width of the otolith usually corresponds with daily somatic growth rate, when

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Table 1 Trachurus japonicus juveniles collected in Kunda Bay Date

Number of fish

27 June 2005 14 July 2005 03 August 2005 15 September 2005 30 September 2005 18 October 2005 02 November 2005 17 November 2005 02 December 2005 16 December 2005 02 February 2006 17 February 2006 03 March 2006 27 March 2006 10 April 2006 19 April 2006 12 June 2006

48 37 30 26 29 26 26 23 27 30 30 34 26 36 24 22 31

SL range (mm) 33.5–54.3 39.9–67.8 60.8–100.6 61.2–97.5 63.0–103.9 73.6–107.0 79.0–100.9 62.6–98.7 63.9–98.1 57.7–99.9 47.0–98.1 46.6–98.3 43.0–96.6 44.5–84.8 54.8–75.9 57.1–80.9 36.4–58.8

the relationship between otolith radius and standard length (SL) are linear (Campana, 1990). In early studies on Trachurus otoliths, it was difficult to read otolith microstructure after the formation of secondary primordia because previous analyses used an unreliable procedure (Waldron and Kerstan, 2001). However, a new method has now been established (Xie et al., 2005). In the present study, we detected multiple cohorts in jack mackerel juveniles collected in a local bay area by examining hatchdate distributions and growth trajectories in early life history. Spawning grounds of these cohorts were inferred by comparing information from otolith microstructures with those of larvae and juveniles collected in five local areas. Contributions of each cohort to the Tsushima Warm Current stock of jack mackerel are analysed. 2. Materials and methods 2.1. Sample collection

Fig. 1. Sampling location of Trachurus japonicus larvae and juveniles in the East China Sea and in the southern Sea of Japan (top) and in Wakasa Bay (bottom), during June 2005 and June 2006. Solid and open circles represent sampling locations in 2005 and 2006, respectively. The East China Sea was divided into southern, central and northern ECS after Sassa and Konishi (2002).

Kunda Bay is a local small inlet in Wakasa Bay facing the Sea of Japan (Fig. 1). Seventeen samplings of jack mackerel juveniles were collected using a 17.8 mm mesh set net in this bay from June 2005 to June 2006 (Table 1). Fish samples were stored frozen until analysis. Larvae and early juveniles were collected using an ORI net with 160 cm mouth diameter and 0.33 mm mesh on the RV Ryokuyomaru of Kyoto University and a small set net with 17.8 mm mesh in Maizuru Bay next to Kunda Bay between May and June 2005 (Table 2; Fig. 1). Larvae and early juveniles were also captured in the southern Sea of Japan during the cruise of the RV Tottori-Maru 1 (Tottori Prefectural Fisheries Experimental Station) in June 2005 using a midwater trawling net with 12 m × 12 m mouth opening and

mesh size of 7.0 mm (Table 2; Fig. 1). Fish samples were collected in the southern, central and northern ECS during the two cruises of the RV Kaiyo-maru 7 (Nippon Kaiyo Co., Ltd.) in April 2005 and the Yokomaru (Seikai National Fisheries Research Institute) in April 2006 (Table 2; Fig. 1). A midwater trawling net with 7 m × 7 m mouth opening and mesh size of 7.0 mm was used in 2005, and a neuston net with 1.3 m × 0.75 m mouth opening and mesh size of 1.0 mm (Oozeki et al., 2001) was used in 2006. Larvae and small juveniles were fixed in 99% ethanol.

