Aquaculture Sci. 60（3），341－348（2012）

Biological Aspects of Large Whitespotted Conger (Conger myriaster) in the Akashi Strait, Eastern ,

1, 2,＊ 2 Shigeaki GORIE and Kazuya NAGASAWA

Abstract: This paper reports on various biological aspects of whitespotted conger (Conger myriaster) on the basis of a long-term pot survey conducted in the Akashi Strait from 1982 to 2009. The fish caught consisted of one or two major size groups, and large females (>500 mm total length) were predominant. Both the gonadosomatic index and condition factor of large females were higher in winter (January and February) than those in other seasons. Females appear to spend for up to four years in this strait and then begin a spawning migration by age-5 to other waters. While catch per unit effort (CPUE) in the pot survey showed considerably large annual and seasonal fluctuations, it was high from May to August and from November to January when bottom water temperature ranged from 10–25℃ but low from September to October (>25℃) and from February to April (<10℃). The Akashi Strait is likely to be an important habitat of large whitespot- ted conger for their early maturation.

Key words: Conger myriaster; Length frequency distribution; Seto Inland Sea; Pot survey

The whitespotted conger (Conger myriaster) This strait is small (only 4 km between the two is a benthic, commercially important marine fish islands) but has a strong tidal current (Kunishi and is usually caught by the small trawl fishery 1976), and the maximum water depth is about in the eastern Seto Inland Sea. They migrate 120 m (Kaseno 1976). While various fisheries to this region as leptocephali and recruit to the have been conducted there using different types fishery at about 250 mm in total length (TL) of fishing gear, such as small trawls, longlines, in October (Gorie and Ohtani 1998). The male- boat seines, gillnets, and angling, a pot fishery biased occurrence is known for young fish is not commonly operated because of the strong (<45 cm TL) (Takemori et al. 2004) and a sex- current (Uchihashi 1976). Furthermore, the pot biased migratory habit has been suggested fishery to catch whitespotted conger is strictly (Okamura et al. 2000). Large fish caught in this prohibited from December to May under the region are mostly females, some of which move Hyogo Prefectural Fishery Adjustment Rules from the Harima Nada Sea to both outside the common fishery areas off Hyogo and the Kii Channel from January to August: Prefecture, and only a few fisheries cooperative this movement appears to be the onset of their associations are permitted to operate the fishery. spawning migration (Gorie et al. 2010). It is generally known that large whitespotted The Akashi Strait is part of the eastern Seto conger are commercially caught with longlines Inland Sea and located between (the in the Akashi Strait, but we have little informa- main island of Japan) and (Fig. 1). tion on them. As those large fish have been

Received 2 December 2011; Accepted 12 March 2012. 1 Fisheries Technology Institute, Hyogo Prefectural Technology Center for Agriculture, Forestry and Fisheries, 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). 342 S. Gorie and K. Nagasawa thought to be a spawning stock (Gorie et al. 15.9 mm, respectively) (Nabeshima et al. 1995). 2010), it is important to collected their biologi- Sampling was done from evening to night. Deeply cal data for stock management and spawning frozen sardines were thawed, cut in pieces, and stock conservation. then put as bait in the pots. Pots were usually Since 1982, a monitoring survey of whites- put down to the sea before one hour of sunset potted conger has been conducted using pots and allowed the fish to trap for four hours. The in the Akashi Strait in order to clarify annual effective number of pots in each fishing survey changes in their length frequency distribution ranged from 60–110. The CPUE was expressed and catch per unit effort (CPUE). Although this as a number of the fish caught per pot for four survey must be operated during the neap tide, dipping hours. it can be done regardless of fishing grounds Oceanographic data, such as bottom water tem- and appears to be appropriate to examine the perature (WT), salinity, and depth, were taken with stock assessment in the Akashi Strait. In this STD ( JFE Advantech Co., Ltd., Nishinomiya, paper, biological aspects of whitespotted conger Japan) before fishing. caught through the pot survey are presented. Seasonal changes in gonadosomatic index, con- Fish examination dition factor, and age composition are also dis- The fish caught were transferred to the cussed on the assumption that large females in laboratory, where they were measured for total the eastern Seto Inland Sea including this strait length (TL, mm) and body weight (BW, g). In conduct a spawning migration. 1996 to 2009, they were sexed by the observation of the gonad (Gorie et al. 2004) and the gonad was Materials and Methods weighed (GW, g). For age determination, otoliths were removed, Fish collection cleaned, dried, and stored in small sealable plas- The study area is located in the western Akashi tic bags until otolith structure analysis. They were Strait (Fig. 1). Water depth of the survey area heated at 200℃ for several minutes on a hot ranged from 20–50 m. The bottom consisted of plate until the color turned light brown. Burnt gravel and cobbles. A fishing survey was con- otoliths were embedded in polyester resin ducted almost every month from 1982 to 1999 (Marumoto Struers K. K., Tokyo, Japan) and sec- and 2–5 times a year (usually twice: June and tioned into 0.3 mm thick intervals using a dia- November) from 2000 to 2009, using commer- mond saw (Leica SP1600; Leica Microsystems cially available pots (nominal mesh size, 18 setsu; GmbH, Wetzlar, Germany). Serial transverse sec- mesh size of the upper and bottom parts, 18 and tions were mounted on glass slides with sticky

