Aquaculture Sci. 58（2），167－179（2010） Habitat Preference and Feeding Habits of Juvenile Whitespotted Conger Conger myriaster in the Eastern Seto Inland Sea, Japan 1, 2,＊ 2 Shigeaki GORIE and Kazuya NAGASAWA Abstract: From 2003 to 2008, we tried to collect newly settled juvenile whitespotted conger in four areas of the northeastern Harima Nada Sea, eastern part of the Seto Inland Sea, Japan. We also investigated the relation between the stomach contents of juveniles (78-259 mm TL) and benthos composition at the sampling sites. No juveniles were taken from the sand bottom area, but they were collected from the gravel bottom and sand-mud bottom areas. Main stomach contents of the juveniles were crustaceans, polychaetes and teleosts. Crustaceans were strongly preferred being the most common and numerically important prey. Juveniles first fed on amphipods and, with growth, shifted their prey to decapods, polychaetes and teleosts. CPUEs (catch per unit effort) of the juveniles from the sand-mud bottom were significantly higher than those from the gravel bottom, although the crustacean biomass on the gravel bottom was more abundant than those on the other bottoms. These results suggest that, when the juveniles settle, they prefer the sand-mud bottom to the gravel bottom and choose their habitat not by the food environment but by the sedi- ment type. We consider that the sand-mud bottom area is a nursery ground of the juveniles. Key words: Conger myriaster; Juvenile; Habitat; Stomach content The whitespotted conger Conger myriaster is methods for efficient management, it is indispens- one of the commercially important fishes in the able to obtain information on various aspects of the Seto Inland Sea, Japan. Average annual catch life history of the target species. of this fish species from 1985 to 1997 in the sea Whitespotted conger are known to begin off Hyogo Prefecture was about 1700 metric their demersal life before completely finishing tons (mt) but it has decreased to 607 mt in 2007 metamorphosis and then to shrink in length (Anon. 2009). after settlement (Kubota 1961; Asano et al. With implementation of the fishery resource 1978). In the northeastern Harima Nada Sea, management concept, a management approach eastern part of the Seto Inland Sea, this species was started in the mid-1990s for whitespot- occurs as leptocephali from February to May ted conger in the eastern Seto Inland Sea. (Gorie and Tanda 2005). Juveniles of 0+ year Especially, an efficient and stable stock manage- cohort are about 250 mm in total length (TL) in ment is an important subject during the period August, when fish of the 1+ year cohort reach of stock decline. For the efficient utilization of at least 300 mm TL (Gorie and Ohtani 1998). In fish resources, there are various potential man- aquaria, juveniles can be reared by feeding poly- agement plans as reduction of fishing efforts chaetes and fresh fishes (Takai 1959) or even (especially for protection of juveniles), conserva- commercial diets (both moist and dry pellets) tion or creation of nursery grounds, and mass and grow at a high rate (Gorie and Ohtani 1997; release of seedling. In order to choose appropriate Gorie 2008). Received October 5, 2009: Accepted January 18, 2010. 1 Hyogo Prefectural Technology Center for Agriculture, Forestry, and Fisheries (Fisheries Technology Institute), 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). 168 S. Gorie and K. Nagasawa Only a few studies have been conducted juveniles of whitespotted conger occur in the on the feeding habits of commercial-sized eastern Seto Inland Sea and what environmen- whitespotted conger (Mori and Inoue 1982; tal conditions are related to their habitats. We Matsumiya and Imai 1987; Nabeshima et al. also discuss the feeding habits of the juveniles 1994; Fukuda et al. 1997). They prey mostly and their relation to the fauna of prey. on teleosts, crustaceans and mollusks. Similar results were reported for related species: Materials and Methods European conger Conger conger (Cau and Manconi 1984; Morato et al. 1999; O’Sullivan Fish sampling et al. 2004; Vallisneri et al. 2007) and American The study was conducted in the northeast- conger Conger oceanicus (Levy et al. 1988). ern Harima Nada Sea, eastern part of the Seto However, very little is known about the growth, Inland Sea off Hyogo Prefecture, Japan (Fig. 1). feeding habits and habitat during the juve- The survey region was divided into four areas: nile stage of these species. In Japan, Takai Area A (depth: 13 m), Area B (10 m), Area C (1959) reported that juvenile whitespotted (17 m) and Area D (18 m). conger occurred at 1-17 m in depth on sand- Sampling was conducted to catch juvenile mud bottom to mud bottom areas but detailed whitespotted conger from April to September in data have not been