Table 2 Trachurus japonicus larvae and juveniles collected in the East China Sea and the southern Sea of Japan Sample

Date of capture

Number of fish

2005 Southern ECS Central ECS Northern ECS Southern Sea of Japan Maizuru Bay

SL range (mm)

29 April 25–28 April 21 April 6–9 June 23 May, 7 and 24 June

10 25 14 27 82

18.5–26.7 17.9–40.6 21.2–29.9 21.7–37.3 3.8–29.9

2006 Southern ECS Central ECS Northern ECS

23–26 April 13–29 April 11–19 April

73 60 47

4.3–23.0 4.3–29.7 3.2–24.5

Sampling gear Surface trawler Surface trawler Surface trawler Mid water trawler ORI net, set net Neuston net Neuston net Neuston net

Y. Kanaji et al. / Fisheries Research 95 (2009) 139–145

Fig. 2. Linear relationship between otolith radius (R) and standard length (L) of Trachurus japonicus juveniles collected in the Kunda Bay in 2005–2006. L = 9.14 + 0.038R (n = 505, r2 = 0.91).

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Fig. 3. Hatchdate distribution of Trachurus japonicus juveniles (n = 505) collected in Kunda Bay during June 2005 and June 2006. Solid lines represent seven normal distributions.

2.2. Otolith analyses The SL was measured to the nearest 0.1 mm using digital calipers. Otoliths extracted from each fish were mounted on a glass slide with the proximal side (facing the body axis) facing up, ground with 1000- and 2000-grid sand papers, and then polished with 3.0, 1.0, and 0.5 ␮m lapping films to expose daily increments (Xie et al., 2005; Xie and Watanabe, 2005). For otoliths with the secondary primordia, sagittal plane of each otolith was divided into the primary growth zone around the otolith nucleus and the marginal growth zone outside of the secondary primordia (Xie et al., 2005). Measurements of otolith increment were conducted using an otolith measurement system (RATOC System Engineering, Tokyo, Japan), for the marginal growth zone first in the dorsal area and later for the primary growth zone in the posterior area. The first daily ring is formed on the third day after hatching (Xie et al., 2005). SL values for different ages were back calculated by the biological intercept method (Campana, 1990). Because the relationship between SL and otolith radius was linear (Fig. 2), the calculation procedure for each individual used a linear function determined with SL at first ring deposition being 2.65 mm notochord length (NL) as in Hatakeyama (2006). Hatchdate distributions and growth rates were compared using Sheffe’s F test. 3. Results 3.1. Cohort comparisons A total of 505 juveniles collected in Kunda Bay range from 33.5 to 107.0 mm SL. Hatchdates from March 2005 to April 2006 (Fig. 3) could be separated into seven normal distributions. Each was con-

Fig. 4. Growth trajectories of cohorts A–G of Trachurus japonicus juveniles collected in Kunda Bay in 2005–2006.

sidered as a cohort with a 1–2-month hatching period: Cohort A from mid-March to early May; B from mid-May to early June; C from mid-June to early August; D from mid-August to mid-October; E from late October to mid November; F from late November to mid-January and G from early March to early April. During the sampling period, Cohort A was collected between June and November, B between September and December, C between September and February, D between November and April, E between February and April, F between February and April, and G in June. Growth trajectories differed between the seven cohorts (Fig. 4). The mean SL at age 7 days were significantly larger in cohorts D, E and F than in A, B, C or G (Sheffe’s F test, P < 0.01, Table 3). The mean SL (±S.D.) at age 14 days was 3.81 ± 0.57 for

Table 3 Standard length (SL) at ages 7 and 14 days of the seven cohorts of Trachurus japonicus juveniles collected in Kunda Bay in 2005–2006 Cohort

Standard length (mm) at each age (days) 7 Days old

A B C D E F G

14 Days old

Number

Average

S.D.

Min

Max

Growth

Number

Average

S.D.

Min

Max

Growth

126 77 74 32 68 97 31

2.88 2.87 2.92 3.15 3.20 3.17 2.85

0.06 0.05 0.10 0.18 0.13 0.09 0.04

2.75 2.77 2.76 2.84 2.94 2.92 2.78

3.19 3.04 3.17 3.56 3.72 3.39 2.99

L L L H H H L

126 77 74 32 68 97 31

3.46 3.44 3.81 5.08 5.42 5.13 3.34

0.22 0.20 0.57 1.08 0.76 0.58 0.17

2.95 3.11 3.07 3.33 3.65 4.09 3.12

4.26 4.45 5.39 7.01 8.07 7.06 3.79

L L M H H H L

L, low growth rate; M, middle growth rate; H, high growth rate (Sheffe’s F test, P < 0.01).

