Aquaculture Sci. 58（2），233－242（2010） Movement and Growth of Whitespotted Conger Conger myriaster in the Eastern Seto Inland Sea, Japan 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 Osaka Bay 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 Awaji Island (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.