Aquaculture Sci. 58（2），233－242（2010）

Movement and Growth of Whitespotted Conger myriaster in the Eastern ,

1, 2, ＊ 1 2 Shigeaki GORIE , Minoru TANDA and Kazuya NAGASAWA

Abstract: We conducted a tagging survey to study the movement and growth of whitespotted conger Conger myriaster in the eastern Seto Inland Sea and found that the fish moved to and the coastal waters of the Kii Channel from the northeastern Harima Nada Sea, the release site, through the Akashi Strait from January to August. Total length and gonadsomatic index of females recaptured in the Osaka Bay-Kii Channel waters were larger and higher, respectively, than those in the Harima Nada Sea-Akashi Strait waters, suggesting that the fish conducted a pre-spawning migration. The fish were recaptured up to 420 days after release and exhibited various growth rates ranging from 0.1 to 2 mm day－1. There was no particular trend in age of recaptured females although the age ranged from 1+ to 3+. Not all the released fish passed through the Akashi Strait, nor they did always move to Osaka Bay even when they were at liberty for a long period. Some large-sized fish remained in the Akashi Strait. These results suggest that there are two types of groups (a locally resident group and a migratory group) and that whitespotted conger have a com- plicated migratory behavior.

Key words: Conger myriaster; Tag and release; Growth; Spawning migration

The whitespotted conger Conger myriaster target fish species. inhabits inshore waters off Japan, Korea and Whitespotted conger occur as leptocephali China (Asano 1984). In Japan, this species is from February to May in the northeastern commercially caught, being one of the most Harima Nada Sea, eastern Seto Inland Sea important fishes in the eastern Seto Inland Sea (Gorie and Tanda 2005). They then settle on the off Hyogo Prefecture. The average landing of bottom, grow and usually recruit to a fishery at the species in Hyogo Prefecture was recorded about 250 mm in total length (TL) in October to be 1,700 metric tons (mt) from 1985 to 1997 (Gorie and Ohtani 1998; Tanda et al. 1998). but decreased to 607 mt in 2007 (Anon. 2009). Juveniles can be reared in aquaria by feeding Since the mid-1990s, a fishery management a commercial diet and exhibit a fast growth approach has been implemented for whites- rate under experimental conditions (Gorie and potted conger in the eastern Seto Inland Sea. Ohtani 1997; Nemoto et al. 2004; Gorie 2008). Especially, an efficient utilization of this stock Information on the growth and migratory is an important subject during the period of the behavior of wild whitespotted conger is quite stock decrease. For the efficient utilization of limited. The fish are likely to show a sex-biased any fish stock, resource management should be migratory habit, and female fish tend to move based on accurate stock assessment, and the to deeper waters with maturation (Okamura stock estimation is brought by accurate catch et al. 2000). Females also exhibit a seasonal data and a good knowledge of the biology of the loop migration driven by the avoidance of cold

Received December 4, 2009: Accepted March 25, 2010. 1 Hyogo Prefectural Technology Center for Agriculture, Forestry, and Fisheries (Fisheries Technology Institute), Akashi, Hyogo 674-0093, Japan. 2 Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8528, Japan. ＊Corresponding author: Tel: (+81) 78-941-8601; Fax: (+81) 78-941-8604; E-mail: [email protected] (S. Gorie). 234 S. Gorie, M. Tanda and K. Nagasawa waters (Katayama et al. 2004). using basket trap nets and some were obtained A few tagging studies have been conducted from a commercial small bottom trawl fishery for juvenile whitespotted conger. Based on the operating near the strait. All tagged fish were fact that tag-released juveniles tended to return released at a single site in the coastal waters to the release site in Tokyo Bay, they are con- of the northeastern Harima Nada Sea (Fig. 1; sidered to have a homing behavior (Shimizu 34°41´N, 134°52´E). Before tagged, fish were 2003). In contrary, juveniles (30 cm TL) may anesthetized in 0.04% 2-Phenoxyethanol (eth- remain near the release site in the eastern Seto ylene glycol monophenyl ether) and measured Inland Sea (Takemori et al. 2004). There have for TL (mm) and body weight (BW, g). From been no tagging and growth studies for large- 1997 to 2002, we used conventional T-bar anchor sized individuals (>50 cm TL). tags (20 mm long) which were yellow or red in In this paper, we deal with the movement and color and individually numbered. They were growth of large-sized whitespotted conger based inserted with a tag gun into the midpoint of on a tagging study in the eastern Seto Inland Sea, the caudal muscle just above the lateral line where the fish were caught in the Akashi Strait between the anus and the posterior end of the and released in the northeastern Harima Nada caudal fin. From 2003 to 2008, we employed a Sea near the strait. Also, based on a hypothesis double-tagging method. After an anchor tag was that female whitespotted conger leave from the introduced, an individually numbered yellow eastern Seto Inland Sea to yet unknown spawning tube (25 mm long, 1.75 mm in diameter), which site, we report new findings that may infer their was cut off the dart tag, was inserted into the pre-spawning migration in this region. abdominal cavity near the anus using a scal- pel. We expected that highly visible, external Materials and Methods anchor tags could attract fishermen’s attention and recaptured fish could be exactly identi- Tagging and release fied by internal dart tubes. Especially, as the The study area is located in the eastern number on an anchor tag tended to vanish Seto Inland Sea, Japan (Fig. 1). For tagging, within 2 months, this double-tagging method fish were mostly captured in the Akashi Strait was quite useful to identify individual fish.