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Fig. 5. Hatchdate distributions of Trachurus japonicus larvae and juveniles collected in 2005 and 2006 from the southern, central and northern ECS, the southern Sea of Japan and Maizuru Bay.

Cohort C, which was significantly larger than that of Cohorts A (3.46 ± 0.22), B (3.44 ± 0.20) and G (3.34 ± 0.17) but smaller than Cohorts D (5.08 ± 1.08), E (5.42 ± 0.76) and F (5.13 ± 0.58) (P < 0.01; Table 3). After 14 days of age, the differences in mean SL at a given age extended between the seven cohorts by up to about 60 days. 3.2. Regional comparisons Hatchdates of the larvae and early juveniles collected in the southern, central and northern ECS, the southern Sea of Japan, and Maizuru Bay extended from February to May in both years (Fig. 5). Almost all individuals collected in the southern Sea of Japan and Maizuru Bay were hatched in March and April. This was signifi-

cantly later than for those collected in the southern, central and northern ECS (February and March) in 2005 (Sheffe’s F test, P < 0.01). For fish collected in the central and northern ECS in 2006, hatchdates were mainly in March, earlier than those of the southern ECS in late March and early April (P < 0.01). Multiple comparisons of growth trajectories for larvae and small juveniles collected in the five different areas were conducted (Table 4; Fig. 6). For fish hatched in March 2005, there were no significant differences in SL values at ages 7 and 14 days for the southern, central and northern ECS, whereas those from Maizuru Bay were significantly smaller (P < 0.01). For fish hatched in April 2005, those collected in the southern Sea of Japan and Maizuru Bay showed similar SL values at ages 7 and 14 days (P > 0.01). For fish hatched in March and April 2006, SL values at ages 7 and 14 days

Table 4 Standard length (SL) at ages 7 and 14 days of Trachurus japonicus larvae and juveniles hatched in March and April in 2005 and 2006 and collected in the East China Sea and the southern Sea of Japan Hatch month

Standard length (mm) at each age (days) Age 7 days

Age 14 days

Number

Mean

S.D.

Min

Max

Growth

10 17 9 28 5

3.09 3.02 3.05 2.81 2.90

0.14 0.07 0.10 0.04 0.08

2.94 2.92 2.90 2.76 2.77

3.38 3.18 3.22 2.94 2.99

H H H L L

26 48 104

2.90 2.86 2.89

0.05 0.07 0.06

2.80 2.79 2.76

3.00 3.22 3.20

March 2006 Southern ECS Central ECS Northern ECS Cohort G

22 38 35 28

3.03 3.06 3.02 2.84

0.14 0.13 0.09 0.04

2.88 2.84 2.83 2.78

3.44 3.44 3.21 2.78

April 2006 Southern ECS Central ECS Northern ECS Cohort G

50 14 5 3

3.16 3.13 2.99 2.86

0.12 0.10 0.14 0.02

2.96 2.96 2.81 2.84

3.39 3.31 3.17 2.88

March 2005 Southern ECS Central ECS Northern ECS Maizuru Bay Cohort A April 2005 Southern Sea of Japan Maizuru Bay Cohort A

H is significantly larger than L (Sheffe’s F test, P < 0.01).

Mean

S.D.