Fig. 1. The study area (hatched) for the pot survey in the Akashi Strait, eastern Seto Inland Sea, Japan. Conger myriaster in the Akashi Strait 343 wax (Maruto Instrument Co., Ltd., Tokyo, Japan). observation, sections were covered with clear Their surfaces were ground using abrasive- nail polish and then microscopically observed coated papers (Presi, Grenoble, France), etched under transmitted light or UV light (Katayama with 0.2N HCl for several tens of seconds. Before et al. 2002, 2010).

Fig. 2. Monthly changes in length frequency distribution of whitespotted conger caught in the Akashi Strait based on the pot survey. Monthly data taken from 1982 to 2009 are combined. Vertical lines indicate 500 mm TL. Numerical values represent MTL±SD mm and percentage proportion of large fish (>500 mm TL). n, sample size. 344 S. Gorie and K. Nagasawa

Since the spawning season was estimated to TL (MTL) gradually increased from winter be from August through February (Tanaka et (Jan.–Mar.) to fall (Oct.–Dec.) and showed al. 1987; Lee and Byun 1996; Mochioka 2001; over 500 mm MTL during summer (July–Sep.) Kurogi et al. 2002), age is chronologically and fall. The fall MTL value was the biggest expressed, assuming that the fish hatch every (Fig. 3). There was a significant difference in January. Gonadosomatic index (GSI) and condi- MTL between four seasons (ANOVA, P<0.001): tion factor (CF) were calculated as follows: GSI P-value between winter and spring (Apr.-June) (%)=GW･ BW –1 ･ 100 and CF= BW･ TL–3 ･ 106, was 0.023, and others were less than 0.001 respectively. In this paper, individuals of whites- (Tukey-Kramer test). potted conger are categorized into two groups: large fish (>500 mm TL) and small fish (≤500 mm Gonadosomatic index (GSI) TL). As stated above, the majority of the fish examined for sex were females. The overall GSI Statistical analysis ranged from 0.02–5.06 (mean=1.59, n=245) In this paper, results are presented as mean and 0.14–0.44 (0.25, 6) in females and males, and standard deviation (SD). Overall significant respectively. Also, GSI ranged from 0.18–5.06 differences in TL, GSI, and CF between four (1.65, 233) in large females, while it from 0.02 seasons were determined by one-way analysis –0.86 (0.46, 12) in small ones. In most of large of variance (ANOVA). If the one-way ANOVA females, GSI was less than 4, and mean monthly was significant, differences between each GSI values were 1.5 in April to December and season were estimated using Tukey-Kramer 2.7 in January to February (Fig. 4). Although method. The GSI of fish between the Akashi only a small number (n=4) of large females Strait and other region (Osaka Bay and the Kii were examined in February, their GSI values Channel combined) was examined by Mann- were clearly separated into two (high and low) Whitney’s U test. groups. In large females, there was a significant dif- Results ference in GSI between four seasons (ANOVA, P<0.001): GSI was the highest in winter (Jan.– Total length distribution Feb.) (Tukey-Kramer test, P<0.001), decreased A total of 14,370 whitespotted conger were markedly in spring (Apr.–June), but increased caught during this survey from 1982 to 2009, and 7,829 fish (54%) were larger than 500 mm TL. The overall TL and BW ranged from 217– 953 (mean=519, n=14,370) mm and from 12– 2,028 (283, 14,354) g. The males and females measured 390–506 (446, 17) and 314–950 (575, 773) mm TL, respectively. Of the 829 fish exam- ined for the sex, the vast majority (n=773, 93%) were females, but the remaining 17 and 39 fish were males and unsexed, respectively. There were one or two major modes in length frequency distribution (Fig. 2). In October, fish smaller than 400 mm TL newly appeared. These fish gradually grew up in the following months. The percentages of large fish in January and Fig. 3. Seasonal changes in mean total length of whites- potted conger caught in the Akashi Strait based on the pot February were about 40% and 20%, respec- survey. Seasonal data taken from 1982 to 2009 are com- tively. It then increased gradually and reached bined. Vertical bars represent standard deviation. Sample 76% in December. The overall seasonal mean size is given for each sampling season. Conger myriaster in the Akashi Strait 345 gradually in July to December (Fig. 5). P-values females and males, respectively. CF values between fall (Oct.–Dec.) and spring and between ranged from 1.3–2.8 (1.9, 599) and 1.3–2.2 (1.7, fall and summer (July–Sep.) were 0.003 and 0.01, 173) in large and small females, respectively. respectively. Male’s CF was 1.6 in one large fish (506 mm TL) and ranged from 1.5–2.2 (1.8, 16) in small fish. Condition factor (CF) In large females, a significant difference The overall CF ranged from 1.3– 2.8 was found in CF, like GSI, between four sea- (mean=1.8, n=772) and 1.5–2.2 (1.8, 17) in sons (ANOVA, P<0.001): CF was the highest in winter (Jan.–Feb.) (Tukey-Kramer test, P<0.001) but decreased in spring (Apr.–June). In small females, on the other hand, there was no significant difference in CF between four seasons (Fig. 6).