134ﾟ40' 135ﾟ 135ﾟ20' Sea of Japan

34ﾟ40' Pacific Ocean Akashi St. Osaka Bay Harima Nada Sea Kariya

34ﾟ20' Awaji Is. Naruto St. Yura Seto St. 10 km Kii Channel

Fig. 1. Study area in the eastern Seto Inland Sea, Japan. The release site is shown by an open star. The location of Kariya Fisheries Cooperative Association where the catch data of whitespotted conger were obtained is indicated by an open circle. The regional boundaries are shown by dotted lines. Movement and Growth of Conger myriaster 235

Almost all tagged fish were estimated to be received from fishermen and measured for TL, females because their overall mean total length BW and gonad weight (GW, g). Gonadsomatic was 519 mm (Table 1) (Gorie et al. 2004). A index (GSI) was calculated as follows: GSI (%) small number of tagged and non-tagged fish = GW・BW-1×100. Following Katayama et al. were kept in laboratory aquaria for 2 months, (2002), age determination was conducted using but there was no tag loss in these fish and no otoliths. When frozen fish specimens were significant differences in TL and mortality were received, their TL was compensated because observed between the two groups. such specimens shrank to 97.3 % of their initial TL when thawed in our preliminary test (n=38). Growth and movement analysis Absolute growth rates (G) were calculated as -1 Growth was determined using the fish speci- follows: G=(TLr-TLi)・d , where TLr is the total mens whose tag numbers and dates at recapture length at recapture, TLi is the total length at were available. Freshly-caught specimens were release, and d is the number of days that the fish were at liberty. No data were used when a fish Table 1. Number and total length of whitespotted size at recapture is lower than that at release. conger tagged and released in the northeastern Harima Recapture locations and movement distances Nada Sea from 1997 to 2008 were determined when veracious locations were Month of SD3 Year N1 TL 2 (mm) release (mm) available. All recapture locations were plotted and 1997 June 90 440 (277-749) 87.3 visually validated. Although some fish were recap- July 262 513 (279-784) 93.9 tured very shortly after release near the release 1998 June 40 535 (354-771) 82.8 site, the locations within 5 km from the release July 14 598 (434-657) 60.0 site were eliminated from movement analysis. Aug. 7 645 (573-704) 45.6 The movement distance was calculated as a linear 1999 May 45 417 (344-516) 39.1 June 90 506 (382-755) 75.0 feasible distance from the release site. However, July 66 532 (421-760) 68.3 when a straight line was impossible due to the Nov. 63 571 (289-758) 126.6 presence of (Fig. 1), the detours were Dec. 89 686 (547-943) 72.8 calculated assuming that the fish passed through 2001 Nov. 104 584 (382-818) 83.0 Dec. 63 609 (385-880) 87.2 the central region in one of the three straits 2002 June 45 441 (368-543) 47.9 (Akashi, Naruto and Yura Seto straits; Fig. 1). Nov. 53 575 (411-777) 98.9 Dec. 129 618 (376-891) 88.5 Annual catch survey at a fisheries cooperative 2003 June 63 440 (326-613) 70.6 association Sep. 81 517 (390-693) 77.0 Dec. 107 614 (354-815) 89.6 From 2002 to 2008, we collected monthly catch 2004 Mar. 5 552 (436-725) 107.6 data of whitespotted conger by brand size at May 46 457 (334-708) 87.3 Kariya Fisheries Cooperative Association, where June 96 479 (328-802) 92.9 Sep. 41 561 (413-682) 63.1 most of the fishermen are engaged in small trawl Nov. 42 613 (478-770) 60.6 fishery in Osaka Bay off Hyogo Prefecture (Fig. 2005 June 134 478 (346-743) 74.7 1). The fish is usually marketed as one of the Nov. 58 605 (377-754) 85.1 three fish sizes: small (<30 cm in TL), medium- 2006 June 203 454 (318-720) 68.7 large (30-50 cm) and extra large (>50 cm). Nov. 46 524 (298-749) 102.5 2007 June 116 441 (302-642) 68.9 Nov. 19 466 (338-607) 77.1 Statistical analysis 2008 June 97 462 (330-685) 75.6 Differences in TL and GSI were explored by July 80 460 (320-763) 87.4 the Mann-Whitney U-test between two waters Nov. 45 474 (286-731) 111.8 (Harima Nada Sea-Akashi Strait waters versus Total/Mean 2,439 519 (277-943) 107.9 Osaka Bay-Kii Channel waters) where the fish 1 Number of fish tagged and released. recaptured. Differences in age composition and 2 Mean total length (range). 3 Standard deviation. recaptured waters were explored by χ2 test. 236 S. Gorie, M. Tanda and K. Nagasawa

