Doctoral Thesis

Fishery biology of whitespotted conger Conger myriasterin the eastern Seto Inland Sea, Japan

Shigeaki Gorie
Department of Bioresource Science
Graduate School of Biosphere Science
Hiroshima University
March 2011

TABLE OF CONTENTS

Abstract

1

I.

4

Introduction

II. Migration of whitespotted conger leptocephali to the eastern Seto Inland Sea

11

1. Occurrence of whitespotted conger leptocephali
2. Unintentional bycatch of whitespotted conger leptocephali by the boat seine fishery

III. Habitat preference and feeding habits of whitespotted conger juveniles

27

IV. Age, growth, and movement of whitespotted conger51

1. Growth of whitespotted conger in captivity
2. Growth of whitespotted conger in the wild
1) Growth of whitespotted conger based on the fishing survey2) Growth of whitespotted conger based on otolith reading
3) Movement and growth of whitespotted conger determined by tagging study

V.

Fishery status of whitespotted conger and stock management approach102

1. Whitespotted conger fishery in the eastern Seto Inland Sea off Hyogo Prefecture
2. Mesh selectivity of whitespotted conger in the small trawl fishery

VI. Biological aspects of whitespotted conger in the Akashi Strait

127

VII. General discussion

147

Acknowledgments

151

References

152

Abstract

The whitespotted conger Conger myriaster is a benthic, commercially importantmarine fish and usually caught by the small trawl fishery in the eastern Seto Inland Sea. The
species migrates as leptocephali (transparent willow-leaf-shaped larvae) to this region inearly spring, spends a certain period, and then leaves probably for the spawning ground.
Since 1993, a survey for the fishery stock management of whitespotted conger has been
conducted in the eastern Seto Inland Sea off Hyogo Prefecture. The primary purpose of thisstudy is to clarify biological characteristics of the species in this sea during its life history
from larval recruitment to probable spawning migration in order to get knowledge for thefishery stock management.
The occurrence of whitespotted conger leptocephali in the northeastern Harima Nada
Sea is described. Their migration to the region was observed from February to May (mainly
from March to April) when sea surface temperature measured 8-18°C. The leptocephali
occurred mainly in the bottom layer, but they spread to the surface at night. A half of themmigrating to the region were found to be at metamorphosing stage.
A large number of whitespotted conger leptocephali were caught as bycatch by theboat seine fishery during their migration. The fishery is likely to give a negative impact on
the stock abundance of the species. Bycatch of the leptocephali is not desirable.
The distribution and feeding habits of the newly settled juveniles of whitespottedconger are reported. No juveniles were sampled from the sand bottom, but they occurred on
the gravel and sand-mud bottoms. The main stomach contents of the juveniles were
crustaceans, polychaetes, and teleosts. Crustaceans were strongly preferred, being the mostcommon and numerically important prey. The juveniles initially fed on amphipods and, withgrowth, shifted their prey to decapods, polychaetes, and teleosts. The density abundance ofthe juveniles from the sand-mud bottom was significantly higher than that of the fish fromthe gravel bottom although the crustacean biomass on the gravel bottom was more abundant
than that on the sand-mud bottom. Therefore, it is considered that whitespotted conger
juveniles choose their habitat not by the food environment but by the bottom sediment type.
The sand-mud bottom is a nursery ground for juvenile whitespotted conger.
The growth of whitespotted conger in captivity is studied for their three stages(leptocephali, newly settled juveniles, and young fish) by feeding them on commercially1available moist pellets or dry pellets. The leptocephali completed metamorphosis without
any feeding. Both the newly settled juveniles and young fish could be reared by feeding thepellets of two types. The juveniles grew from less than 100 mm in total length (TL) in May
to 300 mm TL in October, and some fish reached over 500 mm TL within a year. It is thus
possible that the species is cultured from its juvenile stage and, by short-time rearing, to add
a commercial value to those juveniles which are not commercially utilized.
The age and growth of whitespotted conger caught in the eastern Seto Inland Sea are
analyzed. Sagittal otoliths were validated for the seasonality of annuli formation and foundto be used as an age character. The maximum age of the fish examined was 5. They showeda wide variation in TL in the same year class and also a complicated age composition in thesame TL class. The male- and female-biased sex ratios were recorded for the small- andlarge-sized fish, respectively.
The growth and migration of whitespotted conger are investigated by tagging study.
They migrated from the northeastern Harima Nada Sea to Osaka Bay and the Kii Channel
through the Akashi Strait from January to August. The TL and gonadosomatic index of thefemales recaptured in Osaka Bay and the Kii Channel were larger and higher, respectively,
than those in the northeastern Harima Nada Sea and the Akashi Strait, which suggests thatthe females conduct a pre- or spawning migration. Nevertheless, in addition to such
migratory fish, the presence of locally resident fish is suggested, and the species may havecomplicated migratory habits. The fish were recaptured up to 420 days after release andexhibited various growth rates ranging from 0.1-2 mm day-1, which is almost equivalent toabout 20 cm year-1.
The average annual catch of whitespotted conger was 1,700 metric tons (mt) from1984 to 1997 in the eastern Seto Inland Sea off Hyogo Prefecture, but the annual catchdecreased to 562 mt in 1999. Subsequently, it slightly recovered during the early 2000’s. The
fluctuation of the annual catch is considered to be related to annual changes in abundance ofthe leptocephali migrating to the region. Whitespotted conger in this sea were mostly caught
by the small trawl fishery and ranged from 250-450 mm TL. They recruit to the fishery at
250 mm TL in fall and were continuously caught for a year.
The mesh and artificial selectivities of whitespotted conger in the small trawl fisheryare studied in the northeastern Harima Nada Sea, and a possible use of enlarged mesh sizefor the codend is discussed as an option for the stock management. A 50% mesh selected TL

2was estimated as 282 mm, and it is desirable to enlarge the codend mesh size from 23 to 26
mm if the conditions are permitted in the region.
Various biological aspects of whitespotted conger caught in the Akashi Strait aredescribed. There were one or two size groups in monthly length frequency distribution, andthe large-sized females (>500 mm TL) were dominant in the catch. The Akashi Strait islikely to be one of the important habitats for the growth and maturation of the species. The
females seem to spend up to four years there and may begin the pre- or spawning migration
by age-5 to other waters.
Based on this study, several measures are proposed to efficiently utilize the stock ofwhitespotted conger in the eastern Seto Inland Sea as follows: 1) although correlation
between the bycatch and stock abundance of the species is yet unclear, we need to minimize
the bycatch of the leptocephali by the boat seine fishery, which may also reduce
unintentional bycatches of other valuable species; 2) since the sand-mud bottom is an
important habitat for the juveniles, it is necessary to conserve such areas as their nursery
ground; and 3) we need to consider the effective utilization of the small-sized fish (<300 mm
TL), which are mainly caught by the small trawl fishery. Their wholesale price is about ashalf as that of the medium-sized fish, then by enlarging the codend mesh size in the small
trawl fishery, we can reduce the catch of the small-sized fish and expect to increase the catchsales.
While much information has been accumulated through this study on the biology ofwhitespotted conger in the eastern Seto Inland Sea, we need more work as follows: 1) amonitoring survey to determine the annual abundance of the leptocephali migrating to theregion; 2) an investigation into the sex ratio of the species by age (fish size) and region; 3) a
study on the reproductive biology and farming of the species; and 4) a survey on the
spawning ground of the species, which is totally unknown in Japanese waters.

3

I. Introduction

The whitespotted conger Conger myriaster (Brevoort, 1856) is a bottom-dwelling,
marine fish and widely distributed in inshore waters of the Far East Asia, including China,
Korea, and Japan (Asano 1984; Park 2001; Tokimura 2001; Tokimura and Yamada 2001). In
Japan, this species is a popular cooking ingredient for Tempura, Sushi, Yaki-anago (= grilledconger), and Ni-anago (= boiled conger). Yaki-anago is preferred in the Kansai area, while
Ni-anago is popular in the Kanto area. The species migrates as leptocephali (transparent
willow-leaf-shaped larvae) to Japanese waters (Kurogi 2008), probably spends a certain
period in the coastal sea areas, and then leaves there for a yet unknown spawning ground.
Whitespotted conger are commercially caught using different types of fishing gear inJapanese waters, including trawls, longlines, tube traps, and pots. The catch in Japanesewaters declined from 13,000 to 8,200 metric tons (mt) during 1995 to 1999. It decreased
gradually from 2000 to 2008 and was 6,339 mt in 2008. The average catch from 1995 to2008 was 8,900 mt. Among various fishing grounds around Japan, the Seto Inland Sea
provides the largest catch of the species (32% of the country catch in 2008), followed by the
mid-Pacific Ocean waters (20%). The catch trend in the Seto Inland Sea well coincides withthat in Japanese waters: the annual catch was 6,000 mt in 1995, but it decreased gradually
and was about 2,000 mt in recent years (Fig. I-1) (Anon. 1997-2010).
The eastern Seto Inland Sea consists of three regions (i.e., the Harima Nada Sea,
Osaka Bay, and the Kii Channel) (Fig. I-2). Both in the Harima Nada Sea and Kii Channel,
whitespotted conger are mostly caught by the small trawl fishery, while in Osaka Bay, they
are mainly taken by the pot fishery (Nabeshima et al. 1995). The catch in the Kii Channel isconsiderably lower than those in the two other waters (Fig. I-3) (Anon. 1997-2007).
Several studies have been conducted on the biology of whitespotted conger in Japan.
Previous works, however, mainly focused on the early life history of the species (Tanaka etal. 1987; Yamano et al. 1991; Lee and Byun 1996; Otake et al. 1997; Kimura et al. 2004).
There are only a few fishery biological studies which provide preliminary information on thefeeding habit (Takai 1959; Mori and Inoue 1982; Matsumiya and Imai 1987; Fukuda et al.
1997), age and growth (Ishida et al. 2003; Nemoto et al. 2004; Takashima and Takahashi2006), and ecology (Shimizu 2003; Katayama et al. 2004; Nemoto 2007). In addition,
several works currently have focused on the reproductive biology (Okamura and Motonobu

4

1999; Utoh et al. 2003; Chiba et al. 2005). Although no sexually matured fish have beenfound to date in inshore waters around Japan, ovulation was induced with hormonal
treatment (Horie et al. 2001), and spermiation was observed (Utoh et al. 2004) in captivity.
Research on the spawning ground also has been tried. Takai (1959) suggested that a single
spawning ground exists in the East China Sea off northern Taiwan. While no significant
genetic differentiation was detected among the specimens from off the east coast of central
Japan, Kimura et al. (2004) suggested the presence of multi-spawning stocks with different
spawning grounds, spawning seasons, and/or migration routes. In contrast to thesesuggestions, it has been currently inferred that the spawning ground is present in the western
North Equatorial Current region and that the larvae are transported westward to the Kuroshio
Current and dispersed by it and its branch, the Tsushima Warm Current (Ma et al. 2007;
Kurogi 2008). Nevertheless, the exact location of the spawning ground and spawning
behavior of whitespotted conger are still unknown.
Like whitespotted conger in East Asian waters, two other related species, the European
conger Conger conger (Linnaeus, 1758) and the American conger Conger oceanicus(Mitchill, 1818), are also commercially important. The European conger is distributed in the
Mediterranean Sea to the northeastern Atlantic Ocean, while the American conger occurs off
the east coast of North America (Katayama and Kurogi 2008). Various aspects of the biologyof European conger have been investigated, including the early life history (Correia et al.
2002a), feeding habits (Cau and Manconi 1984; Morato et al. 1999; O’Sullivan et al. 2004;
Vallisneri et al. 2007), age, growth, and reproductive status (Sbaihi et al. 2001, O’Sullivan etal. 2003; Correia et al. 2009). For American conger, the metamorphosis, otolith
microstructure, and otolith microchemistry have been studied (Bell et al. 2003; Correia et al.
2004). Further, the distribution, age, growth, and reproduction of the species were discussed(Hood et al. 1988; McCleave and Miller 1994), and the food habit was also reported (Levy etal. 1988). However, knowledge of the spawning ground and spawning behavior is quite
limited for both species.
We have very little information on the whitespotted conger population in the eastern
Seto Inland Sea. Nabeshima et al. (1994) described the feeding habit of thecommercial-sized fish from Osaka Bay. Nishikawa et al. (1994) studied a mesh selectivity inthe small otter trawl fishery for the species. Utsumi (2009) reported the fishing status andoccurrence of the leptocephali in the eastern Kii Channel.

5In 1988, a fishery resource management project started for the four fish species,
including marbled flounder Pseudopleuronectes yokohamae (Günther, 1877), ridged-eye
flounder Pleuronichthys cornutus (Temminck & Schlegel, 1846), bastard halibut
Paralichthys olivaceus (Temminck & Schlegel, 1846), and red seabream Pagrus major(Temminck & Schlegel, 1843), in cooperation with the six prefectures (Okayama, Hyogo,
Osaka, Wakayama, Kagawa, and Tokushima) facing the eastern Seto Inland Sea (Anon.
1991). Also, since 1993, a survey has been conducted in this sea off Hyogo Prefecture for the
resource management of whitespotted conger and other fishes caught by the small trawl
fishery (Anon. 1996).
A stock management approach to the tube trap fishery for whitespotted conger has
been employed in Tokyo Bay (Shimizu 2003; Uchida et al. 2005; Harada et al. 2007). A
management also has been conducted for the species in Ise-Mikawa Bay for various fisheries,
including the small trawl fishery, pot fishery, and boat seine fishery (Anon. 2010).
As shown in Fig. I-3, the current catch of whitespotted conger in the eastern Seto
Inland Sea remains at a low level, and an efficient utilization of the fishery stock is very
important during such a low catch period. The stock management for optimum utilization ofwhitespotted conger should be done based on the accurate stock assessment and sufficientbiological knowledge of the species, and the stock estimation can be completed using theaccurate catch data. Much information on the life history of whitespotted conger in theeastern Seto Inland Sea is necessary for the stock management approach.
The primary purpose of the present study is to clarify various aspects of the biology ofwhitespotted conger in the eastern Seto Inland Sea from their larval recruitment to probablespawning migration in order to implement an efficient stock management of the species. Partof the results presented in this thesis was published earlier by me and my colleagues (Gorie
and Ohtani 1997, 1998a, 1998b; Gorie and Tanda 2004; Gorie et al. 2004, 2010; Gorie andNagasawa 2010).
Following this introduction (Chapter I), the migration of whitespotted conger
leptocephali into the Harima Nada Sea is reported in Chapter II. Biological information on
whitespotted conger juveniles during a period from post-settlement to recruit is provided in
Chapter III. In Chapter IV, the age and growth are analyzed using the aquarium-reared andwild-caught fish, and the migration and growth of the species are also given based on atagging study. In Chapter V, the catch and fishing status of the species in the eastern Seto

6

Inland Sea are analyzed, and a possible use of an enlarged mesh size is discussed for future
resource management. In Chapter VI, monthly changes in length frequency distribution andcatch per unit effort (CPUE) of whitespotted conger in the Akashi Strait through a pot survey
are reported, and some biological aspects, including changes in gonadosomatic index,
relative condition factor, and age composition of the species, especially the large-sized
females, in this area are also discussed. This thesis concludes a general discussion in Chapter
VII, which contains suggestions for the stock management and proposals for future work ofwhitespotted conger in the eastern Seto Inland Sea.

7

15

Catch (1000 mt)

12

Japan
9
6
3
Seto Inland Sea
01995 1997 1999 2001 2003 2005 2007
1996 1998 2000 2002 2004 2006 2008

Fig. I-1. Annual changes in catch of whitespotted conger in Japanese waters and the Seto
Inland Sea from 1995 to 2008.

8134ﾟ20'
Sea of Japan

134ﾟ40'135ﾟ135ﾟ20'34ﾟ40'

Pacific Ocean

Harima Nada Sea

Osaka Bay

34ﾟ20'

Awaji Is.
10 km

Kii Channel

Fig. I-2. The eastern Seto Inland Sea, a study area in this study.

9

1,4001,200

Harima Nada Sea

1,000
800
600
400
200
0
1,400
1,200

Osaka Bay

Catch (mt)

1,000
800
600
400
200
0
1,400
1,200

Kii Channel

1,000
800
600
400
200
0
1995

1997

1996

1999

1998

2001

2000

2003

2002

2005

2004

Fig. I-3. Annual changes in catch of whitespotted conger by various fisheries in the Harima
Nada Sea, Osaka Bay, and the Kii Channel from 1995 to 2005. Open squares, total catch;closed circles, catch by small trawl fishery; open circles, catch by longline fishery; closed
triangles, catch by other fisheries.

10II. Migration of whitespotted conger leptocephali to the eastern Seto Inland Sea

Whitespotted conger occur as leptocephali (transparent willow-leaf-shaped larvae) ininshore waters around Japan. Both the pre- and metamorphosing leptocephali have beenrecorded from various regions in different seasons: the Suo Nada Sea from October to June
(Takai 1959), the Kii Channel from November to April (Utsumi 2009), both Ise and Suruga
bays from February to May (Kubota 1961; Mochioka et al. 2006), and the southern Tohoku
waters from January to July (Onuma 1995; Katayama and Shimizu 2006). Based on theseinformation, it is evident that whitespotted conger leptocephali tend to emerge earlier at thelower latitudes of Japan. They also have been considered to conduct multiple migrations
intermittently (Shimizu 2005; Katayama and Shimizu 2006; Mochioka et al. 2006; Kurogi2008).
Clarifying their migration ecology can provide basic knowledge to choose options forthe stock management (e.g., conservation of the leptocephali), but we have only limited
information on the migration and its duration in the eastern Seto Inland Sea.
In this chapter, I report the occurrence of whitespotted conger leptocephali in thenortheastern Harima Nada Sea. I also describe unintentional bycatch of the leptocephali by
the boat seine fishery.

1. Occurrence of whitespotted conger leptocephali

In this section, various aspects of the migration of whitespotted conger leptocephali,such as migrating duration, vertical distribution, and metamorphic stage are presented basedon a fishing survey in the northeastern Harima Nada Sea and PAL/TL (preanal length/total
length) analysis of the sampled leptocephali.