Min

Max

Growth

10 17 9 28 5

4.72 4.25 4.50 3.17 3.52

0.69 0.30 0.31 0.12 0.28

3.86 3.75 4.02 3.05 3.06

6.23 4.94 5.06 3.58 3.77

H H H L L

26 48 104

3.59 3.41 3.46

0.18 0.25 0.20

3.28 3.12 3.05

3.96 4.65 4.13

H H H L

22 38 35 28

4.35 4.51 4.32 3.33

0.55 0.63 0.49 0.18

3.71 3.41 3.30 3.12

6.03 6.22 5.40 3.79

H H H L

H H

50 13 3 3

4.99 4.49 4.06 3.40

0.60 0.43 0.21 0.12

3.97 3.73 3.84 3.27

6.21 5.34 4.26 3.48

H

L

Number

L

Y. Kanaji et al. / Fisheries Research 95 (2009) 139–145

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Fig. 6. Growth trajectories of Trachurus japonicus larvae and juveniles hatched in March and April in 2005 and 2006, and collected in the southern ECS (*), the central ECS (♦), the northern ECS (×), the southern Sea of Japan (䊉) and Maizuru Bay (), and Cohort A () and G () collected in Kunda Bay.

were not significantly different among fish collected in the three areas in the ECS (P > 0.01).

3.3. Regional groups and seven cohorts We compared the hatchdate distributions of larvae and early juveniles collected in the five different areas with those of the seven cohorts collected in Kunda Bay (Figs. 3 and 5). Cohort A had a similar hatchdate distribution to those of the fish collected in the southern Sea of Japan and Maizuru Bay (P > 0.01), and this was significantly different from individuals in the three areas in the ECS (P < 0.01). The hatchdate distribution of Cohort G was similar to those collected in the central and northern ECS, but significantly different from those in the southern ECS (P < 0.01).

Growth trajectories of cohorts hatched in March and April and collected in Kunda Bay were compared with fish collected in the five different areas (Table 4; Fig. 6). SL values at ages 7 and 14 days of March-hatched fish in Cohort A were not significantly different from those caught in Maizuru Bay (P > 0.01), but were smaller than those collected in the three areas in the ECS (P < 0.01). SL values at ages 7 and 14 days of April-hatched fish of Cohort A were similar from those in the southern Sea of Japan and Maizuru Bay (P > 0.01). Fish hatched in March in Cohort G were significantly smaller than those of the three areas in the ECS in SL at ages 7 and 14 days (P < 0.01). SL values at ages 7 days of April-hatched fish of Cohort G were smaller than those collected in the southern and central ECS (P < 0.01). Those at 14 days were smaller than the southern ECS (P < 0.01). 4. Discussion 4.1. Population structure of the Tsushima Warm Current stock

Fig. 7. Catch of Trachurus japonicus juveniles by a set net in Kunda Bay.

Fish collected in Kunda Bay from June 2005 to June 2006 had hatched almost around the year between March 2005 and April 2006. We separated them into seven cohorts, which probably hatched in different areas. Catches of jack mackerel juveniles by the small set net in Kunda Bay varied seasonally with higher catches in summer than in fall or spring (Fig. 7). Considering the timing of occurrence of each cohort, the large catch in summer was mainly composed of Cohort A in 2005 and Cohort G in 2006. Small catches from fall to spring appeared to be of Cohorts B–F. As Cohort A from Kunda Bay, fish collected in Maizuru Bay (3.8–29.9 mm SL) and the southern Sea of Japan (21.7–37.3 mm SL) showed similar hatchdate distributions and growth trajectories (Figs. 4 and 6), they probably originated from the same area. The spawning area of Cohort