Fig. 2. Growth increments of whitespotted conger recaptured in four waters in the eastern Seto Inland Sea. Left, released in May, June, July and September; right, released in November, December and March. Dotted lines show guide growth increments day-1 (mm). Open circle, Harima Nada Sea; closed circle, Akashi Strait; open triangle, Osaka Bay; closed triangle, Kii Channel.

900

Results 800

Tag-release and recapture 700 From 1997 to 2008, a total of 2,439 fish were 600 tagged and released. The overall mean TL was 519 mm (Table 1). No mortality was observed 500 during the tagging and release operations. Of 400 the 2,439 fish released, 181 were recaptured. Total length at recaptureTotal (mm) Of them, 63 fish were single tagged while 118 300 Harima Nada Sea Osaka Bay were double tagged. A half of the recaptured Akashi Strait Kii Channel fish were caught during our basket trap net Fig. 3. Total length at recapture of male and female surveys in the Akashi Strait. The remaining whitespotted conger recaptured in four waters in the recaptures were mostly caught by a small trawl eastern Seto Inland Sea. Open circle, male; closed circle, fishery and reported from fishermen, but a few female; triangle, unidentified. specimens were sent by fish brokers. When a tag was identified by these people, only infor- 2 mm day-1 during May to September (month mation on the date and location of recapture or at release), and from 0.1 to 1 mm day-1 during the tag number was sometimes obtained. Of the November to March (Fig. 2). The fish released 100 specimens we closely examined, 4 fish were from May to September showed larger growth found to have lost an internal tube. The recap- rate than those released from November to tured fish could be individually identified up to March. The growth rate of the fish when recap- 420 days after the release, using the numbers tured over 300 days after release ranged from on the internal tubes. 0.3 to 0.6 mm day-1 (mean: 0.5, n=5). One fish showed as high as 233 mm growth increment Growth and biological aspects for 394 days (from 502 mm at release to 735 mm The growth rates were determined from 131 at recapture). Females recaptured in the Osaka recaptured fish. As the number of day at lib- Bay-Kii Channel waters was larger than those erty was longer, the growth increment of the in the Harima Nada Sea-Akashi Strait waters fish at recapture was bigger. The fish exhib- (P=0.009), but no significant difference was ited various growth rates ranged from 0.25 to found in males (Fig. 3). Movement and Growth of Conger myriaster 237