Materials and Methods

Fish collection
A survey was conducted from January to May in 2003 and February to May in
2004-2009 (except for 2005 and 2007) at depths of 20-25 m near Kami Island in thenortheastern Harima Nada Sea (Fig. II-1-1).

11

Whitespotted conger leptocephali were collected by a small midwater beam trawl net
with a 2.4 mm mesh aperture of a codend (Fig. II-1-2). The net was towed in the middle andbottom layers for 30 min at a speed of 1-2 knots using a small boat (9.1 mt) in the daytime.
In 2003, three layers (surface, middle, and bottom) were towed both in the daytime and night.
All the night surveys were conducted from 30 min lag time after sunset. The net was held ineach layer during the net towing by monitoring the position of a net recorder unit (TypeCN-8: Furuno Electric Co., Ltd., Nishinomiya, Japan) attached on the square net. In thesurface, middle, and bottom layer samplings, the net recorder unit was kept 2-3 m downfrom the surface, at an intermediate depth, and 3-4 m up from the sea bottom, respectively.
The ground rope was usually 1-2 m above the sea bottom when towing the bottom layer. The
catch per unit effort (CPUE) was expressed as mean number of the leptocephali collected per
30 min towing.
Before the net was towed, oceanographic data, such as water temperatures, salinities,
and depths, were collected using STD (JFE Advantech Co., Ltd., Nishinomiya, Japan).

Fish measurements and determination of metamorphic stage
Whitespotted conger leptocephali were fixed in 5-10% buffered formalin solution on
board immediately after capture and then brought to the laboratory, where they were
measured for total length (TL) and preanal length (PAL) (i.e., length from the tip of the snout
to the posterior margin of the anus). All measurements were taken in millimeters. In thisstudy, those larvae with a value of PAL/TL<0.88 were identified as being ofmetamorphosing stage, based on a preliminary analysis (see the Discussion section).

Statistical analysis
Statistical difference between CPUEs in each towing layer as well as sampling time
(daytime vs. night) was analyzed by Kruskal-Wallis test. Difference in PAL/TL between thedaytime and night was determined by t-test.

Results

Occurrence and distribution
Whitespotted conger leptocephali were collected in the northeastern Harima Nada Sea

12from February through May with being abundant especially between March and April. The
leptocephali were sampled when sea surface temperature (SST) ranged from 8-18°C (Fig.
II-1-3). They were usually collected in the bottom and middle layers but not in the surface
layer in the daytime. CPUEs in the bottom layer were higher than those in the middle layer.
Night CPUEs were also higher than those in the daytime, and the leptocephali were caught
even in the surface layer at night (Table II-1-1). There was a significant difference (P=0.037,
Kruskal-Wallis test) between CPUEs in each towing layer in the daytime, but not in thenight.

Total length and metamorphic stage
The pre-metamorphosing leptocephali (PAL/TL>0.88) ranged from 94-132 mm TL,
and the latter size was the largest recorded in this study (Fig. II-1-4). The mean TL (MTL)
tended to decease from March to May although no particular tendency in TL was observed
between the towing layers in the daytime and night. A half of the leptocephali caught inMarch were already at metamorphic stage, and the rate of pre-metamorphic leptocephali
decreased coincidentally with decrease in TL (Fig. II-1-4). While almost all the leptocephalicaught in the daytime in May were at metamorphosing stage (98%, N=43), the rate of themetamorphic leptocephali caught in the night was only 44% (N=9). In addition, PAL/TL
values in the leptocephali taken at night were significantly higher (P<0.001) than those in
the daytime, indicating that the pre-metamorphosing leptocephali spread more widely in thenight (Fig. II-1-5).

Discussion

To differentiate the metamorphic stage of whitespotted conger leptocephali, PAM/TM(number of preanal myomere/number of total myomere) ratio has been used in the studies of
leptocephalus development (Lee and Byun 1996; Otake et al. 1997; Utsumi 2009), andPAM/TM<0.8 was employed as a criterion for metamorphosing stage (Katayama andShimizu 2006; Mochioka et al. 2006). Meanwhile, like in the present study, PAL/TL ratio
also has been successfully used in classifying the metamorphic stage for whitespotted conger(Yamano et al. 1991) and two other related species, European conger Conger conger(Correia et al. 2002a, 2002b, 2003) and American conger C. oceanicus (Bell et al. 2003;

13

Correia et al. 2004). In this study, I used PAL/TL=0.88 instead of PAM/TM=0.80 as acriterion for metamorphic stage, based on the analysis between PAL/TL and PAM/TM. In
my preliminary study (Gorie unpublished), PAL/TL was found to be strongly correlated with
PAM/TM: [PAL/TL=0.986×(PAM/TM)+0.087, r2=0.98, N=361]. Therefore, as an indicatorof the developmental stage of whitespotted conger, PAL/TL has a higher advantage than
PAM/TM in easily obtaining measurement data and avoiding myomere counting errors,
especially when formalin preserved specimens are examined. In addition, although
PAL/TL=0.88 was used as a threshold of metamorphic stage in this study, it should be notedthat the threshold must be determined based on both external and physiological events.
Further study is needed to determine the metamorphic stage based on PAL/TL.
This study has revealed that whitespotted conger leptocephali migrate to thenortheastern Harima Nada Sea from February to May (abundantly from March to April),when SST ranged from 8-18°C. In the nearby, southerly-located Kii Channel, theleptocephali appeared from November to April with SST ranging from 8.8-20.0°C and weremost abundant when SST was at 10°C (Utsumi 2009). Although no data were presented,
Nabeshima et al. (1995) mentioned that the leptocephali were found also in the nearby
Osaka Bay from January to April. Further, in the northeastern Harima Nada Sea, theleptocephali were not collected in January and June (Gorie unpublished). These results
suggest that whitespotted conger leptocephali are drifted from the offshore Pacific waters to
the Kii Channel in November, when SST are between 8 and 20°C, and are transferred and/or
move to the northeastern Harima Nada Sea through Osaka Bay with lag time. Unfortunately,
I cannot refer to the possibility that the leptocephali reach the Harima Nada Sea through thewestern Seto Inland Sea and/or the Naruto Strait because no survey has been conducted bothin the western and southeastern Harima Nada Sea. It is, however, reasonable to infer that the
leptocephali migrate through the Naruto Strait as well.
Multiple migrations have been suggested for whitespotted conger leptocephali inTokyo Bay (Shimizu 2005), the southern Tohoku area (Katayama and Shimizu 2006),
Suruga Bay (Mochioka et al. 2006), and Sagami Bay (Kurogi 2008). It also has been
assumed that the migration is not continuous but intermittent. In this study, length frequency
distribution of the leptocephali and their rate of pre-metamorphic stage did not fluctuate but
decreased consistently from March to May (Fig. II-1-4), which suggests that, unlike in otherregions, the migration of the leptocephali into the Harima Nada Sea was not multiple. Also,14

it might be possible that the intermittent migration was hidden by buffering effects of thewaters because, as above discussed, the leptocephali are thought to pass through the Kii
Channel and Osaka Bay to reach the northeastern Harima Nada Sea.
This study has shown for the first time that whitespotted conger leptocephali do notoccur in the surface layer but in the bottom and middle layers in the daytime. They, however,spread their distribution to the surface layer in the night (Table II-1-1). Also, both thepre- and early-metamorphosing leptocephali may stay just above the sea bottom because thepre-metamorphosing individuals were collected at night in May (Fig. II-1-4, right) and
PAL/TL in the leptocephali caught in the night is significantly higher (P<0.001, t-test) thanthat in the daytime (Fig. II-1-5). Consequently, these results may suggest that the
pre-metamorphosing leptocephali migrate intermittently into the Harima Nada Sea even in
May and/or that such leptocephali stay just above the sea bottom in the daytime but spreadwidely in the night.
It is known that almost all the leptocephali caught in the southern Tohoku area
(Katayama and Shimizu 2006), Suruga Bay (Mochioka et al. 2006), and the Kii Channel(Utsumi 2009) were at pre-metamorphic stage. However, the results obtained in this study
was different form those in such regions. A half of the leptocephali caught during the earlymigration period (February-March) were already at metamorphic stage, which implies thatmany of the leptocephali migrate to the northeastern Harima Nada Sea with metamorphosing.
The difference in migration period and metamorphic stage between the Harima Nada Sea
and Kii Channel can be explained by the following reasons: 1) the former region is slightly
more northerly located than the latter region in the eastern Seto Inland Sea; and 2) when the
leptocephali are about to arrive in the Harima nada Sea after migrating from the Kii Channel,
they may recognize that they have reached appropriate waters to begin metamorphosis(Kurogi 2008).
The leptocephali caught in this study ranged from 91-132 mm TL. In other inshore
waters around Japan, the similarly sized leptocephali (80-140 mm TL) were captured(Shimizu 2005; Mochioka et al. 2006; Utsumi 2009), and a specimen of up to 155 mm TL
was collected in the open waters facing the Pacific Ocean (Katayama and Shimizu 2006).
The largest size of the species at pre-metamorphic stage may vary between the regions, and,as Kurogi (2008) suggested, the leptocephali appear to continue growing until they are
drifted to appropriate waters to settle down.

15More information is needed on the relationship between horizontal distribution of the
leptocephali and oceanographic environment to clarify their occurrence patterns in theeastern Seto Inland Sea. Also, for forecasting the fishing conditions of the species there, it isimportant to assess the abundance of the migrating leptocephali near the Kii Channel. We
need more work to understand the migration ecology of the leptocephali for the efficient
stock management of the species.

16134°50´E

10m

135°00´E

Kakogawa

20m

10 km

Kami Is.
34°40´N

AkashiNo.10 buoy

Basket trap net

Harima Nada Sea

Fig. II-1-1. A survey area (crosshatched) to collect whitespotted conger leptocephali in thenortheastern Harima Nada Sea.

17

Net recorder transmitter unit
FURUNO Type CN-8

Float

FRP Beam 12m

Chain (8mm)h=2.3m

12m

4.5m

17 Setsu,
Inner diameter 18.2mm

3m
21 Setsu,
14.2mm

4.5m
28 Setsu,
10.2mm

3m
K 120

3m
K200

Minnow net

Fig. II-1-2. Diagram of a midwater beam trawl net to collect whitespotted conger
leptocephali in the northeastern Harima Nada Sea.

18

2003
15

10
10
5
5
20

0
20
15

20

80
60

2003 Night
15

40
10
20
5

0
Jan.

Feb.

Mar.

Apr.

May

2004
15

10
10

5
20

0
80
60

2006
15

40
10
20
5
20

0
100
80

Sea surface temperature

CPUE (mean number of leptocepali)

5

2008
15

60
40

10

20
5
20

0
40
2009
30

15
20
10
10
5

0
Jan.

Feb.

Mar.

Apr.

May

Fig. II-1-3. Monthly changes in CPUE (mean number of whitespotted conger leptocephalicaught per haul for 30 min) using a midwater beam trawl net in the northeastern Harima
Nada Sea in the daytime (left) and night (right) from 2003 to 2009 (except for 2005 and2007). Open triangles, surface layer; open circles, middle layer; closed circles, bottom layer;line with dots, sea surface temperature (°C).

19

Sea surface temperature

15

CPUE (mean number of leptocepali)

20

20

130

0.6

110

0.4

100

0.2

90
140

0.0
1.0

2004

0.6

110

0.4

100

0.2

90
140

0.0
1.0

2006

0.8

120

0.6

110

0.4

100

0.2

90
140

0.0
1.0

130

2008

0.6

110

0.4

100

0.2

90
140

0.0
1.0

2009

120

0.6

110

0.4

100

0.2

90

0.0
Jan.

Feb.

Mar.

Apr.

May

0.8

120

0.6

110

0.4

100

0.2

90

0.0
Jan.

0.8

0.8

120

130

2003 Night

0.8

120

130

1.0

Feb.

Mar.

Apr.

May

Fig. II-1-4. Monthly changes in total length distribution and rate of pre-metamorphic stage
in whitespotted conger leptocephali caught with a small midwater beam trawl net in thenortheastern Harima Nada Sea in the daytime (left) and night (right) from 2003 to 2009
(except for 2005 and 2007). Open triangles, surface layer; open circles, middle layer; closed
circles, bottom layer; line with dots, rate of pre-metamorphic stage (PAL/TL>0.88).

20

Rate of pre-metamorphic stage

0.8

120

130

Total length (mm)2003 

140

Rate of pre-metamorphic stage

130

1.0

Total length (mm)

140

1.0

PAL/TL

0.8

0.6

0.4

0.2
80

90

100

110

120

130

140

Total length (mm)Fig. II-1-5. Relationship between total length and PAL/TL of whitespotted conger
leptocephali caught in the northeastern Harima Nada Sea in the daytime and night in 2003.
The value of PAL/TL less than 0.88 is used as a criterion for metamorphosing stage. Opencircles, daytime caught specimens (N=64); closed circles, night caught specimens (N=142).

21

22

6

5

15

0

31 Mar.

11 Apr.

12 May25 May

14.9

13.7

10.4

9.0

8.3

WT

0

6

3

4

23
9.2

8.3

WT

14.9

13.7

10.4

Night1
CPUE

Bottom

1

0

0

4

10

0

CPUE

15.4

14.6

10.4

9.5

8.1

WT2

Daytime

Night

0

5

0

63

17

CPUE

Middle

16.1

14.3

10.6

9.4

8.3

WT2

0

0

0

0

0

CPUE

18.3

15.7

10.5

10.0

8.2

WT

Daytime

Night

0

0

19

3

1

CPUE

Surface2

Water temperature at 10 m in depth.

8.3

WT

18.3

18.0

10.7

10.7

CPUE, mean number of leptocephali per haul for 30 min. The night surveys were conducted from 30 min lag time after sunset.

11

CPUE

Daytime13 Mar.

Date
in 2003

Table II-1-1. CPUE of whitespotted conger leptocephali caught with a midwater beam trawl net in the daytime and night in thenortheastern Harima Nada Sea in 2003

2. Unintentional bycatch of whitespotted conger leptocephali by the boat seine fisheryIn the eastern Seto Inland Sea, the boat seine fishery commonly operates fromFebruary through December. This fishery is conducted by a lot of fishing unities commonly
consisting of three boats (two towing boats and a carrying boat). While two boats are towing
the net, a carrying boat transports the catch to land it at fish markets.
The juveniles of the Pacific sandlance Ammodytes personatus Girard, 1856 are themain target of the boat seine fishery from February through April which is just the period
when whitespotted conger leptocephali abundantly migrate to the eastern Seto Inland Sea(see Chapter II-1).
When Pacific sandlance juveniles are auctioned at fish markets, purity of product withless contamination of other organisms is strongly important for a high auction price. To
avoid contamination of non-target animals in the catch, fishermen usually use a filtration
system attached to the fishing net. This filtration system has a powerful effect on both
removing non-target organisms and selecting the high-valued juveniles of appropriate size.
Unfortunately, however, the filtration system is not perfect but still catches non-target
organisms as bycatch (= unintentional capture of non-target species), which becomes a major
problem in fishery resource management in the eastern Seto Island Sea. Numerous valuable
larvae and juveniles of other marine animals are caught but discarded dead or dying backinto the sea because bycatches are usually released only when retrieved by the carrying boat.
So far, there has been no information on bycatch of whitespotted conger leptocephali
by the boat seine fishery in the eastern Seto Inland Sea. Since the bycatch is implied to waste
stocks of other commercially important organisms, including whitespotted conger, it isessential to clarify the present status of bycatch to estimate its impact on the population ofwhitespotted conger and to improve the fishing equipment for avoiding bycatch.
In this section, the bycatch of whitespotted conger leptocephali through the boat seine
fishery is described.

Materials and Methods

Sample collection
The surveys were conducted from February to April in 2003-2007 when the boat seine

23

fishery operated in the eastern Seto Inland Sea with targeting Pacific sandlance juveniles. I
requested some of the fishermen engaged in the fishery in the Harima Nada Sea and Osaka
Bay to preserve a haul of bycatches found in the filtration net before they released the
bycatches from the net. These bycatch samples were taken from the carrying boats, fixed in
5-10% buffered formalin solution, and preserved until analysis.

Identification and measurements of bycatch species
The collected animals were identified and counted by species. They were classified
into one of the five abundant species: rockfish inferred as Sebastes cheni Barsukov, 1988, fat
greenling Hexagrammos otakii Jordan & Starks, 1895, whitespotted conger leptocephali
Conger myriaster, marbled rockfish Sebastiscus marmoratus Cuvier, 1829, Japanese flying
squid Todarodes pacificus (Steenstrup, 1880), and others. Total length (TL, mm) or mantle
length (ML, mm) and body weight (BW, g) were recorded for individuals of the major fivespecies.

Results

Rockfish (7-35 mm TL) and fat greenling (21-73 mm TL) were abundantly found in
the bycatch, accounting for 15-85% and 29-44%, respectively, of the annual bycatch innumber although there was no bycatch of the latter species in 2003 and 2007 (Table II-2-1).
Whitespotted conger leptocephali (72-135 mm TL) followed these two species andaccounted for 5-21% in the annual bycatch. Marbled rockfish (6-33 mm TL) and Japanese
flying squid (9-117 mm ML) were caught only in two years of the five years surveyed.