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A was probably near Maizuru Bay and the southern Sea of Japan. These waters are considered to be significant spawning grounds for the juvenile jack mackerels found in Kunda Bay. Jack mackerels are known to spawn in the shelf break region of the southern ECS (Sassa and Konishi, 2002; Sassa et al., 2006). Larvae hatched there are transported by the Kuroshio and its branch current to the coastal waters of southern and central Japan. There were no significant differences in hatchdate distributions (Fig. 5) or growth trajectories (Fig. 6) in early life stages among fish collected in the southern, central and northern ECS, except for those from the southern ECS in 2006. These results suggest that almost all collected in the ECS in April 2005 and 2006 had hatched in the southern ECS and been transported to the central and northern areas. Individuals collected in the southern ECS in 2006 had hatched later than fish caught in the central and northern ECS. This could be explained by the transportation of fish; individuals hatched earlier in the southern ECS were transported northeastward and collected in the central and northern ECS, but younger larvae hatched later were caught in the southern ECS. Sassa et al. (2006) reported that a high density of early larvae was collected in the southern ECS, but larger larvae and juveniles were rarely collected in that area. This supports the above view. Jack mackerel juveniles collected in the ECS had hatched in February and March both in 2005 and 2006, but only Cohort G had hatched in these months. Back calculated SL values at ages 7 and 14 days of Cohort G were smaller than for those fish collected in the ECS. Therefore, Cohort G had not spawned in the southern ECS. Although the southern ECS was found to constitute a principal spawning grounds of jack mackerel (Sassa and Konishi, 2002; Sassa et al., 2006), none of the cohorts collected in Kunda Bay were from the southern ECS between June 2005 and June 2006. Based on otolith microstructure analyses, Xie and Watanabe (2007) found that juvenile T. japonicus collected during November 2003 and October 2004 in Sagami Bay along the Kuroshio Current had hatched in winter and early spring, and suggested that they had been transported from the southern ECS. By contrast, juveniles collected between June and September 2002 in Fukawa Bay upstream of the Tsushima Warm Current comprised fish both from the southern ECS and from coastal waters in Japan (Xie et al., 2005), suggesting different population structures between the Tsushima Warm Current stock and the Pacific stock of jack mackerel. From the use of satellite-tracked drifters and simulation models, larvae hatched in the southern ECS are believed to be transported swiftly to the Pacific coast of Japan and to the northern ECS in one month by the Kuroshio Current and its branches (Kim and Sugimoto, 2002; Kim et al., 2007). However, particles released in the southern ECS took more than two months to be transported to the Sea of Japan (Kasai et al., 2008), because of the complicated structure of the Tsushima Warm Current (Isobe, 1999; Lee and Cho, 2002). Jack mackerel juveniles obtain a high swimming ability at about 50 mm fork length (FL) and then migrate to coastal waters (Kazihara, 1957; Azeta and Ochiai, 1962). In this study, we found that juveniles could reach an FL of 50 mm within about 60 days (Fig. 4); so most fish hatched in the southern ECS would grow to 50 mm FL and migrate to the coastal areas of northern ECS before being transported to the Sea of Japan. 4.2. Growth and survival Cohorts A, B, C and G from Kunda Bay showed significantly slower growth rates in the larval stage than the other three Cohorts, D, E and F. These four cohorts occurred in large numbers in Kunda Bay (Fig. 3). In the Atlantic cod (Gadus morhua) and Japanese anchovy (Engraulis japonicus), it has been suggested that individuals with high growth rates in their early life stages have a higher prob-