Age composition Of the 291 fish examined for age, 276 (95%) were females, but the remaining one and 14 fish were a male and unsexed, respectively. This single male was age-2 (395 mm TL). The females were age-1 to 4 but did not contain any fish of age-5 and more (Fig. 7). There was a wide TL variation in females of such ages.

Fig. 4. Monthly changes in gonadosomatic index of female whitespotted conger caught in the Akashi Strait CPUE during the pot survey. Monthly data taken from 1996 to CPUE was high from May to August and 2009 are combined. Closed circles, actual values from from November to January (highest in June) large fish (>500 mm TL, n=233); open circles, mean values but low from September to October and from from large fish; open triangles, actual values from small fish (≤500 mm TL, n=12). February to April (lowest in March) (Fig. 8).

Fig. 6. Seasonal changes in mean condition factor of Fig. 5. Seasonal changes in mean gonadosomatic index female whitespotted conger caught in the Akashi Strait of large female whitespotted conger (>500 mm TL) caught during the pot survey. Shaded bars, large fish (>500 mm in the Akashi Strait during the pot survey. Seasonal data TL); open bars, small fish (≤500 mm TL). Seasonal data taken from 1996 to 2009 are combined. Vertical bars indi- taken from 1996 to 2009 are combined. Vertical bars indi- cate standard deviation. Sample size is given for each sam- cate standard deviation. Sample size is given for each sam- pling season. No data in March are available. pling season. No data in March are available. 346 S. Gorie and K. Nagasawa

These monthly changes in CPUE well coincided with those of WT: CPUE values were usually high when WT ranged from 10–25℃ but low when WT was very high (over 25℃) or very low (under 10℃). While CPUE showed considerably large annual and seasonal fluctuations, it showed a decreasing trend from 1982 to 1999 but a gradual recovery after 2000 (Fig. 9). Although annual commercial catches of the species in the eastern Seto Inland Sea off Hyogo Prefecture were correlated with annual CPUE values from 1982 to 1999, no sig- Fig. 7. Total length of female whitespotted conger of vari- ous ages caught in the Akashi Strait during the pot survey nificant correlative relationship was detected from 1996 to 2009. from 2000 to 2008: commercial catch continued to show a decreasing trend, but annual CPUE had an increasing trend.

Discussion

The Akashi Strait is considered to be an impor- tant region for whitespotted conger before the onset of their spawning migration. Unlike those in the nearby Osaka Bay and the northeastern Harima Nada Sea (Gorie and Ohtani 1998; Tanda et al. 1998), individuals caught in the Akashi Strait are mostly large females, implying that the fish over 400 mm TL aggregate and/or grow there. The present study also supports our previ- Fig. 8. Monthly changes in mean CPUE (a number of the ous hypothesis (Gorie et al. 2010) that large fish caught in a pot per four dipping hours) of whitespot- females move from the Akashi Strait to other ted conger and bottom water temperature in the Akashi Strait based on the pot survey. Monthly data taken from regions for spawning during January to August 1982 to 2009 are combined. Closed circles, CPUE; open and the remaining other individuals continue circles, bottom water temperature (WT, ℃). to grow in the strait. After recruitment in