Discussion

This study has shown for the first time that a certain number of larvae and juveniles ofcommercially important organisms were unintentionally caught in the eastern Seto Inland
Sea by the boat seine fishery targeting Pacific sandlance juveniles. It has been alreadyrecognized in other Japanese waters that the bycatch can affect the stock of whitespottedconger. Ishida et al. (2003), who surveyed coastal fisheries of the Tohoku region, found thatthe boat seine fishery selectively caught the leptocephali of the species and gave a negative24

impact on its resource. Since 2006, in Ise-Mikawa Bay, a stock management has been
actually performed in the boat seine fishery by setting a catch limit for the leptocephali(Anon. 2010). It is, however, difficult to accurately estimate an actual impact of bycatch on
the fishery resources.
While whitespotted conger leptocephali may be found at fish markets, it is yet
unknown whether the leptocephali are selectively caught in the eastern Seto Inland Sea. It
may be also possible that fishermen sell the incidentally caught leptocephali of the species
by themselves. This study has revealed that a numerous number of the leptocephali werecaught as bycatch, which suggests that the boat seine fishery is likely to give a negative
impact on the stock of whitespotted conger in the eastern Seto Inland Sea. Under thissituation, at least the selective catch of whitespotted conger leptocephali should beprohibited.
Since we have very limited information on the unintentional bycatch of marine
animals, including whitespotted conger leptocephali, further study is desirable. Research toestimate a negative impact of the bycatch on the fishery resources will provide the basic
knowledge for the stock management of whitespotted conger and all other bycatch species inthe eastern Seto Inland Sea.

25

26

1

2.8

Others
5,709

18.5

8,036

4.3

24.4

5,088

18,066

16.2

18.6

9,673

11.6

4.2

0.0

17.5
0.0

12.4

15.1

50.8

Mean

5.9

11.7

0.0

61.3

19

5

2007

10.8

4.6

44.1

15.2

43

13

2006

Inferred as Sebastes cheni ; mean number of fish per a haul. All the bycatches were larvae or juveniles.

2

11,465

0.7

Japanese flying squid

Total number of bycatch species2

0.0

0.0

Marbled rockfish
0.0

13.8

20.7

11.7

Whitespotted conger leptocephalus
0.0

42.7

28.7

0.0

Fat greenling

14.8

34

32.2

40

8

2005

84.8

35

Number of species occurred

12

2004

Rockfish1

16

2003
Number of samples

English name/Year

Table II-2-1. Number percentage of unintentional bycatches with a boat seine fishery in the eastern Seto Inland Sea
from 2003 to 2007

III. Habitat preference and feeding habits of whitespotted conger juveniles

In Japanese waters, larval whitespotted conger are spread by the Kuroshio Current andits branch, the Tsushima Warm Current. When the leptocephali reach appropriate coastal
waters, they actively move into inshore waters and begin metamorphosis (Kurogi 2008). In
the northeastern Harima Nada Sea, whitespotted conger occur as leptocephali from February
to May and complete metamorphosis. They display shrinkage in total length withmetamorphosis, settle down to the ground, and begin the demersal life (see Chapters II-1,
IV-1).
There are a few studies on the feeding habits of commercial-sized whitespotted conger(Mori and Inoue 1982; Matsumiya and Imai 1987; Nabeshima et al. 1994; Fukuda et al.
1997): this species preys mostly on teleosts, crustaceans, and mollusks. Similar results werereported for two related species: European conger Conger conger (Cau and Manconi 1984;
Morato et al. 1999; O’Sullivan et al. 2004; Vallisneri et al. 2007) and American conger
Conger oceanicus (Levy et al. 1988). On the other hand, very little is known about thefeeding habits and habitat during the juvenile stage of these three species. In Japan, Takai(1959) reported that juvenile whitespotted conger occurred at 1-17 m in depth on the
sand-mud to mud bottoms, but detailed data have not been published. He also mentioned thatthe juveniles of 100-200 mm in total length fed on shrimps, fishes, crabs, and polychaetes. In
Korea, Huh and Kwak (1998) found that the juveniles of the same species fed mainly on
fishes, shrimps, and crabs in a Zostera bed and showed a very constant prey selection.
Previously, I reported that the diet of whitespotted conger changed with fish size (Gorie andTanda 2004). Nevertheless, information on the habitat and food habit of their juveniles isquite limited, and there have been no studies on relations between prey fauna and thestomach contents of the juveniles.
In this chapter, I report: 1) where the newly settled juveniles of whitespotted conger
are distributed in the northeastern Harima Nada Sea; and 2) what environmental conditions
are related to their habitats. I also discuss the feeding habits of the juveniles and their relation
to the fauna of prey.

Materials and Methods

27

Fish sampling
The study was conducted in the northeastern Harima Nada Sea (Fig. III-1). The survey
region was divided into four areas: Area A (depth: 13 m), Area B (10 m), Area C (17 m), andArea D (18 m).
Juvenile whitespotted conger were sampled using a small trawl net and pots. The trawl
survey was performed from April to September in 2003-2008 with a 2.4 mm mesh apertureof a codend (Fig. III-2). Since no juvenile was caught during the daytime and few juveniles
were obtained within 30 min after sunset in the trawl survey, sampling was carried out atnight (30 min lag time after sunset). The net was towed for 10-20 min at a speed of 2 knots
using a small boat (9.1 mt). Catch per unit effort (CPUE) was expressed as mean number ofthe juveniles per haul for 20 min.
The pot survey was conducted using commercially available pots (Fig. III-3: nominal
mesh size, 18 setsu; mesh size of the upper and bottom parts, 18 and 15.9 mm, respectively)
in October and December from 2004 to 2008. The survey was carried out from evening tonight in Areas A and B. After thawed from a deep freezer, sardines were put as bait inside thepots. The pots were usually devoted into the sea before 1 hour of sunset and allowed the fish
to trap for 4 hours. The effective numbers of the pots for the survey was 35 in each survey
area. Catch per unit effort (CPUE) was expressed as mean number of the fish caught for 4
hours dipping time per pot.
Before the surveys were conducted in each survey area, oceanographic data, including
water temperatures (WT), salinities, and depths, were recorded with STD (JFE Advantech
Co., Ltd., Nishinomiya, Japan).

Bottom sediment sampling and grain size analysis
Bottom sediments were sampled with a SK Type bottom sampler (Rigo Co., Ltd.,Tokyo, Japan) in Areas B-D on 3 July 2003 and in Area A on 22 August 2003. Grain sizes
were analyzed based on the Japanese Geotechnical Society Standard (JSF T 131-1990) andsorted by the Wentworth grain size scale. Losses on ignition were measured after the
sediments were heated at 600°C for 2 hours.

Benthos sampling and their composition
Benthos were sampled before towing the trawl net in 2004 with a Smith-MacIntyre

28bottom sampler (22×22 cm, Rigo Co., Ltd., Tokyo, Japan) and with a sledge net (Fig. III-4)
for 1 min towing at a speed of 2 knots. The samples caught using the bottom sampler weresieved with a 1 mm mesh aperture size and stored in 5-10% buffered formalin solution. The
benthos composition in these samples was expressed as numerical and weight percent forfive categories: polychaetes, gastropods, bivalves, crustaceans, and cephalochordates. Since
these animals accounted for more than 95% of the total, other organisms were excluded fromthe analysis. The benthos sampled with the sledge net were categorized into one of eightgroups: nematodes, polychaetes, gastropods, bivalves, crustaceans, chaetognaths, ophiuroids,
and teleosts.

Stomach content analysis
The juveniles were fixed soon after caught in 5-10% buffered formalin solution from
May to August in 2004. For stomach content analysis, 30, 106, and 120 juveniles wereselected randomly from Areas A-C, respectively (Table III-1). These juveniles were
measured for total length (TL, mm) and body weight (BW, g), and the stomachs were
removed and examined for contents. Prey items from each stomach were identified to one offour groups (polychaetes, crustaceans, teleosts, and others) and expressed as numerical andweight percent by sampling date and fish size. Moreover, crustaceans were classified into
amphipods, decapods, or others.
Ivlev’s selectivity index (Ei) (Ivlev 1955) was calculated as follows: Ei =(ri-pi)·(ri+pi)-1, where ri is the percentage of prey item i in the fish stomachs, and pi is thepercentage of item i in the benthos samples collected with the Smith-MacIntyre bottom
sampler. The index can take values between -1 and 1. A value between 0 and 1 means that afish has a positive preference for the prey item i, while a value between 0 and -1 shows anegative preference. If a value is 0, a fish has no preference to the item i.

Statistical analysis
Differences in CPUE and prey density were explored using the Steel-Dwass test or
Mann-Whitney’s U test between the survey areas where the juveniles were collected.

Results29

Juvenile distribution and oceanographic features
In the trawl survey, whitespotted conger juveniles were collected in Areas A-C, but nojuvenile was taken in Area D. The juveniles were collected from May to September (Table
III-2). CPUEs in Areas B and C were significantly higher than that in Area A (P ≈ 0.003,
Steel-Dwass test), but no significant difference was observed between Areas B and C.
In the pot survey, the CPUE in Area B was significantly higher than that in Area A(P=0.01, U test) (Table III-3).
Bottom WTs and salinities where the juveniles were caught ranged from 14.6-27.4°C
and 30.2-32.7, respectively (Tables III-2, III-3).

Bottom sediment typeThe grain size composition of the bottom was different among the survey areas (Fig.
III-5). Gravel and pebbles were the major components in Area A, but their proportions were
low in Areas B and C, where medium-coarse sand predominated. A small amount of graveland pebbles were also found in Area D, where medium sand was abundant. Some silt andclay were found in Areas B and C, but they were not observed in Area D. Both median andmaximum size of particles were the largest in diameter in Area A and the smallest in Area D(Table III-4). Losses on ignition ranged from 2.0-2.9% in the four survey areas. Although thedepths were different, the bottom of Areas B and C was similar, and the bottom types of eachsurvey area were classified as follows: gravel bottom in Area A, sand-mud bottom in Areas
B and C, and sand bottom in Area D.

Benthos composition
There were some differences in composition of the benthos collected by theSmith-MacIntyre bottom sampler between Areas A-C, where whitespotted conger juveniles
were collected (Fig. III-6). In Area A, crustaceans were the most numerous, but
cephalochordates (lancelet Branchiostoma belcheri) and bivalves were the major
components by weight. In Areas B and C, polychaetes predominated by both number andweight. The biomasses of polychaetes were similar in these three areas and that ofcrustaceans in Area A was more abundant than those in the other areas although significant
difference was not observed (P ≈ 0.05, Steel-Dwass test, Table III-5).
A wide variety of the benthos was collected by the sledge net (Fig. III-7). In Area A,

30

crustaceans were dominant by number, but bivalves and teleosts were also important by
weight. In Areas B and C, the proportions of four groups (bivalves, crustaceans, polychaetes,
and nematodes) were high by number, whereas those of crustaceans and ophiuroids werehigh by weight in these areas, respectively. The composition considerably fluctuated bysampling date.
Based on the bottom sediment type and benthos composition, Area A is characterized
by gravel bottom with abundant crustaceans and bivalves, and both Areas B and C bysand-mud bottom with high abundance of various groups of the benthos, such as polychaetes,
bivalves, crustaceans, and ophiuroids. Areas B and C are considered to have a similarbenthos composition and bottom sediment type.

Stomach contents
A total of 256 juveniles were examined for their stomach contents (Table III-1).
Throughout the survey period, crustaceans were very common by number in Areas A-C (Fig.
III-8). Both polychaetes and teleosts were also important by weight in Areas B and C, wherepolychaetes showed the highest percentage in June.
The stomach contents varied between the survey areas and with fish size (Fig. III-9).
Crustaceans were highly predominant in all fish sizes in Area A. They were also abundantly
preyed on in Areas B and C, but the importance of polychaetes and teleosts increased byweight in these areas. Polychaetes were found in the juveniles of 80-210 mm TL but notobserved in the stomach contents of the juveniles over 220 mm TL. Teleosts as prey weredetected in the juveniles over 90 mm TL. Of the crustaceans found, both amphipods anddecapods were very dominant, but their importance was different by size: the proportion ofamphipods and decapods declined and increased, respectively, with an increase in fish size
(Fig. III-10). This trend was found in all survey areas.
The main amphipods preyed on were podocerids, aorids, and pontogeneiids by
number. Oedicerotids were also detected by weight. As for the decapods, three groups(penaeids, crangonids, and processids) mainly occurred by number but pasiphaeids were
important by weight. The teleosts eaten were gobiids and callionymids by number, but
engraulids were also important by weight. The polychaetes that were preyed on consisted oferrantians and sedentarians, and the former predominated (83% by number). Sand grains
were rarely observed.

31Based on the Ivlev’s selectivity index, the juveniles showed a strong preference forcrustaceans (Table III-6). Crustaceans showed a high value throughout the period, regardlessof the survey areas. The percentage of empty stomachs ranged from 10-72% in thesmall-sized juveniles (<100 mm TL), but few empty stomachs were found in the juveniles
larger than 100 mm TL. No particular trend in percentage of empty stomachs was observed
among the three survey areas.
To summarize these results, whitespotted conger juveniles fed mainly on crustaceans,polychaetes, and teleosts and showed a strong preference for crustaceans. The juveniles
preyed on amphipods after beginning their demersal life and shifted their prey, with growth,from amphipods to decapods, polychaetes, and teleosts.

Discussion

This study has shown for the first time that the newly settled juveniles of whitespottedconger occurred at 10-20 m in depth on the gravel (Area A) and sand-mud bottoms (Areas B
and C). They were not sampled from the sand bottom (Area D).
This study also suggests that whitespotted conger juveniles exhibit a higher preference
for the sand-mud bottom than gravel bottom because CPUEs from the sand-mud bottom
were significantly higher than that from the gravel bottom (Area A). There was no
significant difference in CPUE between Areas B and C in the trawl survey. Water depths inthese areas were slightly different (10 and 17 m each), but both the environmental conditions
and benthic fauna were very similar to each other. The observed difference in water depth
may give no impact on the distribution and abundance of the juveniles in these areas.
As for the benthic fauna of the survey areas, polychaetes, crustaceans, and mollusks(gastropods and bivalves) were most commonly sampled (Figs. III-6, III-7). The biomass ofpolychaetes was similar in Areas A-C and that of crustaceans was abundant in Area A(Table III-5). The stomach contents of the juveniles, however, did not accord with thebenthic fauna: the juveniles showed a strong preference for crustaceans (Table III-6) andpreyed mainly on crustaceans, polychaetes, and teleosts, indicating that the juveniles show
selective feeding habits for prey animals (Fig. III-8). Crustaceans are numerically important,while polychaetes and teleosts are quantitatively significant. The juveniles also shifted theirfood items, with growth, from amphipods to decapods, polychaetes, and teleosts (Figs. III-9,

32

III-10). Crustaceans were heavily preyed on in Area A, whereas polychaetes were fed on in
Areas B and C (Figs. III-8, III-9). In these areas, crustaceans and/or polychaetes were themost abundant, and the stomach contents of the juveniles correlated with the available
crustacean-polychaete composition (Figs. III-6, III-8). These results suggest that thejuveniles feed on crustaceans and polychaetes in the areas, where these animals are moreabundant than other prey animals.
Although nothing has been reported on the specific feeding behavior of whitespottedconger, epifaunal polychaetes were commonly found in the juvenile’s stomachs and sandgrains were rarely observed. This implies that the juveniles have a foraging behavior for
epifaunal animals rather than buried infaunal animals.
CPUEs of juvenile whitespotted conger were higher in Areas B and C than in Area A(Tables III-2, III-3) although crustaceans predominated in Area A and their biomass wasmore abundant than in other areas (Fig. III-6, Table III-5). These results suggest that thejuveniles choose their habitats not by the food environment but by the bottom sediment type.
I previously reported the food habits of juvenile whitespotted conger from thenortheastern Harima Nada Sea (Gorie and Tanda 2004). Huh and Kwak (1998) also reported
the stomach contents of the juveniles of the same species caught in an eelgrass (Zostera) bed
in Korean waters. The results of these studies are slightly different from those in the present
study. A higher variation in percentage of prey items was observed in the northeastern
Harima Nada Sea (Gorie and Tanda 2004), and the juveniles in Korean waters consumed
fishes, crustaceans, and gastropods but did not feed on polychaetes. These differences may
be due to the environment of sampling locations between Japanese (sand-mud bottom) andKorean (Zostera bed) waters and/or the smaller sample size [N=1-18 (mean: 6) for each 10
mm size class] in this study than in the previous one [N=1-26 (mean: 12)] (Gorie and Tanda
2004).
In conclusion, the newly settled juveniles of whitespotted conger were collected fromthe gravel and sand-mud bottoms, where crustaceans and bivalves commonly occurred andvarious types of the benthos were abundant, respectively. The sand-mud bottom waspreferred by the juveniles, being considered to be an important nursery ground. The
juveniles showed a strong preference for crustaceans and selective feeding habits forcrustaceans, polychaetes, and teleosts. They potentially choose their habitats by the bottom
sediment type rather than the food environment. Further clarification of the habitat

33

requirements and feeding habits is needed at the different depths and under various
environmental conditions in the Seto Inland Sea to identify the key habitats used bywhitespotted conger.

34134°50´E

10m

B
Sea of Japan

135°00´E

Kakogawa

C
20m

10 km

Kami Is.
34°40´N

A
D
No.10 buoy

Pacific Ocean

AkashiBasket trap net

Harima Nada Sea

Fig. III-1. A location of the survey areas (hatched) in the northeastern Harima Nada Sea.

35

K200
K180

3m

K140

1.7m
3m

K130

3.75m

Minnow net

K130

6m
Float
Chain (8mm)DredgeGround rope

w=5m, h=0.75m, φ11cm

Warp

Weight (60kg)

Fig. III-2. Diagram of a bottom trawl net to collect newly settled whitespotted conger in thenortheastern Harima Nada Sea.

36

Fig. III-3. A commercially available pot used for the pot survey in the northeastern Harima
Nada Sea from 2004 to 2008. Nominal mesh size: 18 setsu; mesh size of the upper andbottom parts, 18 and 15.9 mm, respectively.

37

Outer net, 5mm

Inner net, GG50: 0.35mm

h=0.4m, w=0.6m
2m

Warp

Sled
Chain (6mm)0.7m

Fig. III-4. Diagram of a sledge net used to collect epifaunal benthos in the northeastern
Harima Nada Sea.

38

(%)
100
80
60
40
20
0
Area AB

C

D

Silt and clay (<0.063 mm)

Fine sand (0.063-0.25 mm)

Medium sand (0.25-0.5 mm)Coarse sand (0.5-2 mm)

Gravel and pebble (2 mm<)Fig. III-5. Grain size compositions of the survey areas in the northeastern Harima Nada Sea.