10 Age of the recaptured fish varied from 1+ to 3+ (Table 2). Males recaptured in the Harima 8 Nada Sea and Osaka Bay were age-1+. There was no significant difference between female 6 age composition and displacement waters (χ2 test), although the age of recaptured females 4 ranged from 1+ to 2+ in the Harima Nada Sea, + + + 2 from 1 to 3 in the Akashi Strait and from 2 to Gonadsomatic index (%) 3+ in Osaka Bay. 0 300 400 500 600 700 800 900 Movement Total length (mm) at recapture The recapture locations were plotted and Fig. 4. Relationship between gonadsomatic index and validated using the data of 144 fish. The fish total length at recapture of female whitespotted conger moved to Osaka Bay and the Kii Channel from recaptured in four waters in the eastern Seto Inland Sea. the Harima Nada Sea through the Akashi Strait. Open circle, Harima Nada Sea; closed circle, Akashi Strait; The most of the fish that exhibited the eastward open triangle, Osaka Bay; closed triangle, Kii Channel. movement were recaptured near the Akashi Strait. From January to August, the fish were 10 recovered widely from Osaka Bay (Fig. 6). Of the fish recovered in the Osaka Bay-Kii Channel

) 8 % waters, the female fish were recaptured from January to August, while a few male fish were 6 from August to October (Table 3). The recap- ture with the longest movement (about 60 km 4 from the release site) was collected in the Kii Channel. A few fish were recaptured in the 2 Gonadsomatic index ( waters west of the release site and one fish was 0 near the Naruto Strait. Jan. Mar. May July Sep. Nov. Not all the released fish passed through the Feb. Apr. June Aug. Oct. Dec. Akashi Strait, nor they did always move to Osaka Bay through the strait even when they were at Fig. 5. Monthly changes in gonadsomatic index at recapture of female whitespotted conger recaptured in liberty for a long period. Some large-sized fish four waters in the eastern Seto Inland Sea. Open circle, remained in the Akashi Strait (Fig. 3). Some fish Harima Nada Sea; closed circle, Akashi Strait; open moved to Osaka Bay soon after the release, but triangle, Osaka Bay; closed triangle, Kii Channel. others remained around the Akashi Strait. The recaptured fish were divided into two groups by Table 2. Age and number of whitespotted conger displacement waters (Fig. 7): a locally resident recaptured in the eastern Seto Inland Sea from 1997 to 2008 group in the Harima Nada Sea-Akashi Strait + + + Age 1 2 3 waters, and a migratory group in the Osaka Bay- Sex Male Female Female Female Kii Channel waters, which indicates that these Harima Nada Sea 1 5 2 groups have different migratory habits. Akashi Strait 34 11 2 Osaka Bay 1 7 2 Seasonal catch of whitespotted conger Extra large-sized fish (>50 cm TL) were The female GSI values were correlated with caught in Osaka Bay off Hyogo Prefecture from their size (Fig. 4, P<0.001, r=0.59, n=90). The fish January to July with a peak in April (Fig. 8). with more than 4 in GSI values were caught only Such fish were not caught from August. in the Osaka Bay-Kii Channel waters (Fig. 5). 238 S. Gorie, M. Tanda and K. Nagasawa

134° 20’ 134° 40’ 135° 00’ 135° 20’ 134° 20’ 134° 40’ 135° 00’ 135° 20’

4 34° 40’ 2 11

2

5

34° 20’

Jan.-Feb. July-Aug.

° 3 34 40’ 9

34° 20’

Mar.-Apr. Sep.-Oct.

15 34° 40’ 53 9

34° 20’

May-June Nov.-Dec.

Fig. 6. Bimonthly recapture positions (dots) of whitespotted conger released at a single site (an open star) in the northeastern Harima Nada Sea. The number of fish recaptured is shown at recapture positions where more than one fish were collected.

body against the burrow wall. Due to this, it was Discussion often difficult to identify individual fish only by the external anchor tags. On the other hand, as In the present study, we employed a double- the numbers printed on the internal tags were tagging method, using conventional, external well preserved in the fish’s abdominal cavity anchor tags and individually numbered internal and clearly identifiable more than one year, the tubes. The numbers printed on the anchor tags double-tagging method is highly recommended were subject to vanish because whitespotted to study the biology of whitespotted conger. conger usually hide in the bottom and rub their Whitespotted conger moved to Osaka Bay Movement and Growth of Conger myriaster 239