ability of survival and that this early growth determines the success or failure of recruitment (Meekan and Fortier, 1996; Takahashi et al., 2001; Takahashi and Watanabe, 2004; Takasuka et al., 2004). On the other hand, the results of the present study show that individuals of jack mackerel with slower growth rates could survive and be recruited successfully to the stock. This may be explained by the high survival ability of jack mackerel in their early life stages. The survival rates of laboratory reared fish up to 20 mm total length (TL) were reported to be 45% in jack mackerel, less than 1% in yellowtail (Seriola quinqueradiata) and bluefin tuna (Thunnus orientalis) and 10% in red seabream (Pagrus major) and Striped beakperch (Oplegnathus fasciatus), as in Fujita (1975) and Ochiai et al. (1984). Behavioral experiments using larval and juvenile fish demonstrated that larval jack mackerel had a higher ability to escape from predation than other small pelagic species (Masuda, 2006). Mortality depending on the growth rate may occur in larval jack mackerel, but mortality rate of individuals with lower growth is supposed to be less than other fish species because whole amount of mortality at early life stage is considerably smaller. Acknowledgements This research was financially supported by the Dynamics of Commercial Fish Stocks (DoCoFis) program from the Ministry of Agriculture, Forestry and Fisheries to Y. Watanabe. We thank M. Kishida for invaluable help in preparing the otolith samples. Samples in Maizuru Bay were provided by Dr R. Masuda. Kyoto Institute of Oceanic and Fishery Science provided catch data for Kunda Bay. References Azeta, M., Ochiai, A., 1962. A study on the race of jack mackerel found in Wakasa Bay. Bull. Jpn. Soc. Sci. Fish. 28, 967–978, in Japanese with English abstract. Campana, S.E., 1990. How reliable are growth back-calculations based on otoliths? Can. J. Fish. Aquat. Sci. 47, 2219–2227. Fisheries Agency and Fisheries Research Agency, 2006. Annual report of fisheries stock assessment in Japan. pp. 69–112, (in Japanese). Fujita, Y., 1975. A large scale breeding of juveniles. In: Feeding and Growth of Juveniles. The Japanese Society of Fisheries Science, Koseisha Koseikaku, Tokyo, pp. 90–96, in Japanese. Fukataki, H., 1960. Consideration of the recruiting process of the jack mackerel population in the Tsushima Current region. I. Consideration from occurrence and distribution of larvae. Ann. Rep. Jpn. Sea Reg. Fish. Lab. 6, 69–85, in Japanese with English abstract. Hatakeyama, R., 2006. Studies in early life history traits and their adaptive significance in clupeid fishes. PhD thesis, The University of Tokyo. 134 p., (in Japanese). Hotta, H., Nakajima, J., 1971. Studies on the structure of the population of jack mackerel, Trachurus japonicus, in the western seas of Japan. V. Analyses based on the spawning and maturity. Bull. Seikai Reg. Fish. Res. 38, 87–100, in Japanese with English abstract. Isobe, A., 1999. On the origin of the Tsushima Warm Current and its seasonality. Continental Shelf Res. 19, 117–133. Kasai, A., Komatsu, K., Sassa, C., Konishi, Y., 2008. Transport and survival processes of eggs and larvae of jack mackerel Trachurus japonicus in the East China Sea. Fish. Sci. 74, 8–18. Kazihara, T., 1957. Ecological studies on young jack mackerel, Trachurus japonicus (Temminch et Schlegel), with special reference to behavior and feeding habit. Bull. Fac. Fish. Nagasaki Univ. 5, 13–22, in Japanese with English abstract. Kazihara, T., Yamada, T., 1958. Juvenile jack mackerel collected by larval net. Rep. Dev. Res. Tsushima Warm Current 2, 79–105, in Japanese. Kijima, A., Taniguchi, H., Ochiai, O., 1985. Degree of genetic divergence and breeding structure in jack mackerel, Trachurus japonicus. Rep. Usa Mar. Boil. Inst. Kochi Univ. 7, 49–60, in Japanese with English abstract. Kim, H.Y., Sugimoto, T., 2002. Transport of larval jack mackerel (Trachurus japonicus) estimated from trajectories of satellite-tracked drifters and advective velocity fields obtained from sequential satellite thermal images in the eastern East China Sea. Fish. Oceanogr. 11, 329–336. Kim, H.Y., Kimura, S., Sugimoto, T., 2007. Transport of jack mackerel (Trachurus japonicus) larvae inferred from the numerical experiment in the East China Sea. Bull. Jpn. Soc. Fish. Oceanogr. 71, 9–17. Kobata, T., 1972. The habits of important fish species in Sagami Bay. II. Jack mackerel Trachurus japonicus. Ann. Rep. Kanagawa Pref. Fish. Exp. Stn. Sagami Bay Branch. 1971., 43–52, in Japanese. Koto, T., 1990. Stock status of jack mackerel in the Pacific coast. Bull. Jpn. Soc. Fish. Oceanogr. 54, 47–49, in Japanese.

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