39

(%)
100
80
60
40
20
0
14 May24 June22 July

25 Aug.

14 May24 June22 July

25 Aug.

12 May23 June21 July

24 Aug.

12 May23 June21 July

24 Aug.

Area A

(%)
100
80
60
40
20
0
12 May23 June21 July

24 Aug.
Area B

(%)
100
80
60
40
20
0
12 May23 June21 July

24 Aug.
Area C

Number percent
Cephalochordates

Weight percent
Bivalves

Crustaceans

Gastropods

Polychaetes

Fig. III-6. Compositions of various groups of benthos collected with the Smith-MacIntyre
bottom sampler. Left, number percent; right, weight percent.

40

(%)
100
80
60
40
20
0
14 May24 June22 July

25 Aug.

14 May24 June22 July

25 Aug.

12 May23 June21 July

24 Aug.

12 May23 June21 July

24 Aug.

Area A

(%)

100
80
60
40
20
0
12 May23 June21 July

24 Aug.
Area B

(%)

100
80
60
40
20
0
12 May23 June21 July

24 Aug.
Area C

Number percent

Weight percent

Teleosts

Ophiuroids

Chaetognaths

Gastropods

Polychaetes

Nematodes

Crustaceans

Bivalves

Fig. III-7. Compositions of various groups of benthos collected with the sledge net. Left,
number percent; right, weight percent.

41

(%)
100

1

20

4

5

14 May24 June22 July

25 Aug.

1

20

4

5

14 May24 June22 July

25 Aug.

10

19

38

39

12 May23 June21 July

24 Aug.

5

40

40

35

12 May23 June21 July

24 Aug.

80
60
40
20
0
Area A
(%)

10

19

38

39

12 May23 June21 July

24 Aug.

5

40

40

35

12 May23 June21 July

24 Aug.

100
80
60
40
20
0
Area B

(%)

100
80
60
40
20
0
Area C
Number percent

Others

Weight percent

Teleosts

Crustaceans

Polychaetes

Fig. III-8. Compositions of stomach contents of juvenile whitespotted conger. Left, number
percent; right, weight percent. Sample size is given for each sampling date.

42

(%)
100

1 2 4 6 5 4 2 2 1 2 1

1 2 4 6 5 4

2 2 1 2 1

80
60
40
20
0
Area A(%)

18 8

1 8 7 8 8 8 8 8 8 8 8 8 8 7 1 0 1 0 1

7 8 8 8 8 8 8 8 8

8 8 7

1 0 1 01 7 14 18 11 10 10 11 12 7 7

5 3 1

0 1 1 0 1

1

100
80
60
40
20
0
Area B(%)

1 7 14 18 11 10 10 11 12 7

7 5 3 1 0 1 1 0 1

100
80
60
40
20
0
70- 90- 110- 130- 150- 170- 190- 210- 230- 250-

70- 90- 110- 130- 150- 170- 190- 210- 230- 250-

80- 100- 120- 140- 160- 180- 200- 220- 240-

80- 100- 120- 140- 160- 180- 200- 220- 240Area C

Number percent

Others

Weight percent

Crustaceans

Teleosts

Polychaetes

Fig. III-9. Compositions of stomach contents of juvenile whitespotted conger of different
total length classes. Left, number percent; right, weight percent. Sample size is given foreach size class.

43

(%)
100

1 2 4 6 5 4 2 2 1 2 1

1 2 4 6 5 4 2

2 1 2 1

80
60
40
20
0
Area A
(%)
1 8 7 8 88 8 8

8 8 8 88 7 1 0 1 0 1

1 8 7 8 8 8 8 8 8 8

8 8 8 7 1 0

1

0 1

100
80
60
40
20
0
Area B(%)
1 7 14 18 11 10 10 11 12 7 7 5

1 7 14 18 11 10 10 11 12 7 7 5 3 1 0 1

3 1 0 1 1 0 1

1 0 1

100
80
60
40
20
0
70- 90- 110- 130- 150- 170- 190- 210- 230- 250-

70- 90- 110- 130- 150- 170- 190- 210- 230- 250-

80- 100- 120- 140- 160- 180- 200- 220- 240-

80- 100- 120- 140- 160- 180- 200- 220- 240-

Number percent

Other crustaceans

Area C

Decapods

Weight percent

Amphipods

Fig. III-10. Compositions of amphipods, decapods, and other crustaceans in the stomach
contents of juvenile whitespotted conger of different total length classes. Left, number
percent; right, weight percent. Sample size is given for each size class.

44

45

9

15

122

21, 22 July24, 25 Aug.

Total

164 (18.6, 133-208)

152 (22.8, 128-188)

120 (13.4, 94-152)

86 (3.5, 81-91)

TL (SD，range)

Area A

30

5

4

20

1

N examined

883

231

374

181

97

N caught

N , sample size; TL, mean total length (mm); SD, standard deviation (mm).

917

N caught

23, 24 June

12, 14 May

Date
in 2004

169 (20.7, 105-262)

139 (19.7, 84-195)

112 (13.6, 86-151)

85 (3.7, 76-95)

TL (SD，range)

Area B

106

39

38

19

10

N examined

460

142

154

146

18

N caught

Table III-1. Number and size of fish examined for stomach contents

167 (23.7, 124-258)

135 (21.5, 100-195)

104 (10.5, 85-147)

84 (3.4, 77-91)

TL (SD，range)

Area C

120

35

40

40

5

N examined46
16

24, 25 Sep.

926, 26 May
36

20

27 Sep.

25 June925, 26 July023, 24 May

25.5

17.7

16.4

27.3

23.1

16.7

26.2

32.6

31.8

32.2

31.9

31.5

32.4

31.5

242

58

78

210

262

25

212

103

29

30.4

14, 15 Sep.

23.0

362

19, 20 July

31.8

16

17.7

8

342

82

11

231

374

5 June

32.1

32.2

31.8

32.0

31.7

0

26.0

25.3

17.2

26.5

24.3

18 May1221, 22 Sep.

3

30, 31 May
7

824, 25 Aug.

24 Aug.

521, 22 July

181

44

23, 24 June

31.8

97

4

12, 14 May
20.6

57

25 Aug.
32.2

94

10 July

16.7

11

32.8

13 June

11.7
2

0

26.1

20.0

16.4

27.4

23.2

15.8

26.6

22.0

16.9

13.9

26.1

25.7

16.6

27.2

24.2

20.8

15.6

25.5

21.8

19.5

15.6

32.4

31.7

32.2

31.7

31.7

32.5

30.9

31.2

31.7

32.1

32.1

32.1

31.7

31.5

31.6

31.6

32.2

31.0

31.5

32.0

32.2

CPUE WT (°C) Salinity

CPUE WT (°C) Salinity
0

Area B

Area A

16 May22 Apr.

9 Apr.

Date

CPUE, mean number of fish caught per 20 min towing; blank, no data.
WT and salinity profiles express the data near the sea bottom.
All of the surveys were conducted at night (30 min lag time after sunset).

2008

2007

2006

2005

2004

2003

Year

42

40

23

40

89

31

102

129

38

0

180

294

58

142

154

146

18

47

52

8

9

0

26.0

19.3

15.8

27.3

23.0

15.8

26.5

21.3

16.2

13.9

25.9

25.0

16.2

26.9

23.8

20.0

15.2

25.0

21.3

18.3

10.3

32.5

31.9

32.3

31.6

32.1

32.5

31.0

31.5

31.9

32.2

32.2

32.1

31.7

31.7

31.7

31.9

32.4

31.3

31.9

32.3

32.9

CPUE WT (°C) Salinity

Area C

Table III-2. Mean CPUE, WT, and salinity of the bottom net surveys

0

0

0

0

0

0

16.7

25.7

22.3

19.3

31.8

31.3

31.9

32.4

CPUE WT (°C) Salinity

Area DTable III-3. CPUE, WT, and salinity of the pot surveys
Year

Date

Area A

Area BCPUE

WT (°C)

SalinityCPUE

WT (°C)

Salinity

26 Oct.

0.1

22.6

30.8

2.1

22.9

30.9

22 Dec.

1.6

15.6

31.1

1.2

15.6

30.2

2005 25 Oct.

0.3

22.4

32.4

2.2

21.2

31.8

24 Oct.

0.5

23.0

31.5

2.9

23.0

31.3

22 Dec.

3.0

14.6

31.7

8.8

14.2

31.5

2007 23 Oct.

0.0

22.9

32.0

4.1

23.1

32.0

21 Oct.

0.0

23.3

32.6

1.8

23.5

32.3

9 Dec.

0.1

14.9

32.7

1.3

14.1

32.4

2004

2006

2008

CPUE, mean number of fish caught per 4 hour dipping time per pot.
WT and salinity profiles express the data near the sea bottom.
All of the surveys were conducted at night.

47

Table III-4. Bottom sediment of the survey areas
Area ABCDMedian particle diameter (mm)

2.39

0.47

0.41

0.38

Max diameter (mm)

26.5

19.0

19.0

9.5

Loss on Ignition (%)

2.9

2.3

2.3

2.0

48

Table III-5. Benthos density of the survey areas
Date

Polychaetes

Crustaceansin 2004

N/m2

W/m2

N/m2

W/m2

14 May3,079

16,41935,31034,24424 June

3,574

16,95910,4968,281

22 July

1,033

3,963

9,897

2,163

25 Aug.

579

5,901

579

2,754

12 May

826

3,112

413

138

23 June

4,566

6,182

124

23

21 July

4,773

3,886

165

43

24 Aug.

2,748

44,727165

1,128

12 May

682

839

248

184

23 June

723

4,585

289

314

21 July

1,281

919

227

68

24 Aug.

1,426

2,713

124

450

Area A

Area B

Area CN, number; W, weight (mg).

49

Table III-6. Ivlev's selectivity index
Date

Polychaetes

Crustaceansin 2004

Number Weight

Number Weight

ri

0.0

0.0

100.0

97.1

pi

7.3

5.2

83.7

10.8

Ei

-1.00

-1.00

0.09

0.80

ri

1.0

2.0

98.3

77.9

pi

23.1

12.1

67.8

5.9

Ei

-0.92

-0.72

0.18

0.86

ri

0.0

0.6

98.7

52.1

pi

8.5

23.7

81.0

13.0

Ei

-1.00

-0.95

0.10

0.60

ri

0.0

0.0

87.5

87.5

25 Aug. p i

28.3

5.3

28.3

2.5

Ei

-1.00

-1.00

0.51

0.94

ri

0.0

47.0

100.0

47.7

pi

54.8

65.3

27.4

2.9

Ei

-1.00

-0.16

0.57

0.89

ri

8.3

52.9

63.9

13.5

pi

87.0

14.2

2.4

0.1

Ei

-0.83

0.58

0.93

0.99

ri

7.2

32.6

68.1

10.3

pi

49.1

35.0

1.7

0.4

Ei

-0.74

-0.04

0.95

0.93

ri

4.1

21.1

80.8

63.7

24 Aug. p i

76.9

82.3

4.6

2.1

Ei

-0.90

-0.59

0.89

0.94

ri

0.0

42.3

92.9

34.9

pi

30.0

40.0

10.9

8.8

Ei

-1.00

0.03

0.79

0.60

ri

10.7

52.6

85.7

43.2

pi

61.4

47.0

24.6

3.2

Ei

-0.70

0.06

0.55

0.86

ri

4.3

21.7

83.9

41.1

pi

77.5

41.3

13.8

3.1

Ei

-0.89

-0.31

0.72

0.86

ri

1.9

12.2

91.6

77.1

24 Aug. p i

86.3

84.2

7.5

14.0

Ei

-0.96

-0.75

0.85

0.69

14 May24 JuneArea A
22 July

12 May23 JuneArea B
21 July

12 May23 JuneArea C
21 July

N

1

20

4

5

10

19

38

39

5

40

40

35

r i , percent of prey item i from the fish stomachs; p i , percent of item
i in the benthos samples with the Smith-MacIntyre bottom sampler;
E i , Ivlev's selectivity index; N , number of fish examined.

50

IV. Age, growth, and movement of whitespotted conger

For efficient fishery resource management; information on the age, growth, andmovement of target fishes is essential. The relation between age and total length of fishes is
one of the basic knowledge, which can provide us useful information (e.g., growth patterns,
growth difference between male and female, and fishing status). The most valuableadvantage of getting these parameters may be collection of the information on age at
recruitment and age composition of catch. These parameters enable us to estimate the catchin number by age, which is important information for stock estimation.
Some approaches are possible to study the age and growth of whitespotted conger by:1) rearing experiments; 2) chasing changes in total length distribution; 3) age determination;and 4) release and recovery of tagged fish. Rearing experiments are practicable in thelaboratory, while the other works must be done using wild fish.
In this chapter, the age and growth of aquarium-reared and wild-caught whitespottedconger are analyzed. The movement and growth of this species in the wild are also studied
through a tagging survey.

1. Growth of whitespotted conger in captivity

Data from rearing experiments are likely to provide us with estimation of the growthof whitespotted conger inhabiting the sea. Recently, a few studies have been conducted on
the growth of the species in captivity. The fish were reared experimentally by feeding
commercial dry pellets (Nemoto et al. 2004) or dry pellets combined with fresh foods(Takashima and Takahashi 2006). The growth process of the species, unfortunately, was yet
poorly understood in these studies, so that our knowledge of the growth is limited, especiallyfrom the viewpoint of fish farming.
In this section, the growth of whitespotted conger in the laboratory tank is studied
during their three stages, leptocephali (pelagic phase), newly settled juveniles (demersal
phase), and young fish (demersal phase), by feeding moist pellets or commercial dry pellets
in order to help estimating the growth of the species in the wild and to search for thefeasibility of fish farming.

51

Materials and Methods

Leptocephali
Whitespotted conger leptocephali were obtained on 8 April 1997 from a commercial
fishing boat operating in the northeastern Harima Nada Sea. The leptocephali weretransferred alive to the laboratory and released to a 0.5 kl round-shaped, black-colored FRP
tank. Although many of the leptocephali died from injury and/or damage on their bodysurface within a few days after transfer, the experiment was continued using the surviving
leptocephali. Sand-filtered seawater was supplied to the tank by an open flow system with aflow rate of 2-10 kl per day (Fig. IV-1-1). Based on a preliminary experiment (Gorie
unpublished), the rearing seawater temperature (WT) was kept below 26°C from July to
September because high WT might actually cause a negative effect, and also no juveniles
were found to be fed until they finished metamorphosis and settled to the bottom. Afteralmost all the leptocephali completed metamorphosis, a moist pellet was fed every evening
from 23 April 1997. The moist pellet was a commercial formula powder feed, originally forJapanese eel Anguilla japonica Temminck & Schlegel, 1847 (“Tokai”, mash for adult eel,
Marubeni Nisshin Feed Co., Ltd., Tokyo), added with equal quantity of tap water (w/w). The
powder feed and water were well mixed, flattened thinly (easy to bite), and then fed. A few
polyvinyl chloride (PVC) pipes (75 mm in nominal diameter and 30-50 cm in length) were
set in the water. The feeding trial finished on 26 December 1997 (Day 262).

Newly settled juveniles
The newly settled juveniles of whitespotted conger were caught by a small trawl net(see Fig. III-2) on 18 May and 5 June 2006 and also on 18-19 May 2009 in the northeastern
Harima Nada Sea. The net was towed for 10-20 min at a speed of 1.5-2.5 knots using a small
boat (9.1 mt) with a 2.4 mm mesh aperture of a codend, and the juveniles were transferred
alive to the laboratory.
The first feeding experiment started on 23 June 2006 using 46 juveniles and ended on
28 September 2007 (Day 462). After the juveniles were acclimated in an aquarium forseveral days, they began to feed a commercial diet (dry pellets). The second feeding
experiment started on 19 May 2009 with 47 fish and finished on 14 July 2010 (Day 420).
This experiment was conducted without feeding acclimation because the first experiment52

demonstrated that no feeding acclimation was necessary for the juveniles. The fish were
initially reared in a 0.2 kl round-shaped, polycarbonate aquarium, then, with growth,transferred to a 0.5 kl round-shaped, black-colored FRP tank, and finally released to a 1 kl
tank. A few PVC pipes (40 or 75 mm in nominal diameter, 10-50 cm in length) were set in
the water. Sand-filtered seawater was supplied to the tank every hour with an equal volumeof the effective storage capacity. In these feeding experiments, commercial dry pellets,originally for Japanese amberjack Seriola quinqueradiata Temminck & Schlegel, 1845(“Marin” No. 1-6, Hayashikane Sangyo Co., Ltd., Shimonoseki), were fed every hour during
the night from 6 p.m. to 6 a.m. using an automatic feeder. The feed dosage was controlled
every day to avoid residual pellets. After the fish were anesthetized in 0.04%
2-Phenoxyethanol (ethylene glycol monophenyl ether), they were usually measured monthly
for total length (TL, mm) and body weight (BW, g). In some cases, total BW of all surviving
fish was also measured.

Young fish
Young whitespotted conger were sampled on 25 August 1995 from a small trawl
fishing boat which commercially operated in the northeastern Harima Nada Sea. The fish
were transferred alive to the laboratory and released to a 0.5 kl round-shaped, black-colored
FRP tank. Forty-four fish were acclimated but were not fed until 1 September 1995. A
feeding trial started on 2 September 1995 and finished on 12 August 1996 (Day 346). Three
PVC pipes (75 mm in nominal diameter, 30-50 cm in length) were set in the water of thetank, which was covered by a transmissive clear PVC board (4 mm thick) to prevent the fishfrom escapement. A total of 10 kl sand-filtered seawater was supplied to the tank a day. In
this feeding experiment, moist pellets with the same component as that for the leptocephali
were fed in the evening every two days. The feed dosage was determined each time to avoid
residual pellets. At monthly intervals, all the fish were anesthetized with 0.01% ethyl
3-aminobenzoate methanesulfonic acid salt and measured for TL and BW.