Table 3. Number of male and female whitespotted conger and the Kii Channel from the northeastern recaptured in Osaka Bay and the Kii Channel from 1997 to Harima Nada Sea through the Akashi Strait 2008 (Fig. 6). We think this movement may be a pre- Male Female Month spawning migration to yet unknown spawning Osaka Bay Osaka Bay Kii Channel areas. In addition, we suggest a new hypothesis Jan. 6 Feb. 1 1 that such movement begins in January and Mar. 5 1 continues until August based on the following Apr. 3 May information: (1) Most of the tagged fish were June females and reached to a sexually maturing size July 1 (based on an experiment of Horie et al. 2001); Aug. 3 1 Sep. (2) The spawning season of whitespotted conger Oct. 1 is estimated to be from September to February Nov. (Tanaka et al. 1987; Mochioka et al. 1988; Lee Dec. and Byun 1996; Kurogi et al. 2002), and the indi- viduals tagged and released in this study moved 70 to Osaka Bay and the Kii Channel from January 60 to August, which is before the estimated spawn- 50 ing season; (3) Both TL and GSI values of 40 the females recaptured in the Osaka Bay-Kii Channel waters were significantly higher than 30 those in the Harima Nada Sea-Akashi Strait 20 waters (Figs. 3-5). From January to April, GSI

Movement distance (km) 10 values of the females recaptured in the Osaka 0 Bay-Kii Channel waters were also significantly 0 100 200 300 400 500 higher than those of the females caught with Days after release backset trap nets in the Akashi Strait (Gorie Fig. 7. Movement distance from the release site of whites- unpublished), indicating that the fish had more potted conger recaptured in four waters in the eastern Seto developed gonads in the Osaka Bay-Kii Channel Inland Sea. Open circle, Harima Nada Sea; closed circle, waters; and (4) No fish of extra large size was Akashi Strait; open triangle, Osaka Bay; closed triangle, Kii Channel. caught in Osaka Bay from August by a small trawl fishery (Fig. 8), which implies that such

100 5 fish left Osaka Bay for other waters. During our survey, we could not obtain any 80 4 evidence of spawning of whitespotted conger in the eastern Seto Inland Sea, which is the same 60 3 as in other inshore waters of Japan (Takai 1959;