published. He also men- 2003-2008 by a bottom trawl net with a 2.4 mm tioned that the fish of 100-200 mm in TL fed mesh size of a cod end (Gorie and Tanda 2004). on shrimps, fishes, crabs and polychaetes. In Since no fish was caught during daytime and Korea, Huh and Kwak (1998) found that juve- few fish were obtained in civil twilight, sampling niles of the same species feed mainly on fishes, was carried out at night (30 min after sunset shrimps and crabs in a Zostera bed and show and later). The net was towed for 10-20 min at a very constant prey selection. Previously, we a speed of 2 knots using a small boat (9.1 mt). reported that the diet of whitespotted conger Before towing in each survey area, oceano- changed with fish size (Gorie and Tanda 2004). graphic data including water temperatures Nevertheless, information on the habitat and (WT), salinities and depths were recorded with food habit of their juveniles is quite limited, STD (JFE Alec Co., Ltd., Kobe, Japan). and there have been no studies on relations between prey fauna and stomach contents of Bottom sediment sampling and grain size analysis juveniles. Bottom sediments were sampled with a SK In this paper, we report where newly settled Type bottom sampler (Rigo Co., Ltd., Tokyo, 134°50´E 135°00´E 10m Kakogawa B Sea of Japan C 20m Kami Is. 10 km 34°40´N A D Akashi No.10 buoy Pacific Ocean Harima Nada Sea Fig. 1. Location of the survey areas in the eastern Seto Inland Sea, Japan. Habitat and Feeding Habits of Conger myriaster 169 Japan) in Areas B-D on 3 July 2003 and in Area Stomach content analysis A on 22 August 2003. Grain sizes were analyzed In May to August 2004, juveniles soon after based on the Japanese Geotechnical Society being caught were fixed in 5-10% buffered sea- Standard (JSF T 131-1990) and sorted by the water formalin. For stomach content analysis, Wentworth grain size scale. Losses on ignition 30, 106 and 120 fish were selected randomly were measured after the sediments were heated from Areas A-C, respectively (Table 1). The fish at 600℃ for 2 hours. were measured for total length (TL) and body weight (BW) and the stomachs were removed Benthos sampling and their composition and examined for contents. Prey items from Benthos were sampled before towing a trawl each stomach were identified to one of four net in 2004 with a Smith-MacIntyre bottom sam- groups (polychaetes, crustaceans, teleosts pler (22×22 cm, Rigo Co., Ltd., Tokyo, Japan) and others) and expressed as numerical and and by a sledge net (Fig. 2) for 1 min towing at weight percent by sampling date and fish size. a speed of 2 knots. The samples caught using Moreover, crustaceans were classified into the bottom sampler were sieved with a 1 mm amphipods, decapods or others. aperture size and stored in 5-10% buffered sea- Ivlev’s selectivity index (Ei) was calculated -1 water formalin. The benthos composition in as follows: Ei = (ri-pi)・(ri+pi) , where ri is the these samples is expressed as numerical and percentage of prey item i in the fish stomachs, weight percent for five categories: polychaetes, and pi is the percentage of item i in the benthos gastropods, bivalves, crustaceans and cephalo- samples collected with the Smith-MacIntyre chordates. Since these animals accounted for bottom sampler. The index can take values more than 95% of total individuals, other organ- between -1 and 1. A value between 0 and 1 isms were excluded from the analysis. The means that a fish has a positive preference for benthos sampled with the sledge net were cat- the prey item i, while a value between 0 and -1 egorized into one of eight groups: nematodes, shows a negative preference. If a value is 0, a polychaetes, gastropods, bivalves, crustaceans, fish has no preference to the item i. chaetognaths, ophiuroids and teleosts. Statistical analysis Differences in mean CPUE (catch per unit Outer net, 5mm effort) and in prey density were explored using the Steel-Dwass test between the three survey Inner net, GG50: 0.35mm areas where juveniles were collected. h=0.4m, w=0.6m Warp 2m Results Sled 0.7m Chain (6mm) Fish distribution and oceanographic features Juvenile whitespotted conger were collected Fig. 2. The sledge net used to collect epifaunal benthos in in Areas A-C, but no fish was taken in Area the eastern Seto Inland Sea. D. The juveniles were collected from May to Table 1. Number and size of fish examined for stomach contents Date Area A Area B Area C in 2004 N caught TL (SD, range) N examined N caught TL (SD, range) N examined N caught TL (SD, range) N examined 12, 14 May 7 86 ( 3.5, 81- 91) 1 97 85 ( 3.7, 76- 95) 10 18 84 ( 3.4, 77- 91) 5 23, 24 June 91 120 (13.4, 94-152) 20 181 112 (13.6, 86-151) 19 146 104 (10.5, 85-147) 40 21, 22 July 9 152 (22.8, 128-188) 4 374 139 (19.7, 84-195) 38 154 135 (21.5, 100-195) 40 24, 25 Aug.