Evaluation of rearing experiments
In the experiments conducted in 1995 and 2006, the following characteristic values ofthe feeding trials were calculated: body weight gain (W, g) = W2+W3-W1, feed conversion
efficiency (%) = W·F -1·100, daily growth rate (%) = (X2-X1)·[(X1+X2)/2·t)]-1·100, and daily

53

feeding rate (%) = F·[(N1+N2)/2·(X1+X2)/2·t]-1·100, where W1 = initial total BW, W2 = final
total BW, W3 = BW of dead and missing fish, F = amount of feeding, X1 = initial mean BW,
X2 = final mean BW, N1 = initial number of fish, N2 = final number of fish, and t = feeding
period. The feeding amounts were assumed to be equal to the food intakes by the fish.

Results

Leptocephali
The initial mean TL (MTL) of the leptocephali was 110 mm on 8 April 1997. With
metamorphosing, they exhibited their body shrinkage and showed a minimum size (69 mm
MTL) on 2 May 1997 (Day 24). Although WT was controlled below 26°C, they increased
their length and reached 365 mm MTL at the end of the experiment on 26 December 1997
(Day 262) (Fig. IV-1-2, Table IV-1-1).
Many leptocephali died for a short period around 21 April 1997, when metamorphosis
was in progress and the settled juveniles emerged. The leptocephali schooled marginallyfrom the surface to the bottom, preferred the dark side, and moved near the tank bottom withmetamorphosing. After finishing metamorphosis, they tended to hide themselves in PVC
pipes.
The settled juveniles were observed to bite the edge of the flattened moist pellets, rollquickly their body, and then pluck the diet. Cannibalization was commonly found: the settled
juveniles swallowed the leptocephali from its head, especially at night.

Newly settled juveniles
In the first experiment conducted in 2006, the juveniles were 108 mm initial MTL in
June, grew linearly during the summer, and attained 344 mm MTL on 26 October 2006 (Day125) (Fig. IV-1-3, Table IV-1-2). Although TL increments remained low during a low WT
period, the juveniles slightly grew in 2007 and finally reached 454 mm MTL (range:
361-607 mm) on 28 September 2007 (Day 462). The total feed eaten was 25.5 kg and the
body weight gain was 8.0 kg. The cumulative feed conversion efficiency, daily growth rate,and daily feeding rate were 31.2, 0.4, and 1.3%, respectively. The overall survival rate was
89% (Table IV-1-3).
In the second experiment conducted in 2009, the juveniles were 88 mm initial MTL

54

and grew to 406 mm MTL in a year (Day 363). These fish attained 422 mm MTL on 14 July2010 (Day 420), measuring 346-605 mm TL. All the fish survived until Day 363 (Table
IV-1-4).
During the daytime, almost all the juveniles hid themselves in PVC pipes. When being
fed, they came all together from the pipes and ate the diet. During the night, almost all thefish stayed outside of the pipes, swam around, and foraged.

Young fish
The young fish took about two weeks to begin feeding well on the moist pellets after
starting the experiment. The fish of 239 mm initial MTL in September 1995 grew 325 mm
MTL in December 1995 (Day 91). They reached 422 mm MTL (range: 353-528 mm TL) in
August 1996 (Day 346) (Fig. IV-1-4, Table IV-1-5). Although a high growth rate wasrecorded in the fall (September to November) and spring (April to July) months, it remained
low in winter and summer months when WTs were below 14°C and over 23°C, respectively.
The feed conversion efficiency, daily growth rate, and daily feeding rate were 31.1, 0.4, and
1.5%, respectively. The overall survival rate was 82% (Table IV-1-6).

Discussion

I found in this study that whitespotted conger leptocephali completed metamorphosis
without any feeding. Takashima and Takahashi (2006) also observed that the leptocephali of
the species did not feed before finishing metamorphosis. Mochioka et al. (1993), however,reported the food intake action of larval whitespotted conger. Mochioka and Iwamizu (1996)
also found that the leptocephali fed selectively on larvacean houses of Oikopleura spp., theirand other zooplankters’ fecal pellets. In addition, a mass mortality occurred in this study
during the final phase of metamorphosis which may have been associated with non-feeding
during metamorphosis. Anyway, there are two different results regarding the feeding ofwhitespotted conger leptocephali. Further study is needed to clarify whether or not thefeeding is essential for metamorphosis.
In this study, the leptocephali spent about two weeks at 13°C to transfer them to thedemersal phase. It is known that the metamorphic duration of whitespotted conger ranges
from 10 to 70 days (Kubota 1961; Asano et al. 1978; Lee and Byun 1996, Otake et al. 1997;55

Takashima and Takahashi 2006). This large variation in the duration of metamorphosis may
depend on differences in initial stage of the larvae used for the experiments and in rearing
WT. Also, differences in criteria (e.g., TL shrinkage or otolith increment analysis) to judge
the metamorphic duration may have effect on the results. Since it is difficult and ambiguous
to identify the beginning and completion of metamorphosis, it is highly desirable to establish
the standardized criteria in order to determine the metamorphic duration of whitespottedconger leptocephali.
This study has revealed that whitespotted conger can be reared by feeding oncommercial dry pellets or moist pellets: the newly settled juveniles (88-108 mm MTL) grew
up to over 420 mm MTL in a year and attained as large as 607 mm TL on Day 462 (Tables
IV-1-2, IV-1-4). From the experiments using the moist pellets for the young fish in 1995 andthe dry pellets for the juveniles in 2006, similar results were observed in the feed conversion
efficiency (30%), daily growth rate (0.4%), and daily feeding rate (1.4%) (Tables IV-1-3,
IV-1-6). These values may be a normal value when the commercial incorporated diets areused. It is also considered that the optimum WT range is between 14 and 23°C. The fish
showed a wide range of variation in TL at the end of the feeding trials, and this may becaused by differences in growth rate between individuals and between sexes because thefemale of the species grows faster significantly than the male for the first one and a half yearof its life (Nemoto et al. 2004).
It is evident that whitespotted conger can be reared in captivity from the pelagic
leptocephalus stage to the commercial size. This study also has given us the following
suggestions: 1) the species probably can be farmed commercially; 2) experimental works torequire rearing are feasible; and 3) rearing the juveniles may be one of the options for future
stock management of the species. It may be possible that those non-commercial sizedjuveniles caught in the fishery are reared until they grow to the commercial size, without
releasing them back to the sea from fishing boats. Because fresh fish and bivalves are alsoutilizable as feed for rearing the juveniles (Takashima and Takahashi 2006), even fishermen
may use part of the discards from the haul.
While there is greater advantage in workability to use commercial dry pellets thanboth moist pellets and fresh food, it was quite difficult to rear the wild-caught young fishover 200 mm TL by dry pellets (Gorie unpublished). This is probably because the wild fish
more than 200 mm TL are completely imbued with fresh food. It may be easier to make the56smaller fish take commercial dry pellets. In addition, it is very difficult to rear the fish from
leptocephalus stage because most of the leptocephali die within a few days after capture, andloss of the fish due to cannibalization is inevitable during the rearing.
In conclusion, this study has clarified that whitespotted conger leptocephali finishedmetamorphosis without any feeding. The newly settled juveniles and young fish could bereared by feeding commercial dry pellets and moist pellets. The fish of less than 100 mm TL
caught in May grew 300 mm TL in October, some of which reached over 500 mm TL within
a year. Therefore, the species has a potential to be cultured commercially and to add a value
to its juveniles by a short-term rearing as an alternative option to utilize the stock efficiently
in the eastern Seto Inland Sea.

57

seawater
inflow

outflow(excrements andleftovers)

strainer
Fig. IV-1-1. Diagram of a rearing tank for the leptocephali, newly settled juveniles, andyoung whitespotted conger. Water flow (dotted line) entrains waste materials.

58

30

Total length (mm)

25
300
20
200

15
10

100
5
0

Water temperature (ºC)

400

0
Apr.

June
May

Aug.
July

Oct.
Sep.

Dec.
Nov.

1997

Fig. IV-1-2. Changes in total length of whitespotted conger leptocephali in captivity fed withmoist pellets and in water temperature in the tank. Open circles, water temperature; closed
circles, total length.

59

30

500

25

400

20

300

15

200

10

100

5

0
June
2006

Water temperature (ºC)

Total length (mm)

600

0
Aug.

Oct.

Feb.

Dec.
Apr.
2007

June

Aug.

Oct.

Fig. IV-1-3. Changes in total length of juvenile whitespotted conger in captivity fed withcommercial dry pellets and in water temperature. Line, water temperature; closed circles,
total length. Vertical bars indicate the standard deviation.

60

500

Total length (mm)

25
400

20
15

300

10

Water temperature (ºC)

30

5
200

0
Aug. Oct.
1995

Dec. Feb. Apr.
1996

June

Aug.

Fig. IV-1-4. Changes in total length of young whitespotted conger in captivity fed with moist
pellets and in water temperature. Line, water temperature; closed circles, total length.
Vertical bars indicate the standard deviation.

61

Table IV-1-1. Growth of whitespotted conger leptocephali
fed with moist pellets in 1997
Date in 1997 (Days)

TL (SD)WTN

8 Apr. (Day 0)

110 (6.0)

11.6

70

18 Apr. (Day 10)

95 (7.3)

12.1

18

26 Apr. (Day 18)

81 (7.1)

13.1

3

2 May (Day 24)

69 (0.0)

14.6

2

13 June (Day 66)

83 (2.8)

20.2

11

16 July (Day 99)

132 (20.8)

22.9

4

30 July (Day 113)

161 (26.4)

23.2

4

3 Sep. (Day 148)

222 (23.0)

25.4

4

25 Sep. (Day 170)

259 (27.9)

24.3

4

27 Oct. (Day 202)

305 (36.2)

20.6

4

26 Dec. (Day 262)

365 (15.8)

12.9

3

TL, mean total length (mm); SD, standard deviation; WT,
water temperature; N , number of fish examined.

62

Table IV-1-2. Growth of juvenile whitespotted conger fed with commercial dry pellets
from 2006 to 2007
Date (Days)

TL (SD, range)

BW (SD, range)N

23 June 2006

(Day 0)108 (6.2)

1 (0.2)927 July(Day 34)

175 (17.0)

8 (2.3)10

29 Aug.

(Day 67)229 (10.7)

18 (2.8)

10

28 Sep.

(Day 97)296 (23.0)

39 (10.7)

10

26 Oct.

(Day 125)344 (14.2)

72 (12.8)

10

15 Dec.

(Day 175)

389 (35.1, 283-450)

107 (32.4, 35-176)

45

7 June 2007 (Day 349)

424 (46.7, 332-552)

129 (65.5, 50-346)

45

28 Sep.

454 (55.8, 361-607)

187 (97.9, 45-450)

41(Day 462)

TL, mean total length (mm); BW, mean body weight (g); SD, standard deviation; N ,number of fish examined.

63

64

N , number of fish examined.

23 June 200627 Sep. 2007

462

113

174

15 Dec.-6 June 2007

7 June-27 Sep.

50

26 Oct.-14 Dec.

30

29 Aug.-27 Sep.

28

33

27 July-28 Aug.

28 Sep.-25 Oct.

34

Days

23 June 2006-26 July

Period25,5398,100

11,090

2,705

1,360

901

832

551

Amount offeeding (g)

7,967

2,193

1,144

1,740

1,359

713

545

275

31.2

27.1

10.3

64.3

99.9

79.1

65.5

49.9

Total body
Feed conversion
weight gain (g) efficiency (%)

0.4

0.3

0.1

0.8

2.1

2.4

2.4

4.2

Daily growthrate (%)

1.3

1.1

1.2

1.3

1.9

2.3

4.3

7.8

Daily feeding
rate (%)

4

4

0

0

0

0

0

0

N dead

1

0

0

0

0

0

1

0

89

91

100

100

100

100

98

100

N
Survival
missing rate (%)

Table IV-1-3. Results of the rearing experiment in juvenile whitespotted conger fed with commercial dry pellets from 2006 to 2007

Table IV-1-4. Growth of juvenile whitespotted conger fed with commercial dry pellets
from 2009 to 2010
Date (Days)

TL (SD, range)

BW (SD, range)N

19 May 2009(Day 0)88 (5.6)

1 (0.1)1217 July(Day 59)

183 (12.6, 153-210)

10 (2.2, 2-5)

47

15 Sep.

(Day 119)296 (17.4, 256-339)

39 (8.3, 25-57)

47

18 Nov.

(Day 183)357 (22.9, 295-421)

70 (16.5, 27-107)

47

16 Mar. 2010 (Day 301)

396 (34.4, 295-490)

102 (34.5, 25-198)

47

17 May(Day 363)

406 (45.1, 292-520)

113 (54.3, 18-288)47

14 July(Day 420)422 (62.1, 346-605)

139 (97.9, 50-507)

45

TL, mean total length (mm); BW, mean body weight (g); SD, standard deviation; N ,number of fish examined.

65

Table IV-1-5. Growth of young whitespotted conger fed with moist pellets from 1995 to
1996
Date (Days)

TL (SD, range)

BW (SD, range)

N

1 Sep. 1995

(Day 0)239 (16.7, 194-271)

15 (3.0, 9-22)

44

3 Oct.

(Day 32)

256 (24.1, 193-298)

22 (8.4, 6-39)

43

2 Nov.

(Day 62)291 (32.8, 216-353)

31 (11.6, 8-59)

43

1 Dec.

(Day 91)325 (33.6, 248-381)

50 (17.3, 17-81)

42

9 Jan. 1996

(Day 130)343 (35.3, 258-404)

66 (22.0, 20-105)

41

13 Feb.

(Day 165)

351 (34.3, 260-413)

65 (21.4, 18-102)

40

14 Mar.

(Day 195)

354 (34.0, 262-416)

67 (21.1, 19-112)

40

12 Apr.

(Day 224)

360 (33.5, 268-427)

72 (22.2, 23-129)

40

13 May(Day 255)375 (34.4, 284-448)

82 (25.6, 28-144)

40

12 June(Day 285)401 (34.5, 321-478)

111 (36.4, 44-188)

40

12 July(Day 315)

419 (36.4, 354-518)

120 (37.9, 62-218)

38

12 Aug.

(Day 346)

422 (39.3, 353-528)

116 (40.5, 56-221)

36

TL, mean total length (mm); BW, mean body weight (g); SD, standard deviation; N ,number of fish examined.

66

67

30

31

12 June-11 July

12 July-11 Aug.

N , number of fish examined.

345

30

13 May-11 June

2 Sep. 199511 Aug. 1996

31

12 Apr.-12 May

35

9 Jan.-12 Feb.

29

39

1 Dec.-8 Jan. 1996

14 Mar.-11 Apr.

29

2 Nov.-30 Nov.

30

30

3 Oct.-1 Nov.

13 Feb.-13 Mar.

31

Days

2 Sep. 1995-2 Oct.

Period13,175

1,390

2,200

2,120

1,065

630

510

740

1,850

1,270

900

500

Amount offeeding (g)

4,096

-149

273

1,182

375

217

87

-37

682

791

399

277

-

-

31.1

12.4

55.7

35.2

34.5

17.0

36.8

62.3

44.4

55.3

Total body
Feed conversion
weight gain (g) efficiency (%)

0.4

-0.1

0.3

1.0

0.4

0.3

0.1

-0.1

0.7

1.6

1.2

1.1

Daily growthrate (%)

1.5

1.0

1.6

1.8

1.1

0.8

0.6

0.8

2.0

2.6

2.6

2.0

Daily feeding
rate (%)

4

1

2

0

0

0

0

1

0

0

0

0

N dead

4

1

0

0

0

0

0

0

1

1

0

1

82

95

95

100

100

100

100

98

98

98

100

98

N
Survival
missing rate (%)

Table IV-1-6. Results of the rearing experiment in young whitespotted conger fed with moist pellets from 1995 to 1996

2. Growth of whitespotted conger in the wild

In the previous section, the growth of whitespotted conger in captivity from thepelagic, larval stage to the young stage is studied. While the absolute relationship between
fish age and size in the wild cannot be identified only through the laboratory work, it is also
important to examine wild-caught specimens in order to clarify the growth of the species.
The aim of this section is thus to study characteristics of the age and growth of wild
whitespotted conger in the eastern Seto Inland Sea. The growth is estimated by studying
changes in total length distribution in a single cohort from continuous fishing surveys and by
age determination. To determine the age of the fish, a new aging technique of UV
observation of burnt otoliths (Katayama et al. 2002) is employed, and the seasonality ofannuli formation in the otolith is validated. The age-total length relationship is determined,
and the male-female ratio is also examined. In addition, the movement and growth of thespecies in this region are discussed based on the data from a tagging study.

1) Growth of whitespotted conger based on the fishing survey

To date, no study has been conducted to examine the growth of whitespotted conger
by chasing changes in total length distribution from the pelagic leptocephali to thoseindividuals recruited to the fishery.
In this section, the growth process of whitespotted conger in the wild is estimated by
the fishing survey.

Materials and Methods

The survey area was located in the northeastern Harima Nada Sea (see Figs. II-1-1,
III-1). Sampling of whitespotted conger was conducted from February to December in 2001,
2003, 2004, and 2008. The leptocephali (pelagic stage) were caught using a midwater beam
trawl net (see Fig. II-1-2) from February to May, while the juveniles and young fish(demersal stage) were taken with a bottom trawl net (see Fig. III-2) from April to September
and with pots (see Fig. III-3) in October and December, respectively. The midwater beam
trawl and bottom trawl surveys were conducted in the daytime and 30 min lag time after68

sunset, respectively. Sampling using pots were carried out from evening to night.
The specimens caught were fixed immediately after capture on board in 5-10%
buffered formalin solution and preserved until analysis. They were examined for total length(TL, mm) and body weight (BW, g).

Results

Whitespotted conger leptocephali were collected from February to May. The juveniles
and young fish were caught in April and afterwards. The leptocephali ranged from 85-132mm TL, and the juveniles and young fish measured 68-417 mm TL. Although the newly
settled juveniles were only 82-86 mm mean TL (MTL) in May, they grew to 183-189 mm
MTL in September and attained 301-340 mm MTL in December, when some large-sizedfish reached over 400 mm TL (Fig. IV-2-1, Table IV-2-1).