40 2 Okamura et al. 2000; Ishida et al. 2003). It is

Extra large (%) thus reasonable to consider that the fish begin 20 1 a pre-spawning migration leaving for the North Small, medium-large (%) Pacific before the estimated spawning season. 0 0 Jan. Mar. May July Sep. Nov. Of the 181 fish recaptured, 110 (60.8%) were Feb. Apr. June Aug. Oct. Dec. taken in the Akashi Strait, where the fish were Fig. 8. Monthly composition of three size classes of whites- first caught for the tagging study. This high potted conger landed at Kariya Fisheries Cooperative recapture rate was partially due to our fishing Association. The fish were caught in a small trawl fishery efforts there, but we think that some fish exhib- in Osaka Bay off Hyogo Prefecture. Data are shown using ited a kind of homing behavior, as is inferred total catch for 7 years from 2002 to 2008. Shaded bar, small fish; unshaded bar, medium-large fish; closed circle, extra with the juvenile stage (Shimizu 2003). The large fish. Akashi Strait may be one of the favorite habitats 240 S. Gorie, M. Tanda and K. Nagasawa of large-sized fish in the eastern Seto Inland Sea. in the eastern Seto Inland Sea showed a wide In the present study, we could not clearly variation in TL within the same year class and take a west-southward detour to the Kii Channel had a complicated age composition in the same through the Naruto Strait. However, such move- TL class (Gorie et al. 2004). A similar result ment may be possible because one fish was was found for the same species caught off recaptured near the strait (Fig. 6, mid-left). Joban, northeastern Japan (Ishida et al. 2003), While no obvious explanation is possible at pres- C. oceanicus in the western Atlantic off North ent why there were few recaptures in the waters America (Hood et al. 1988) and C. conger in west of Awaji Island, this may be due to quite a the European waters (O’Sullivan et al. 2003). low fishing activity and/or the lack in notifica- Whitespotted conger reared in aquaria also show tion to fishermen about the tagging study. various differences in growth (Nemoto et al. Judging from their TL, almost all tagged and 2004; Gorie 2008). This wide variation in growth released fish were females. A few males were rate is probably another specific aspect of con- recovered in Osaka Bay in August and October grid fishes, suggesting that an age-length key (Fig. 3; Tables 2, 3), but this study cannot refer method is more reliable for the stock estimation to the movement of males because of their of the fish than a size-frequency analysis. small sample size. The absence of males or In the present study, we could not take any large-sized males in whitespotted conger from data on the factors to trigger the launch of the coastal Japanese waters has been reported the movement. There was no significant dif- to date (Okamura et al. 2000; Ishida et al. 2003; ference between female age composition and Gorie et al. 2004). A similar phenomenon is also displacement waters where the fish were known for Conger conger (Cau and Manconi recaptured (Table 2). Some large-sized whites- 1983; O’Sullivan et al. 2003). Nonetheless, it has potted conger remained in the Akashi Strait recently found that whitespotted conger males for a long period after the release. Two types occurred at a higher rate in a specific enclosed, of groups were identified in the study area by brackish-water lagoon (percentage of males, the displacement waters (Fig. 7): a local resi- 55%) than in inshore waters (1%) off the Pacific dent group to stay for a certain period near the coast of northeastern Japan (Nemoto 2007). Akashi Strait, and a migratory group to move The male-biased occurrence was also reported to Osaka Bay and the Kii Channel. The spe- for young fish (<45 cm TL) in the eastern Seto cies may have complicated migratory habits in Inland Sea (Takemori et al. 2004). Differences the eastern Seto Inland Sea. As the individuals in various biological aspects between both sexes caught in the Osaka Bay-Kii Channel waters may be a specific character of congrid fishes. were larger than those in the Harima Nada Sea- This study has shown the growth rate of Akashi Strait waters, there is a possibility that wild whitespotted conger in the eastern Seto whitespotted conger conduct a size-dependent Inland Sea. The recaptured fish exhibited migration in the eastern Seto Inland Sea. various growth rates from 0.1 to 2 mm day-1 Alternatively, the pre- and spawning migration (about 20 cm year-1). The fish released from of the local resident group of the fish may begin May to September showed larger growth rate at age-5+ and more in the eastern Seto Inland than those released from November to March Sea, because the fish collected in the Akashi (Fig. 2). This is due to the WT of the habitat Strait were less than 5-year-old (Gorie et al. because the fish reared in aquaria showed WT 2004). However, as a 13-year-old specimen was dependent growth (Gorie and Ohtani 1998). taken in the Joban waters (Ishida et al. 2003), These growth rates are higher than those of the migration of the species is still a compli- two related species, Conger oceanicus and C. cated biological issue for further research. We conger (about 10 cm year-1 for both species, need more information on the age composition Hood et al. 1988; Sbaihi et al. 2001). In addition, and other biological aspects of whitespotted we previously found that whitespotted conger conger in various regions. Movement and Growth of Conger myriaster 241