Fig. 9. Seasonal changes (12-month moving average) in CPUE (a number of the fish caught in a pot per four dipping hours) of whitespotted conger in the Akashi Strait based on the pot survey from June 1982 to November 2009. Conger myriaster in the Akashi Strait 347

October as individuals smaller than 400 mm to estimate their stock abundance if no fishery TL, they grew, gradually replaced large fish, statistic is provided. In 1982 to 1999, annual and were more abundant than small fish from commercial catches of the species in the east- July to December. The percentage of such ern Seto Inland Sea off Hyogo Prefecture were large fish dropped in between December and correlated with annual changes in CPUE, but February and thereafter increased gradually such correlative relationship disappeared from (Figs. 2, 3). Their GSI and CF values peaked in 2000 to 2008. Although it is not easy to explain January to February, dropped in April to June, this reason, one of the possible explanations but then gradually increased (Figs. 4–6). In is that the observed, non-correlative relation- particular, GSI values of the majority of large ship during the latter period might be due to the females caught in the Akashi Strait in January reduction in sampling effort: the survey was con- to February remained less than 4 (2.7±1.21, ducted at almost monthly intervals from 1982 to n=31) which were significantly lower (U test, 1999, but it was done only 2–5 times a year from P=0.005) than those in tag-released and recap- 2000 to 2008. It is thus desirable to analyze in tured females in Osaka Bay and the Kii Channel details a relationship between commercial fisher- (4.2±0.97, n=8, Gorie et al. 2010). Also, GSI ies and catch data from the pot survey. values of large females from the Akashi Strait To summarize, length frequency distribution were significantly lower (U test, P<0.001) than of whitespotted conger caught in the Akashi those of individuals commercially caught in Strait through the pot survey showed the pres- Osaka Bay in the same season (5.1±1.16, n=10, ence of two major size groups, and large females Gorie unpublished). Moreover, females caught were dominant there. The Akashi Strait is likely in the Akashi Strait were less than age-5 (Fig. 7). to be an important region for the early matura- These results suggest that large whitespotted tion of such large females before conducting the conger occurring in the Akashi Strait are a spawning migration. They may stay in the strait spawning stock and spend a certain period for up to four years. Considering the importance in preparation for their spawning migration. of the Akashi Strait for the life cycle of whites- Especially, females with high GSI (>4) may be potted conger, we must consider conservation just about to begin the spawning migration in of individuals in this area: this species has been January to February. Okamura et al. (2000) also targeted by only a few fisheries there, which suggested that whitespotted conger spend a might have contributed to the conservation of certain period off the Pacific coast of the Atsumi the spawning stock. Further study through the Peninsula, central Japan, and move to a deeper pot survey is desirable to collect more informa- area with maturation. tion on the biology and abundance of whitespot- The migration of males, on the other hand, is ted conger in the Akashi Strait. still yet unknown because of the small sample size in this study. More information on the age References composition and other biological aspects of males in the Akashi Strait is necessary to clarify Gorie, S. and T. Ohtani (1997) Growth of the juvenile the trigger to launch their spawning migration. white-spotted congers under experimental conditions. Suisanzoshoku, 45, 485-488 (in Japanese with English Monthly changes in CPUE in the Akashi abstract). Strait were closely related to those in WT, and Gorie, S. and T. Ohtani (1998) Seasonal changes in the high CPUEs were observed when WT ranged length compositions of the white-spotted conger, from 10–25℃ (Fig. 8). This is probably because Conger myriaster, caught by the small trawl fishery in Harima-Nada and the possible use of enlarged mesh the feeding of whitespotted conger is dependent size for resource management. Suisanzoshoku, 46, on water temperature (Gorie and Ohtani 1997). 495-501 (in Japanese with English abstract). A significant correlation between commer- Gorie, S., M. Tanda and S. Katayama (2004) Age and growth cial catch of whitespotted conger and CPUE of of white-spotted conger Conger myriaster collected in northeast Harima Nada, Seto Inland Sea. Suisanzoshoku, the pot survey in the Akashi Strait can help us 348 S. Gorie and K. Nagasawa