Discussion

No study has ever been tried to examine the growth of whitespotted conger in the wild
during the period from newly settled juveniles to individuals recruited to the fishery.
Through the continuous sampling at sea, this study is the first to successfully show thatwhitespotted conger reached up to 400 mm TL within a year after they changed from thepelagic to demersal phase. Although the age of the fish was not determined in the study,
monthly changes in length frequency distribution strongly suggest that all the fish examined
are age-0 (Fig. IV-2-1).

69

500
2001

2004

2003

2008

400
300

Total lenght (mm)

200
100
0
500
400
300
200
100
0
Apr.
Dec.
Aug.
June
Feb.
Oct.
Mar.
July
Nov.
Sep.
May

Feb.
Apr.
June
Aug.
Oct.
Dec.
Mar.
May
July
Sep.
Nov.

Fig. IV-2-1. Monthly changes in total length of whitespotted conger collected in thenortheastern Harima Nada Sea in 2001, 2003, 2004, and 2008. Open circles, leptocephali(pelagic phase); closed circles, juveniles and young fish (demersal phase).

70

Table IV-2-1. Change in mean total length of wild-caught whitespotted conger in 2001, 2003, 2004, and
2008
Date

TL (SD)

N

Stage

Date

TL (SD)N

Stage

21 Mar.

113 (6.3)

33

P

18 Feb.

115

-

1

P

11 Apr.

112 (6.9)

93P

11 Mar.

126 (3.3)

4P

23 Apr.

109 (6.0)

204

P

26 Mar.

114 (5.0)

17P

24 May104 (7.3)

52

P

8 Apr.

109 (7.7)

6P

18 Apr.

83 (10.6)

4D

12, 14 May

85 (3.7)

122

D

29 May82 (6.6)

14D

23, 24 June

111 (13.9)

418

D

2001 11 June

90 (9.9)18

D21, 22 July

138 (20.4)

537

D

26 June113 (7.0)

7D

24, 25Aug.

168 (22.3)

388

D

12 July

126 (3.9)

3D

26 Oct.

307 (30.1)

70D

30 July150 (14.1)

7D

22 Dec.

340 (32.0)

98

D

10 Aug.

158 (25.7)

35D

25 Feb.

101

-

1

P

24 Aug.

167 (33.3)

18

D

7 Mar.

123 (3.2)

7P

12, 13 Sep.

183 (8.6)

2D

28 Mar.

116 (9.5)

9P

13 Mar.

117 (5.4)

11P

7 Apr.

108 (7.1)

29P

31 Mar.

120 (8.9)

17P

21 Apr.

116 (6.1)

102

P

11 Apr.

113 (5.7)

9P

2008 19 May

111 (2.8)

11P

25 Apr.

118 (6.1)

13P

28 May86 (6.8)

18

D

2003 12 May

108 (6.6)

14P

25 June103 (10.0)

11D

16 May82 (5.4)

11D

24, 25 Sep.

189 (26.2)

193

D

13 June92 (4.9)

19D

21 Oct.

290 (36.9)

52

D

10 July

125 (16.5)

146D

9 Dec.

301 (38.0)

47D

25 Aug.

186 (21.4)

104

D

2004

TL, mean total length (mm); SD, standard deviation; P, pelagic phase (leptocephalus); D, demersal
phase (juvenile or young); N , number of fish examined.

712) Growth of whitespotted conger based on otolith reading

The absolute age and growth of whitespotted conger in Japanese waters are still poorly
understood. Only several works were conducted based on otolith reading of the material
from the Suo Nada Sea (Takai 1959), Ise Bay (Kubota 1961), and the Kumano Nada Sea(Kubota 1961). This paucity of research in otolith reading for age determination ofwhitespotted conger was due to the fact that a good age character had not been found until
recently. A new method using burnt otoliths was currently developed by Katayama et al.
(2002) to estimate the age of the species. Using this new method, Ishida et al. (2003)
reported the age and growth of the species collected from the Joban Sea off the Pacific coastof Ibaraki Prefecture. No information is, however, available on the age and growth ofwhitespotted conger in the eastern Seto Inland Sea.
In this section, the growth of whitespotted conger in the eastern Seto Inland Sea isinvestigated using the newly developed technique by reading sagittal otoliths. The validity ofsagittal otoliths as an age character is evaluated, and the relationship between total lengthand body weight of the species is presented. The sex ratio of the fish is also discussed.

Materials and Methods

For age determination, whitespotted conger were sampled between 1999 and 2009
from small trawl fishing boats operating in the Harima Nada Sea and Osaka Bay and fromthe pot survey conducted in the Akashi Strait and Harima Nada Sea. A total of 730 fish were
examined for age determination. In addition, the fish from the pot survey collected in 2006
and 2007 in the Harima Nada Sea (Areas A and B, see Fig. III-1) were used for sex
identification.
After measuring total length (TL, mm) and body weight (BW, g), the fish were sexed.
Sagittal otoliths were removed from each fish, washed with tap water, wiped dry, and thenstored in small sealable plastic bags until treatment. After otoliths were burnt and seriallysectioned, they were observed under UV light by the method of Katayama et al. (2002). The
year of birth and age were determined for each fish by counting annuli of the otoliths. As thehatch period of the species is inferred to continue for a long period, from August throughFebruary (Lee and Byun 1996; Mochioka 2001), a chronologic age (hatch in January) is72

adopted in this study. The sex was identified based on observations by the naked eye of the
gonad (Table IV-2-2).

Results

Otolith observation
The year of birth and sex were obtained from 446 out of 730 fish. Seasonal changes inproportion of otoliths with a bright or dark zone at their margin under UV light are shown in
Fig. IV-2-2. The margin of otoliths from the fish collected in October and February
possessed the dark zone. The bright zone was observed between April and August with apeak of the proportion being in June. The proportion of the bright zone increased anddecreased before and after June, respectively.

Age and growthWhitespotted conger examined from the eastern Seto Inland Sea were found to bemostly age-1 to 4 (Fig. IV-2-3). The fish showed a wide variation in TL in the same year
class and had a complicated age composition in the same TL class. Age-1, 2, 3, and 4
females measured 326-793 (mean: 579, N=105) mm TL, 301-895 (592, N=162) mm TL,
473-890 (665, N=35) mm TL, and 661-851 (782, N=8) mm TL, respectively. Also, thefemales of about 400 mm TL consisted of age-1 to 3 fish. While over 800 mm TL wasrecorded even for age-1 and age-2 females, most of the males were less than 400 mm TL(85%, N=134) and age-1 (85%). In the TL-BW relationship, dispersion of BW increased forthe females larger than about 600 mm TL (Fig. IV-2-4).

Sex ratio
The number of males and females used for age determination was 134 and 312,
respectively (Table IV-2-3). The age of the males ranged from 0-2 (mostly age-1), while thatof the females was up to 5 (age-2 fish were the most abundant, followed by age-1 fish). The
males and females ranged from 284-470 (mean: 376, N=134) mm TL and 301-895 (601,
N=312) mm TL, respectively. However, most of the specimens from the pot survey in 2006
and 2007 were males (91%, N=507) (Table IV-2-4).

73

Discussion

It has been shown in this study that the bright zone at the margin of burnt sagittal
otoliths from whitespotted conger captured in the eastern Seto Inland Sea was formed from
April to August. While the percentage of the bright zone did not reach 100%, it is reasonable
to consider that this zone was formed once a year during the period from April to August.
Similarly, Katayama et al. (2002) found that the bright zone was formed from April to
October in the same species from the coastal Pacific waters off northern Honshu Island,
Japan. Thus, this study has confirmed that the sagittal otoliths can be used as an age
character for whitespotted conger.
Whitespotted conger exhibited a wide variation in TL in the same year class and had acomplicated age composition in the same TL class. While an age-TL relationship was not
displayed, Ishida et al. (2003) found similar results in the fish caught in the Pacific waters off
the coast of Ibaraki Prefecture. In addition, although the male sample size was small in thepresent study, it is likely that difference in growth existed between sexes in the wild (i.e., thefemales grow faster than males) as Nemoto et al. (2004) reported for the captive fish. In
addition, Egusa (1970) reported a growth difference between sexes in European eels
Anguilla anguilla in freshwater ponds. Further, the females have a potential to mature atage-2 (Horie et al. 2001), but the males attained sexual maturity even at age-1 in captivity(Gorie unpublished). Thus, the growth difference between sexes may be derived from theages at maturity and/or life spans. Therefore, whitespotted conger in the wild also show highdiversity in growth not only between individuals but also sexes, as stated for the captive fish(see Chapter IV-1), which may be a characteristic of the life history of the species.
The lack of a clear correlation between the age and TL means that the age compositionof whitespotted conger cannot be accurately estimated only using data of length frequency
distribution. In this study, the method using burnt otoliths was found useful to determine theage of whitespotted conger, which suggests us to apply age-length key method for the
analysis of population dynamics of the species in the future. In addition, the TL-BW
relationship found in this study will contribute to conversion from the total amount of catch
into the number of catch of whitespotted conger in the eastern Seto Inland Sea.
Based on microscopic observations of the whole otoliths, Takai (1959) and Kubota(1961) previously reported the age and growth of whitespotted conger from the Suo Nada

74

Sea, Ise Bay, and the Kumano Nada Sea. The results of these authors were somewhat
different from those found in this study. The range of TL in their fish of the same age class
was at most about 200 mm. In contrast to it, the fish I examined showed a very wide
variation in TL, for example, from 301-895 mm (594 mm in difference) for age-2 fish. It is
unknown at present whether the difference between their and my results is due to themethods employed, the number of the fish examined, and/or the fluctuation in growth
pattern of the species between years and regions.
Whitespotted conger examined for age determination in this study were mostly
females (70%, Table IV-2-3) as previously reported (Takai 1959; Kubota 1961; Okamura etal. 2000; Katayama et al. 2002; Ishida et al. 2003). In 2006 and 2007, nonetheless, a large
number of males appeared in the same length class (91%, Fig. IV-2-5, Table IV-2-4), andmost of them were considered to be age-0 or 1 (see Chapters IV-1, IV-2-1). A similar
male-biased sex ratio has been reported for the individuals in a specific enclosed lagoon(Nemoto 2007) and for the fish less than 450 mm TL in the eastern Seto Inland Sea(Takemori et al. 2004). To date, no adequate explanation is possible for the biased sex ratio(e.g., the absence of males more than 450 mm TL or over age-2 and the male-biased sex
ratio in the young fish). As the large-sized fish are supposed to be females (Fig. IV-2-3), thesex ratio in commercially caught, large whitespotted conger may be female-biased.
Alternatively, the males may conduct a migration to an unknown spawning ground earlier
than females. It may be also possible that the females have already moved somewhere out of
the survey area before they recruit to the fishery as their recruitment is earlier than the males’.
Consequently, it is likely that the biased sex ratio may be caused by differences in growth,
habitat utilization, and movement between sexes of the species.
In conclusion, sagittal otoliths could be used as an age character for whitespottedconger. This species showed a wide variation in TL in the same year class and had acomplicated age composition in the same TL class, which suggests that age-length key
method is more suitable than length-frequency analysis for future research on the population
dynamics of whitespotted conger. More work is needed in various waters of Japan todetermine the causes responsible for the biased sex ratio of the species.

75

1

100

13 5 31 14 46

92

Frequency (%)

80
60
40
20
0
Jan.

Mar.

Feb.

May

Apr.

July

June

Sep.

Aug.

Nov.

Oct.

Dec.

Fig. IV-2-2. Seasonal changes in proportion of otoliths with a bright or dark zone at their
margin under UV light. Open bars, bright zone; shaded bars, dark zone. Sample size is given
for each month. Vertical bars mean 95% confidence interval.

76

1,000Total length (mm)

800
600
400
200
0
0

1

3

2

4

5

Age

Fig. IV-2-3. Age-total length relationship of whitespotted conger collected in the eastern
Seto Inland Sea from 1999 to 2009. Open circles, males (N=134); closed circles, females(N=312); open triangles, fish of unknown sex and probably age-0 (N=475).

77

1,400
Y=6.04·10-7X3.17
R2=0.998

Body weight (g)

1,200
1,000
800
600
400
200
0
0

100 200 300 400 500 600 700 800 900
Total length (mm)Fig. IV-2-4. Relationship between total length and body weight of whitespotted conger
collected in the eastern Seto Island Sea. Allometric relationship regressed with all data
overlaid. Open circles, males (N=8); closed circles, females (N=162); open triangles, fish of
unknown sex and probably age-0 (N=475).

78

700

Total length (mm)

600
500
400
300
200
100
0
2006

2007

Fig. IV-2-5. Sex of whitespotted conger collected in the eastern Seto Inland Sea in 2006 and2007. Open circles, males (N=460); closed circles, females (N=45); open triangles, fish of
unknown sex (N=2).

79

Table IV-2-2. Criteria to differentiate the sex of whitespotted conger based on the gross morphology ofthe gonad
Male

FemaleTestes barely visible as thin strands or visible ascord

Ovary cleary visible as a lamellar structure anddull white

Ventrolateral to the swim bladder

Individual oocytes barely visible to the naked eye

Very smooth and dull white

Subtle granular appearance and bright white

Can be easily pulled out with tweezers

Fragile tender tissue, easy to be pieces whenpulled out

80Table IV-2-3. Sex composition in each age groupof whitespotted conger collected in the eastern
Seto Inland Sea from 1999 to 2009
Age

Male

Female

0

6

1

1

114

105

2

14

162

3

35

4

8

5

1

Total

134

81

312

Table IV-2-4. Sex composition in whitespotted conger collected in the eastern Seto
Inland Sea in 2006 and 2007
Date

Male

Femaleunidentified

Total

22 Dec. 2006

357

40

2

399

23 Oct. 2007

103

5

0

108

Total

460

45

2

507

82

3) Movement and growth of whitespotted conger determined by tagging study

Information on the growth and migration of wild whitespotted conger is quite limited.
The species may have a sex-biased migratory habit (see Chapter IV-2-2), and the females
tend to move to deeper waters with maturation (Okamura et al. 2000). They also exhibit aseasonal loop migration driven by the avoidance of cold waters (Katayama et al. 2004). A tag
and release survey must be a powerful tool to investigate the movement of whitespottedconger in the wild. So far, a few tagging studies have been conducted for young whitespottedconger. Based on the fact that the tag-released young fish tended to return to the release sitein Tokyo Bay, they are considered to have homing behavior (Shimizu 2003). In contrast,
however, the young fish [30 cm in total length (TL)] may remain near the release site in theeastern Seto Inland Sea (Takemori et al. 2004). To date, however, there have been no studies
using tagging on the growth of the large-sized fish (>50 cm TL).
In this section, the movement and growth of large-sized whitespotted conger is shown
based on a tagging study conducted in the eastern Seto Inland Sea. Also, based on the
hypothesis that the females leave from the eastern Seto Inland Sea for a yet unknown
spawning ground, new findings that may infer their pre- or spawning migration arepresented.

Materials and Methods

Tagging and release
The study area was located in the eastern Seto Inland Sea (Fig. IV-2-6). Most of thewhitespotted conger used for tagging were captured in the Akashi Strait using pots, and somewere obtained from a commercial small trawl fishery operating near the strait. All the tagged
fish were released at a single site in coastal waters of the northeastern Harima Nada Sea (Fig.
IV-2-6, 34°41’N, 134°52’E). Before tagged, the fish were anesthetized in 0.04%
2-Phenoxyethanol (ethylene glycol monophenyl ether) and measured for total length (TL,mm) and body weight (BW, g). Conventional T-bar anchor tags (20 mm long), yellow or red
in color and individually numbered, were used from 1997 to 2002. Anchor tags wereinserted with a tag gun into the midpoint of the caudal muscle just above the lateral line
between the anus and the posterior end of the caudal fin. Also, in 2003 to 2008, a83

double-tagging method was employed. After anchor tags were introduced, an individuallynumbered yellow tube (25 mm long, 1.75 mm in diameter), which was cut off the dart tag,
was inserted into the abdominal cavity near the anus using a scalpel. In this double-tagging
method, it was expected that highly visible, external anchor tags could attract fishermen’s
attention, and recaptured fish could be exactly identified by internal dart tubes. Especially, as
the number on an anchor tag tended to vanish within 2 months, this double-tagging method
was quite useful to identify individual fish.
Almost all the tagged fish were estimated to be females because their overall mean TL(MTL) was 519 mm (Table IV-2-5, see Chapter IV-2-2). A small number of the tagged andnon-tagged fish were kept in a laboratory aquarium for 2 months, but there was no tag loss
from these fish, and no significant differences in TL and mortality were observed between
the two groups.

Growth and movement analysis
Growth was determined using the fish specimens whose tag numbers and dates atrecapture were available. Freshly-caught specimens were received from fishermen andmeasured for TL, BW, and gonad weight (GW, g). Absolute growth rate (G) was calculated
as follows: G = (TLr -TLi )·d -1, where TLr is total length at recapture, TLi is total length atrelease, and d is the number of days that the fish were at liberty. When frozen fish specimens
were received, their TL was compensated because those specimens shrank to 97.3% of theirinitial TL in my preliminary test (N=38) when they were thawed. No data were used when afish size at recapture is lower than that at release.
Gonadosomatic index (GSI) was calculated as follows: GSI (%) = GW·BW -1·100. In
addition, following Katayama et al. (2002), age determination was conducted using sagittal
otoliths.
Recapture locations and movement distances were determined when veracious
locations were available. All recapture locations were plotted and visually validated.
Although some fish were recaptured very shortly after release near the release site, thelocations within 5 km from the release site were eliminated from movement analysis. The
movement distance was calculated as a linear feasible distance from the release site.
However, when a straight line was impossible due to the presence of Awaji Island, thedetours were calculated assuming that the fish passed through the central region in one of the84three straits (Akashi, Naruto, and Yura Seto straits, Fig. IV-2-6).