In conclusion, whitespotted conger moved Bull. Hyogo Pref. Tech. Cent. Agr., Forest. Fish. (Fish. to Osaka Bay and the Kii Channel from the Sec.), 40, 101-104 (in Japanese with English abstract). Hood, P. B., K. W. Able and C. B. Grimes (1988) Biology of Harima Nada Sea through the Akashi Strait in the conger Conger oceanicus in the Mid-Atlantic the eastern Seto Inland Sea. The observed dis- Bight I. Distribution, age, growth and reproduction. placement suggests their pre-spawning migra- Mar. Biol., 98, 587-596. tion, which seemed to begin in January and Horie, N., T. Utoh, Y. Yamada, A. Okamura, H. Zhang, N. Mikawa, S. Tanaka and H. P. Oka (2001) Artificial continue until August. This species is likely to induction of ovarian maturation and ovulation in the have two groups showing different migratory common Japanese conger Conger myriaster. Nippon habits. The fish exhibited various growth rates Suisan Gakkaishi, 67, 842-849 (in Japanese with ranging from 0.1 to 2 mm day-1, which is almost English abstract). -1 Ishida, T., A. Yamanobe, K. Goto, S. Katayama and N. equivalent to about 20 cm year . Mochioka (2003) Study on the white-spotted conger Conger myriaster (Brevoort) caught in the coast of Acknowledgments Joban. Bull. Fukushima Pref. Fish. Exp. Stn., 11, 65-79 (in Japanese). Katayama, S., T. Ishida, K. Goto, K. Iizuka and K. Karita We thank to Dr. Satoshi Katayama, National (2002) A new aging technique by UV light observa- Research Institute of Fisheries Science, for his tion of burnt otoliths for the conger eel Conger myri- assistance with age determination of the fish. aster (Brevoort). Ichthyol. Res., 49, 81-84. Thanks also go to Mr. Haruyoshi Nakasuji, the Katayama, S., T. Ishida, Y. Shimizu and A. Yamanobe (2004) Seasonal change in distribution of conger eel captain of the research vessel Chidori, and the Conger myriaster off the Pacific coast south of crews for their help during the survey. Tohoku, north-eastern Japan. Fish. Sci., 70, 1-6. Kurogi, H., N. Mochioka, Y. Takaki and O. Tabeta (2002) References First offshore record of Conger myriaster leptocephali in the East China Sea. Fish. Sci., 68, 1155-1157. Lee, T. W. and J. S. Byun (1996) Microstructural growth in Anonymous (2009) Annual Statistical Report on Agriculture, otoliths of conger eel (Conger myriaster) leptocephali Forestry and Fisheries of Hyogo Prefecture, Hyogo Assoc. during the metamorphic stage. Mar. Biol., 125, 259-268. Agric. Forest. Stats. , pp. 166-212 (in Japanese). Mochioka, N., O. Tabeta and T. Kubota (1988) A pre- Asano, H. (1984) . In “The fishes of the Japanese larva of Conger myriaster (family archipelago” (ed. by H. Masuda, K. Amaoka, C. Araga, Congridae) collected from Suruga Bay, central Japan. T. Uyeno and T. Yoshino), Tokai University Press, Jpn. J. Ichthyol., 35, 184-188. Tokyo, p. 28. Nemoto, Y., T. Kawai and T. Ishida (2004) Survival rate, Cau, A. and P. Manconi (1983) Sex-ratio and spatial dis- growth and sex ratio of reared juvenile conger eel, placement in Conger conger (L.). Rapp. P.-V. Reun. Conger myriaster. Bull. Fukushima Pref. Fish. Exp. CIESM., 28, 93-96. Stn., 12, 7-12 (in Japanese). Gorie, S. and T. Ohtani (1997) Growth of the juvenile Nemoto, Y. (2007) Sex ratio of the conger eel, Conger myri- white-spotted under experimental conditions. aster, in Matsukawaura Lagoon, Fukushima Prefecture. Suisanzoshoku, 45, 485-488 (in Japanese with English Bull. Fukushima Pref. Fish. Exp. Stn., 14, 37-40 (in abstract). Japanese). Gorie, S. and T. Ohtani (1998) Seasonal changes in the Okamura, A., T. Utoh, H. Zhang, Y. Yamada, N. Horie, N. length compositions of the white-spotted conger, Mikawa, S. Tanaka, T. Motonobu and H. P. 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標識放流からみた瀬戸内海東部海域におけるマアナゴの移動と成長

五利江重昭・反田 實・長澤和也

マアナゴの移動と成長を明らかにするため，明石海峡周辺で漁獲されたマアナゴ（平均全長約 50 cm）に標識を付けて放流し，その再捕状況から成長と移動を調べた。放流魚が 1 - 8 月頃に明石海 峡を経由して，播磨灘から大阪湾，紀伊水道へ移動することが明らかになった。年齢は 1 +-3+，また 1 日当たりの成長量は0.1- 2 mm であった。大阪湾・紀伊水道で再捕された雌の再捕時の全長と生殖 腺指数は，播磨灘・明石海峡で再捕された雌の値よりも有意に高く，播磨灘から大阪湾・紀伊水道へ の移動は，産卵回遊の準備段階のものであると推測された。しかし，放流魚全てが大阪湾へ移動する わけではなく，大型個体や放流後日数が経過した個体でも明石海峡周辺にとどまる場合も多かった。 明石海峡の大型マアナゴには，産卵回遊へ向かう群と，ある程度の年齢（大きさ）に達するまで明石 海峡周辺にとどまる群の存在が示唆され，複雑な移動様式が存在すると考えられた。