52, 407-411. Nabeshima, Y., T. Abe, K. Yamamoto, S. Omoto and T. Gorie, S., M. Tanda and K. Nagasawa (2010) Movement Tokai (1995) Establishment of minimum permissible and growth of whitespotted conger Conger myriaster capture length and selection of its appropriate mesh in the eastern Seto Inland Sea, Japan. Aquaculture Sci., size of basket trap targeting white-spotted conger eel 58, 233-242. Conger myriaster (Brevoort) for fisheries oriented Kaseno, Y. (1976) The topography and geological features resource management, and its estimated effect. Bull. of the bottom of the Akashi Strait. In“ Fisheries in the Osaka Pref. Fish. Exp. Stn., 9, 41-55 (in Japanese). Akashi Strait” (ed. by T. Hanaoka, H. Kunishi, T. Okamura, A., T. Utoh, H. Zhang, Y. Yamada, N. Horie, N. Kuroki and K. Uchihashi), Research Committee for Mikawa, S. Tanaka, T. Motonobu and H. P. Oka (2000) the Fisheries in the Akashi Strait, , pp. 1-4 (in Seasonal changes in maturity in the conger eel Conger Japanese). myriaster at the Pacific coast of Atsumi Peninsula, Katayama, S., T. Ishida, K. Goto, K. Iizuka and K. Karita central Japan. Nippon Suisan Gakkaishi, 66, 412-416 (2002) A new aging technique by UV light observa- (in Japanese with English abstract). tion of burnt otoliths for the conger eel Conger myri- Takemori, H., M. Yamamoto and H. Makino (2004) Ecological aster (Brevoort). Ichthyol. Res., 49, 81-84. study of the conger Conger myriaster in the eastern Bisan Katayama, S., M. Yamamoto and S. Gorie (2010) Age com- Seto and mark-recapture experiment of the conger positions of flatfish stocks as determined by a new Conger myriaster. Bull. Kagawa Pref. Fish. Exp. Stn., 5, otolithometric method, its application in the estima- 7-12 (in Japanese). tion of growth, spawning potential and fisheries man- Tanaka, K., O. Tabeta, N. Mochioka, J. Yamada and S. agement. J. Sea Res., 64, 451-456. Kakuda (1987) Otolith microstructure and ecology of Kunishi, H. (1976) Research on the flow conditions in the the conger eel (Conger myriaster) larvae collected in Akashi Strait. In “Fisheries in the Akashi Strait” (ed. the Seto Inland Sea, Japan. Nippon Suisan Gakkaishi, by T. Hanaoka, H. Kunishi, T. Kuroki and K. Uchihashi), 53, 543-549. Research Committee for the Fisheries in the Akashi Tanda, M., T. Nishikawa and S. Gorie (1998) Real state of Strait, Kobe, pp. 89-102 (in Japanese). Japanese conger eel fishery in Hyogo Prefectural Kurogi, H., N. Mochioka, Y. Takaki and O. Tabeta (2002) region of the Seto Inland Sea. Bull. Hyogo Pref. Fish. First offshore record of Conger myriaster leptocephali Exp. Stn., 34, 59-64 (in Japanese). in the East China Sea. Fish. Sci., 68, 1155-1157. Uchihashi, K. (1976) Fisheries in the Akashi Strait based Lee, T. W. and J. S. Byun (1996) Microstructural growth in on a fishery calendar. In “Fisheries in the Akashi otoliths of conger eel (Conger myriaster) leptocephali Strait” (ed. by T. Hanaoka, H. Kunishi, T. Kuroki and during the metamorphic stage. Mar. Biol., 125, 259-268. K. Uchihashi), Research Committee for the Fisheries Mochioka, N. (2001) Early life history of Conger myriaster. in the Akashi Strait, Kobe, pp. 363-408 (in Japanese). Nippon Suisan Gakkaishi, 67, 111-112 (in Japanese).

明石海峡における大型マアナゴの資源生物学的特徴

五利江重昭・長澤和也

明石海峡におけるマアナゴの資源生物学的情報を明らかにするため，1982－2009年にかけて，あな ご籠を用いた試験操業を実施し，当海域におけるマアナゴの月別 CPUE，全長組成の推移，GSI およ び肥満度の変化を調べた。マアナゴの全長組成には 1－2 個の大きなモードが存在し，毎年10月頃に 全長40 cm 以下の新規加入群が認められ，この群は 1 年かけて，より大きなモードへと移行していく 様子がみられた。CPUE は，水温変動期である春と秋に高く（底層水温：10－25℃），夏と冬に低かっ た（25℃以上および10℃以下）。明石海峡には主に大型の個体が分布し，全長50 cm 以上の雌の占め る割合が多かったため，明石海峡は産卵回遊に向かうまでの集積地の一つであると考えられた。雌の GIS および肥満度は，冬（1－2月）に高く，春（ 4月）になると低下し，年齢は 0－4 歳であった。 マアナゴの雌は，5 歳までの冬以降に産卵回遊を行うと考えられた。