Annual catch survey at a fisheries cooperative association
In 2002 to 2008, monthly catch data of whitespotted conger by brand size werecollected at Fisheries Cooperative Association in Kariya, where most of the fishermen are
engaged in the small trawl fishery in Osaka Bay off Hyogo Prefecture (Fig. IV-2-6). The fish
are usually marketed as one of the three fish sizes: small (<30 cm TL), medium-large (30-50
cm TL), and extra large (>50 cm TL).

Statistical analysis
Differences in TL and GSI were explored by Mann-Whitney’s U test between the twowaters (Harima Nada Sea-Akashi Strait waters vs. Osaka Bay-Kii Channel waters) where thefish were recaptured. Differences in age composition and recaptured waters were exploredby χ2 test.

Results

Tag-release and recapture
In 1997 to 2008, a total of 2,439 fish were tagged and released. The overall MTL was
519 mm (Table IV-2-5). No mortality was observed during the tagging and release
operations. Of the 2,439 fish released, 181 were recaptured. Of these recaptured fish, 63
were single tagged, while 118 were double tagged. A half of the recaptured fish were caught
during the pot surveys in the Akashi Strait. The remaining recaptures were mostly caught by
the small trawl fishery and reported from fishermen, but a few specimens were sent by fish
brokers. When a tag was identified by these people, only information on the date andlocation of recapture or the tag number was sometimes obtained. Of the 100 specimens
closely examined, 4 fish were found to have lost internal tubes. The recaptured fish could be
individually identified up to 420 days after the release, using the numbers on the internal
tubes.

Growth and biological aspects
The growth rates were determined from 131 recaptured fish. As the number of days at85

liberty was longer, the growth increment of the fish at recapture was bigger. The fishexhibited various growth rates ranging from 0.25-2 mm day-1 during May to September(month at release) and from 0.1-1 mm day-1 during November to March (Fig. IV-2-7). The
fish released from May to September showed larger growth rates than those released from
November to March. The growth rate of the fish, which were recaptured over 300 days after
release ranged from 0.3-0.6 mm day-1 (mean: 0.5, N=5). One fish showed as large as 233
mm growth increment for 394 days (from 502 mm TL at release to 735 mm TL at recapture).
The females recaptured in the Osaka Bay-Kii Channel waters were larger than those in the
Harima Nada Sea-Akashi Strait waters (P=0.009, U test), but no significant difference wasfound in the males (Fig. IV-2-8).
GSIs of the females were correlated with their size (Fig. IV-2-9, P<0.001, r=0.59,
N=90). The fish with more than 4 in GSI were caught only in the Osaka Bay-Kii Channelwaters (Fig. IV-2-10).
The age of the recaptured fish varied from 1-3 (Table IV-2-6). The males recaptured in
the Harima Nada Sea and Osaka Bay were age-1. There was no significant difference
between female age composition and displacement waters (χ2 test) although the age of therecaptured females ranged from 1-2 in the Harima Nada Sea, from 1-3 in the Akashi Strait,
and from 2-3 in Osaka Bay, respectively.

Movement
The recapture locations were plotted and validated using the data of 144 fish. The fish
moved to Osaka Bay and the Kii Channel from the Harima Nada Sea through the Akashi
Strait. Most of the fish that exhibited the eastward movement were recaptured near the
Akashi Strait. In January to August, the fish were recovered widely from Osaka Bay (Fig.
IV-2-11). Of the fish recovered in the Osaka Bay-Kii Channel waters, the females wererecaptured from January to August, while a few males were from August to October (Table
IV-2-7). The recapture with the longest movement (about 60 km from the release site) wascollected in the Kii Channel. A few fish were recaptured in the waters west of the release site,and one fish was near the Naruto Strait.
Not all the released fish passed through the Akashi Strait, nor they did always move to
Osaka Bay through the strait even when they were at liberty for a long period. Somelarge-sized fish remained in the Akashi Strait (Fig. IV-2-8). Some fish moved to Osaka Bay

86

soon after the release, but others remained around the Akashi Strait. The recaptured fish weredivided into two groups by displacement waters (Fig. IV-2-12): a locally resident group in
the Harima Nada Sea-Akashi Strait waters and a migratory group in the Osaka Bay-Kii
Channel waters, which indicates that these groups have different migratory habits.

Seasonal catch of whitespotted conger
Extra large-sized fish (>50 cm TL) were caught in Osaka Bay off Hyogo Prefecturefrom January to July with a peak in April (Fig. IV-2-13). Such fish were not caught from
August.

Discussion

A double-tagging method, using conventional, external anchor tags and individuallynumbered internal tubes, was employed in this study. The numbers printed on the anchor
tags were subject to vanish because whitespotted conger usually hide in the bottom and rub
their body against the burrow wall. Due to this, it was often difficult to identify individual
fish only by the external anchor tags. On the other hand, since the numbers printed on theinternal tags were well preserved in the fish’s abdominal cavity and clearly identifiable morethan one year, the double-tagging method is highly recommended to study the biology ofwhitespotted conger.
This study has shown that whitespotted conger moved to Osaka Bay and the Kii
Channel from the northeastern Harima Nada Sea through the Akashi Strait (Fig. IV-2-11).
This movement may be a pre- or spawning migration to a yet unknown spawning ground. In
addition, a new hypothesis that such movement begins in January and continues until Augustis herein proposed based on the following information: 1) most of the tagged fish werefemales and reached to a sexually maturing size (based on an experiment of Horie et al.
2001); 2) the spawning season of whitespotted conger is estimated to be from August to
February (Tanaka et al. 1987; Lee and Byun 1996; Mochioka 2001; Kurogi et al. 2002), andthe individuals tagged and released in this study moved to Osaka Bay and the Kii Channel
from January to August, which is before the estimated spawning season; 3) both TL and GSIof the females recaptured in the Osaka Bay-Kii Channel waters were significantly higher
than those in the Harima Nada Sea-Akashi Strait waters (Figs. IV-2-8, IV-2-9, IV-2-10). In

87

January to April, GSIs of the females recaptured in the Osaka Bay-Kii Channel waters werealso significantly higher than those of the females caught with pots in the Akashi Strait (seeChapter VI), indicating that the fish had more developed gonads in the Osaka Bay-Kii
Channel waters; and 4) no fish of extra large size was caught in Osaka Bay from August by
the small trawl fishery (Fig. IV-2-13), which implies that such fish left Osaka Bay for otherwaters.
During the survey, no spawning evidence of whitespotted conger in the eastern Seto
Inland Sea was obtained, which is the same as in other inshore waters of Japan (Takai 1959;
Okamura et al. 2000; Ishida et al. 2003). It is thus reasonable to consider that the fish begin apre- or spawning migration to the North Pacific before the estimated spawning season.
Of the 181 fish recaptured, 110 (61%) were taken in the Akashi Strait, where the fish
were originally caught for the tagging study. This high recapture rate was partially due to thefishing efforts there, but some fish are thought to have exhibited a kind of homing behavior,as is inferred with the young fish (Shimizu 2003). The Akashi Strait may be one of thefavorite and important habitats for the large-sized fish in the eastern Seto Inland Sea.
A west-southward detour to the Kii Channel through the Naruto Strait was notconsidered in this study. Such movement, however, may be possible because one fish wasrecaptured near the strait (Fig. IV-2-11, mid-left). While no obvious explanation is possible
at present why there were few recaptures in the waters west of Awaji Island, this may be due
to quite a low fishing activity and/or the lack in notification to fishermen about the tagging
study.
Judging from their TL, almost all the tagged and released fish were females. A few
males were recovered in Osaka Bay in August and October (Fig. IV-2-8, Table IV-2-7), but
this study cannot refer to the movement of the males because of their small sample size. The
absence of the males or large-sized males of whitespotted conger from coastal waters ofJapan has been reported to date (Okamura et al. 2000; Ishida et al. 2003). A similar
phenomenon is also known for European conger Conger conger (Cau and Manconi 1983;
O’Sullivan et al. 2003). It has, however, been recently found that whitespotted conger males
occurred at a higher rate in a specific enclosed, brackish-water lagoon (percentage of males,55%) than in inshore waters (1%) along the Pacific coast of northeastern Japan (Nemoto2007). The male-biased occurrence was also reported for the fish less than 45 cm TL in theeastern Seto Inland Sea [see Chapter IV-2-2 (percentage of males, 91%); Takemori et al.

88

2004 (73%)]. Differences in various biological aspects between sexes may be a specificcharacter of congrid fishes.
This study has shown for the first time the growth rate of wild whitespotted conger inthe eastern Seto Inland Sea. The recaptured fish exhibited various growth rates from 0.1-2
mm day-1 (about 20 cm year-1). The fish released from May to September showed a larger
growth rate than those released from November to March (Fig. IV-2-7). This is due todifferences in WT in the habitat because the fish reared in aquaria showed WT dependent
growth (see Chapter IV-1). These growth rates are higher than those of two related species,
American conger Conger oceanicus and European conger C. conger (about 10 cm year-1 forboth species, Hood et al. 1988; Sbaihi et al. 2001). In addition, whitespotted conger in theeastern Seto Inland Sea showed a wide variation in TL within the same year class and had acomplicated age composition in the same TL class (see Chapter IV-2-2). Similar results were
found for the same species caught off Joban, northeastern Japan (Ishida et al. 2003), C.
oceanicus in the western Atlantic off North America (Hood et al. 1988), and C. conger in theEuropean waters (O’Sullivan et al. 2003). Whitespotted conger reared in aquaria also show
various differences in growth (see Chapter IV-1; Nemoto et al. 2004). This wide variation in
growth rate is probably another specific aspect of congrid fishes, suggesting that age-length
key method is more reliable for the stock estimation of the fish than length-frequency
analysis.
No data on the factors to trigger the launch of the movement were taken in this study.
There was no significant difference between the female age composition and displacement
waters where the fish were recaptured (Table IV-2-6). Some large-sized fish remained in the
Akashi Strait for a long period after release. Two types of groups were identified in the study
area by the displacement waters (Fig. IV-2-12): a locally resident group to stay for a certain
period near the Akashi Strait and a migratory group to move to Osaka Bay and the Kii
Channel. The species may have complicated migratory habits in the eastern Seto Inland Sea.
Since the individuals caught in the Osaka Bay-Kii Channel waters were larger than those in
the Harima Nada Sea-Akashi Strait waters, the species may conduct a size-dependent
migration. Alternatively, the pre- or spawning migration of the locally resident group of the
fish may begin by age-5 in the eastern Seto Inland Sea because the fish collected in this sea
were up to age-5 (see Chapters IV-2-2, VI). However, as an age-13 specimen was taken in
the Joban waters (Ishida et al. 2003), the migration of the species is still a complicated89biological issue for further research. More information on the age composition and otherbiological aspects of whitespotted conger in various regions are needed.
In conclusion, whitespotted conger moved to Osaka Bay and the Kii Channel from theHarima Nada Sea through the Akashi Strait in the eastern Seto Inland Sea. The observed
displacement suggests their pre- or spawning migration, which is considered to begin in
January and continue until August. This species is likely to have two groups showing
different migratory habits. The fish exhibited various growth rates ranging from 0.1-2 mmday-1, which is almost equivalent to about 20 cm year-1.

90134ﾟ20'
Sea of Japan

134ﾟ40'135ﾟ135ﾟ20'34ﾟ40'

Pacific Ocean

Akashi St.

Osaka Bay

Harima Nada Sea

Kariya34ﾟ20'

Naruto St.

Awaji Is.
Yura Seto St.
10 km

Kii Channel

Fig. IV-2-6. A study area in the eastern Seto Inland Sea, Japan. The release site is shown by
an open star. The location of Fisheries Cooperative Association in Kariya, where the catch
data of whitespotted conger were obtained is indicated by an open circle. The regionalboundaries are shown by dotted lines.

91

Growth increment (mm)250

1 mm/day

2 mm/day

2 mm/day

1 mm/day

200
150

0.5 mm/day

0.5 mm/day

0.25 mm/day

0.25 mm/day

100
50

0.1 mm/day

0.1 mm/day

0
0

100

200

300

400

500 0

100

200

300

400

500

Days after release

Fig. IV-2-7. Growth increments of whitespotted conger recaptured in four regions of theeastern 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 circles, recaptures in the Harima Nada Sea; closed circles, recaptures in the Akashi
Strait; open triangles, recaptures in Osaka Bay; closed triangles, recaptures in the Kii
Channel.

92

Total length at recapture (mm)

900
800
700
600
500
400
300
Harima Nada Sea
Osaka Bay
Akashi Strait
Kii Channel

Fig. IV-2-8. Total length at recapture of male and female whitespotted conger from four
regions of the eastern Seto Inland Sea. Open circles, males; closed circles, females; open
triangles, fish of unknown sex.

93

Gonadosomatic index (%)

10
8
6
4
2
0
300

400

500

600

700

800

900

Total length (mm) at recapture

Fig. IV-2-9. Relationship between total length and gonadosomatic index at recapture offemale whitespotted conger from four regions of the eastern Seto Inland Sea. Open circles,
recaptures in the Harima Nada Sea; closed circles, recaptures in the Akashi Strait; open
triangles, recaptures in Osaka Bay; closed triangles, recaptures in the Kii Channel.

94

Gonadosomatic index (%)

10
8
6
4
2
0
Jan.

Sep. Nov.
July
Mar. May
Aug. Oct. Dec.
Apr. June
Feb.

Fig. IV-2-10. Monthly changes in gonadosomatic index at recapture of female whitespottedconger from four regions of the eastern Seto Inland Sea. Open circles, recaptures in the
Harima Nada Sea; closed circles, recaptures in the Akashi Strait; open triangles, recaptures in
Osaka Bay; closed triangles, recaptures in the Kii Channel.

95134°20

134°

1Fig. IV-2-11. Bimonthly recapture positions (dots) of whitespotted conger released at asingle 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.

96Movement distance (km)

70
60
50
40
30
20
10
0
0

100

200

300

400

500

Days after release

Fig. IV-2-12. Movement distance from the release site of whitespotted conger from four
regions of the eastern Seto Inland Sea. Open circles, recaptures in the Harima Nada Sea;
closed circles, recaptures in the Akashi Strait; open triangles, recaptures in Osaka Bay;closed triangles, recaptures in the Kii Channel.

97

5

80

4

60

3

40

2

20

1

Extra large (%)

Small, medium-large (%)

100

0

0
Jan.

Mar.

Feb.

May

Apr.

July

June

Sep.

Aug.

Nov.

Oct.

Dec.

Fig. IV-2-13. Monthly composition of three size classes of whitespotted conger landed at
Fisheries Cooperative Association in Kariya. The fish were caught in a small trawl fishery inOsaka Bay off Hyogo Prefecture. Data are shown using total catch for 7 years from 2002 to
2008. Shaded bars, small fish; unshaded bars, medium-large fish; closed circles, extra large
fish.

98

Table IV-2-5. Number and total length of whitespotted conger
tagged and released in the northeastern Harima Nada Sea from1997 to 2008
Year

Month ofreleaseN

TL (mm)SD (mm)

1997

June

90

440 (277-749)

87.3

July

262

513 (279-784)

93.9

June

40

535 (354-771)

82.8

July

14

598 (434-657)

60.0

Aug.

7

645 (573-704)

45.6

May

45

417 (344-516)

39.1

June

90

506 (382-755)

75.0

July

66

532 (421-760)

68.3

Nov.

63

571 (289-758)

126.6

Dec.

89

686 (547-943)

72.8

Nov.

104

584 (382-818)

83.0

Dec.

63

609 (385-880)

87.2

June

45

441 (368-543)

47.9

Nov.

53

575 (411-777)

98.9

Dec.

129

618 (376-891)

88.5

June

63

440 (326-613)

70.6

Sep.

81

517 (390-693)

77.0

Dec.

107

614 (354-815)

89.6

Mar.

5

552 (436-725)

107.6

May

46

457 (334-708)

87.3

June

96

479 (328-802)

92.9

Sep.

41

561 (413-682)

63.1

Nov.

42

613 (478-770)

60.6

June

134

478 (346-743)

74.7

Nov.

58

605 (377-754)

85.1

June

203

454 (318-720)

68.7

Nov.

46

524 (298-749)

102.5

June

116

441 (302-642)

68.9

Nov.

19

466 (338-607)

77.1

June

97

462 (330-685)

75.6

July

80

460 (320-763)

87.4

Nov.

45

474 (286-731)

111.8

Total/Mean2,439

519 (277-943)

107.9

1998

1999

2001

2002

2003

2004

2005

2006

2007

2008

N , number of fish tagged and released; TL, mean total length(range); SD, standard deviation.

99

Table IV-2-6. Age and number of whitespotted conger recaptured
in the eastern Seto Inland Sea from 1997 to 2008
Age

1

2

3

Sex

Male Female

FemaleFemaleHarima Nada Sea

1

Akashi Strait
Osaka Bay

5

2

34

11

2

7

2

1

100

Table IV-2-7. Number of male and female whitespottedconger recaptured in Osaka Bay and the Kii Channel from1997 to 2008
Month

FemaleMale
Osaka Bay

Osaka BayKii Channel

Jan.

6

Feb.

1

1

Mar.

5

1

Apr.

3

May
June
July
Aug.

1
3

1

Sep.
Oct.

1

Nov.
Dec.

