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Biological Aspects of Large Whitespotted Conger (Conger Myriaster) in the Akashi Strait, Eastern Seto Inland Sea, Japan

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Biological Aspects of Large Whitespotted Conger (Conger Myriaster) in the Akashi Strait, Eastern Seto Inland Sea, Japan Aquaculture Sci. 60（3），341－348（2012） Biological Aspects of Large Whitespotted Conger (Conger myriaster) in the Akashi Strait, Eastern Seto Inland Sea, Japan 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 Osaka Bay 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 Honshu (the in the Akashi Strait, but we have little informa- main island of Japan) and Awaji Island (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.
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