101

V. Fishery status of whitespotted conger and stock management approach

A fishery resource management project started in the eastern Seto Inland Sea in 1988.
The six prefectures (Okayama, Hyogo, Osaka, Wakayama, Kagawa, and Tokushima) facing
the eastern Seto Inland Sea formed the eastern Seto Inland Sea block for the purpose of thefishery resource management (Anon. 1991). In this project (phase I), four species weretargeted: marbled flounder Pseudopleuronectes yokohamae (Günther, 1877), ridged-eye
flounder Pleuronichthys cornutus (Temminck & Schlegel, 1846), bastard halibut
Paralichthys olivaceus (Temminck & Schlegel, 1846), and red seabream Pagrus major(Temminck & Schlegel, 1843). The project addressed avoiding and reducing the discards ofthese fishes in various fisheries. Following the phase-I project, since 1993, a survey offishery management for the fish species caught by the small trawl fishery has been
conducted as a phase-II project (Anon. 1996). This phase-II project has focused onwhitespotted conger because this species is one of the most commercially important fishes
caught by the small trawl fishery in the eastern Seto Inland Sea.
In this chapter, the catch and fishing status of whitespotted conger in the eastern Seto
Inland Sea are reported. The possible use of enlarged mesh size in the small trawl fishery forthe stock management of the fish is discussed based on mesh and artificial selectivity
experiments in the northeastern Harima Nada Sea. Knowledge of the mesh selectivity ofwhitespotted conger will provide fishermen with useful information on how to takeappropriate stock management options, such as the feasibility of enlarging the mesh size inthe codend.

1. Whitespotted conger fishery in the eastern Seto Inland Sea off Hyogo Prefecture

Whitespotted conger are usually caught by three fisheries (small trawl, longline, andothers) in the eastern Seto Inland Sea off Hyogo Prefecture. Of these fisheries, the small
trawl fishery is the main one in this region and includes some variations, such as beam trawl,
dredge trawl, otter trawl, etc. Chin-kogi, a variant of the dredge trawl equipped with chainsor net-sinkers on a ground rope, is usually operated in the northeastern Seto Inland Sea,
while ita-biki, a small otter trawl, and/or other trawl fisheries are commonly operated both in
the western Harima Nada Sea and Osaka Bay off Hyogo Prefecture. On the other hand, in102

Osaka Bay off Osaka Prefecture, the species is commercially caught by “other fishery”,
which is considered to be the pot fishery (Nabeshima et al. 1995).
In this section, annual and monthly changes in the catch as well as monthly changes inlength frequency distribution of whitespotted conger caught in the Harima Nada Sea andOsaka Bay off Hyogo Prefecture are described.

Materials and Methods

Fishery statistics
Information on the catch of whitespotted conger and cumulative fishing effort of thesmall trawl fishery was collected from Annual Statistical Reports on Agriculture, Forestry,
and Fisheries of Hyogo Prefecture published from 1986 to 2010 (Anon. 1986-2010). The
catch data of “congrid fishes” found in these annual reports are herein regarded as those ofwhitespotted conger because congrids commercially caught in the eastern Seto Inland Sea
are a single species (i.e., whitespotted conger Conger myriaster).

Annual catch survey at fisheries cooperative associations
Monthly catch data of whitespotted conger were collected from 1994 to 1997 at thethree fisheries cooperative associations in Tasakago, Kariya, and Nushima, Hyogo Prefecture.
Most of the fishermen belonging to these three associations are engaged in the small trawl
fishery and catch the species in the northeastern Harima Nada Sea, Osaka Bay, and thenorthern Kii Channel, respectively (Fig. V-1-1).

Fishing status in Osaka Bay
A monthly sampling was carried out for a year from January to December in 1994 at
Fisheries Cooperative Association in Kamaguchi (Fig. V-1-1). Whitespotted conger caught
by the small trawl fishery (an otter trawl, called ita-biki) were collected there from fishingboats and brought to the laboratory, where they were measured for total length (TL, mm) andbody weight (BW, g).

Fishing status in the northeastern Harima Nada Sea
A monthly fishing survey was conducted in the northeastern Harima Nada Sea for one

103

year from October 1994 to October 1995, using a chartered small trawl fishing boat (adredge trawl, called chin-kogi) belonging to Fisheries Cooperative Association in Takasago(Fig. V-1-1). The fish caught through this survey were divided by a fisherman into twogroups, fish to be landed or fish to be discarded. All the fish of these groups were separatelytransferred to the laboratory, where they were measured for TL (mm) and BW (g). Artificial
selectivity was calculated by 10-mm TL class as follows (Nishikawa et al. 1994): Artificial
selectivity (%) = (Number of fish for landing)·(Number of fish caught in the codend)-1·100.
In addition, following Chen et al. (1992), an artificial selectivity master curve (logistic
model) was also calculated by a least square method: Sa = 1·[1+exp-k(L-m)]-1, where Sa is
artificial selectivity, L is TL class (mm), k and m are constant numbers of logistic curve. The
25, 50, and 75% artificially selected TLs were deduced from the following formulae: L25 =(km-ln3)k-1, L50 = m, and L75 = (km+ln3)k-1, respectively.

Results

Fishery statistics
Whitespotted conger are one of the commercially important fishes in the small trawl
fishery as well as in the fisheries in the eastern Seto Inland Sea off Hyogo Prefecture (Tables
V-1-1, V-1-2). The average annual catch of the fish in this region was 1,700 metric tons (mt)from 1984 to 1997, but the catch decreased to 562 mt in 1999. After the catch slightly
recovered, it was rather stable from 2000 to 2008 and was 582 mt in 2008 (Fig. V-1-2, top).
In this region, whitespotted conger were mostly caught by the small trawl fishery. The
annual percentage of the species catch from the small trawl fishery in the total catch wasstable as high as 70% during the whole survey period from 1984 to 2008 (Fig. V-1-2,
bottom). The catch in the longline fishery decreased gradually and that in other fisheries
showed a very little increase trend.
The cumulative fishing effort of the small trawl fishery decreased gradually from 1984
to 2006 (Fig. V-1-3). This trend, however, did not coincide with annual changes in catch of
the fish.
The catch of whitespotted conger at the three fisheries cooperative associations in
Takasago, Kariya, and Nushima exhibited a clear seasonal fluctuation every year (Fig.
V-1-4). It was constantly high both in May-August and November-January, while it always

104remained low both in September-October and February-April.

Length frequency distribution of whitespotted conger from the small trawl fishery
A. Osaka Bay
Whitespotted conger caught in Osaka Bay by the small trawl fishery ranged from198-891 (mean: 319, N=3,702) mm TL and from 14-1,356 (50, N=2,934) g BW (Fig. V-1-5).
Almost all the fish measured 250 to 450 mm TL, but only some fish were over 500 mm TL.
In January 1994, there were two (small and large) modes of the fish size (Fig. V-1-5).
The large-sized fish were not recognized in other months, but the small-sized fish
predominated in the catch from February to September 1994 with growth. In October 1994,
however, length frequency distribution of the fish suddenly changed (Fig. V-1-5): the mode
shifted smaller from 360 mm TL in September to 270 mm TL in October. These, newly
recruited small-sized fish accounted for almost all catches from October to December 1994.
The mean percentage of whitespotted conger less than 300 mm TL was 47% fromJanuary to April and November to December in 1994 (Fig. V-1-6). It declined from April toJune and remained at a low level until September but, in October, jumped up dramatically tomore than 60%. Those fish were composed of 40% in the overall samples (N=4,036).

B. The northeastern Harima Nada Sea
Whitespotted conger caught in the northeastern Harima Nada Sea ranged from
224-473 (mean: 313, N=917) mm TL and from 15-176 (46, N=917) g BW (Fig. V-1-7).
There was no catch of the fish larger than 500 mm TL. Although the shift of length
frequency distribution of the fish was not as clear as in Osaka Bay, there was a shift in
September 1995, when catch of the small-sized fish increased (Fig. V-1-7). The 25, 50, and
75% artificially selected TLs were calculated to be 250, 262, and 274 mm TL, respectively,
using the estimated parameters (m=261.7, k=0.092), and almost all the fish examined werelarger than 250 mm TL because the small-sized fish were discarded at sea (Fig. V-1-8).

Discussion

Annual changes in total catch of whitespotted conger in the eastern Seto Inland Sea off
Hyogo Prefecture are well consistent with those in catch from the small trawl fishery105because the most of the fish are caught by this fishery (Fig. V-1-2). One of the reasons why
the catch is mostly by the small trawl fishery in this region is because the pot fishery to catch
whitespotted conger is strictly prohibited from December to May under Hyogo Prefectural
Fishery Adjustment Rules outside the common fishery right areas, and only a few fisheries
cooperative associations are permitted to operate the fishery. There was a clear seasonalvariation in monthly catch: high in May-August and November-January but low in
September-October and February-April (Fig. V-1-4). Although it is not easy to explain its
causes, one of the possible explanations is that water temperature is related to such
fluctuation because the feeding activity of the species declines over 23°C and under 14°C
(see Chapter IV-1).
This study has shown that only one or two modes were usually recognized in length
frequency distribution of whitespotted conger commercially caught in the eastern Seto
Inland Sea (Figs. V-1-5, V-1-7). Most of those fish measured 250 to 450 mm TL. In Osaka
Bay, the small-sized fish (<300 mm TL) newly appeared in October (Fig. V-1-5), and theirpercentage showed a sharp rise in October (Fig. V-1-6). Similarly in the Harima Nada Sea,
the mode of the fish size shifted smaller in September (Fig. V-1-7). As described in Chapters
II and IV, this study has shown that whitespotted conger occur as leptocephali from February
to May in the Harima Nada Sea, and then they settle down to the ground and can grow toabout 300 mm TL in October. These results strongly suggest that the fish being recognizable
in spring as leptocephali begin recruiting to the fishery at 250 mm TL in September-Octoberand become a target for the catch of the small trawl fishery. Those fish are also considered tobe caught continuously all year round and to account for most of the catch. Since annualchanges in fishing effort of the small trawl fishery did not coincide with those in the total
catch (Figs. V-1-2, V-1-3), it is most likely that fluctuations in the catch largely depends on
the amount of the newly recruited fish (i.e., that of the leptocephali migrating to the eastern
Seto Inland Sea).
In Osaka Bay, the large-sized fish were recognized in January but soon disappeared.
Further, a few and no fish over 500 mm TL were caught in Osaka Bay and in the Harima
Nada Sea, respectively, which suggests that the those fish inhabit the area that is not fishedby the small trawl fishery. As reported in Chapter IV-2-3, the large-sized fish (>500 mm TL)
seem to leave Osaka Bay for other regions in January and afterwards
To summarize, in the eastern Seto Inland Sea off Hyogo Prefecture, the average catch106

of whitespotted conger was 1,700 mt from 1984 to 1997, but the annual catch decreased to
562 mt in 1999. Subsequently, it slightly recovered and was rather stable from 2000 to 2008.
The monthly catch showed a seasonal fluctuation every year, being high both in May-Augustand November-January but low both in September-October and February-April.
Whitespotted conger were mostly caught by the small trawl fishery and measured 250 to 450
mm TL. Only a few large-sized fish (>500 mm TL) were caught. Both in Osaka Bay and theHarima Nada Sea, a size of 250 mm TL was obviously the lower limit for catch and landing.
The fish arrived in this region as leptocephali in spring and became a target of the small
trawl fishery at 250 mm TL in fall. After recruitment, they were caught continuously
throughout the year and accounted largely for the annual catch. The fluctuation of the annual
catch might be related to the amount of the leptocephali migrating to the region.

107

134ﾟ20'

134ﾟ40'135ﾟ135ﾟ20'Takasago
34ﾟ40'

Osaka Bay

Harima Nada Sea

KariyaKamaguchi
34ﾟ20'

Sea of Japan

Awaji Is.

Nushima

Pacific Ocean

10 km

Kii Channel

Fig. V-1-1. A study area (hatched) in the eastern Seto Inland Sea. The locations of the four
fisheries cooperative associations in Takasago, Kariya, Kamaguchi, and Nushima, where thecatch data of whitespotted conger were collected, are indicated by open circles. The fishingsurvey was carried out in the hatched area.

108

2,500Catch (mt)

2,000
1,500
1,000
500
0
100

Percentage

80
60
40
20
0
1984 1988

1992 1996 2000

2004 2008

Fig. V-1-2. Annual changes in catch [top: metric tons (mt); bottom: percentage] ofwhitespotted conger from various fisheries in the Seto Inland Sea off Hyogo Prefecture from1984 to 2008. Closed circles and bars, small trawl fishery; open circles and bars, longline
fishery; closed triangles and hatched bars, other fisheries; open squares, total catch.

109

Fishing effort (1,000 days)

300

250

200

150

100
1984

1988

1992

1996

2000

2004

2008

Fig. V-1-3. Annual changes in cumulative fishing effort of the small trawl fishery in the Seto
Inland Sea off Hyogo Prefecture from 1984 to 2006.

110

10

40

Catch (mt)

6
20
4
10

Catch (mt)

8

30

2

0

0
Apr. Aug.
1994

Dec

Apr. Aug.
1995

Dec

Apr. Aug.
1996

Dec

Apr. Aug.
1997

Dec

Fig. V-1-4. Monthly changes in catch of whitespotted conger at fisheries cooperative
associations in Takasago, Kariya, and Nushima. Open circles, catch in Takasago; closed
circles, catch in Kariya; open triangles, catch in Nushima; left axis for catch in Kariya; rightaxis for catches in Takasago and Nushima.

111

20
15

19 Jan. 1994, N=502

11 July, N=165

23 Feb., N=382

25 Aug., N=183

17 Mar., N=368

7 Sep., N=79

14 Apr., N=372

13 Oct., N=501

12 May, N=340

10 Nov., N=320

15 June, N=220

7 Dec., N=604

10
5
0
20
15
10
5
0
20
15

Frequency (%)

10
5
0
20
15
10
5
0
20
15
10
5
0
20
15
10
5
0
200

300

400

500

600

700

800 200

300

400

500

600

700

800

Total length class (mm)Fig. V-1-5. Monthly changes in length frequency distribution of whitespotted conger caught
in Osaka Bay by the small trawl fishery from January to December 1994. N, sample size.

112

Percentage of the fish (<300 mm TL)

100
80
60
40
20
0
Jan.

Mar.
Feb.

May

Apr.

July

June

Sep.

Aug.

Nov.

Oct.

Dec.

Fig. V-1-6. Monthly changes in percentage of whitespotted conger less than 300 mm TL
caught in Osaka Bay by the small trawl fishery from January to December 1994.

113

30
31 Oct., 1994 N=89

19 May, N=138

16 Dec., N=161

23 June, N=99

3. Feb., 1995 N=55

13 July, N=123

24 Feb., N=28

25 Aug., N=19

23. Mar., N=46

21 Sep., N=32

11 Apr., N=72

20 Oct., N=55

20

10

0
30

20

10

0
30

20

Frequency (%)

10

0
30

20

10

0
30

20

10

0
30

20

10

0
200

300

400

500

600

700

800 200

300

400

500

600

700

800

Total length class (mm)Fig. V-1-7. Monthly changes in length frequency distribution of whitespotted conger caught
in the northeastern Harima Nada Sea by a small trawl fishing boat from October 1994 to
October 1995. N, sample size.

114

Selectivity (%)

100
80
60
40
20
0
150

200

250

300

350

400

450

500

Total length class (mm)Fig. V-1-8. Artificial selectivity master curve of whitespotted conger caught in thenortheastern Harima Nada Sea. Open circles, eliminated data for the parameter estimation of
the logistic model.

115

Table V-1-1. Overall catch of the main species caught in the
Seto Inland Sea off Hyogo Prefecture in 2008
Common name (main species)

Catch (mt)Pacific sandlance

13,814

Japanese anchovy juveniles

8,807

Octopods

3,234Japanese jack mackerel and Decapterid fishes

1,637

Asian seaperches

1,233

Prawns and shrimps

1,170

Righteye flounders and Tonguefishes

1,131

Largehead hairtail959

Red seabream

868

Congrid fishes

582

Others

9,887

Total

41,872116

Table V-1-2. Catch of main species caught by the small trawl
fishery in the Seto Inland Sea off Hyogo Prefecture in 2008
Common name (main species)

Catch (mt)Octopods

2,159

Prawns and shrimps

1,154

Righteye flounders and Tonguefishes

915

Asian seaperches

821

Squids and cuttlefishes

755Japanese jack mackerel and Decapterid fishes

492

Red seabream

455

Congrid fishes

403

Others

3,995

Total

11,149117

2. Mesh selectivity of whitespotted conger in the small trawl fisheryIn the eastern Seto Inland Sea, a codend with a small mesh size is usually used in thesmall trawl fishery to catch whitespotted conger, and it has been considered that this species
is a mesh-defining species of the fishery in the waters (Tanda 2008). Enlargement of themesh size of the codend is one of the important management options to protect thecommercially unutilized juvenile fish. Nishikawa et al. (1994) reported a mesh selectivity ofwhitespotted conger in the small otter trawl fishery (called ita-biki) which commonly
operates in Osaka Bay off Hyogo Prefecture. However, information on the relationshipbetween the mesh size and total length of whitespotted conger escaping from the codend is
still limited.
In this section, a mesh selectivity of whitespotted conger caught in the Harima Nada
Sea is described, based on a fishing experiment using a small trawl fishing boat (a dredge
trawl, called chin-kogi) from 1994 to 1995. Possible use of the enlarged mesh size in thecodend as an option for future stock management of the species is also discussed.

Materials and Methods

Mesh selectivity survey at sea
A mesh selectivity survey was conducted during the fishing survey described in
Chapter V-1 at almost monthly intervals from October 1994 to October 1995 in thenortheastern Harima Nada Sea. The codend was covered by a cover net, whose mesh size
was smaller than that of the codend to sample the fish escaping through the codend. The
codend consisted of a knotless net with a 23 mm mesh aperture (nominal mesh size: 12
setsu) which is usually used in commercial fishing. The cover net was of a 17 mm mesh
aperture (nominal mesh size: 22 setsu) with threefold long of the codend to avoid a masking
effect (Tokai et al. 1990; Nishikawa et al. 1994). In this study, the net was hauled 6-9 times anight and usually towed for 20-30 min at a speed of 2-3 knots. The fish caught in the codend
were divided into fish for landing and fish for discarding. In addition to these 2 groups, thefish sampled in the cover net were transferred separately to the laboratory, where the 3groups were measured for total length (TL, mm) and body weight (BW, g).

118

Mesh and artificial selectivities
Mesh selectivity was defined by the 10-mm TL size class following Tokai (1993) andNishikawa et al. (1994): Mesh selectivity (%) = (Number of fish caught in thecodend)·(Number of fish caught in the codend and cover net)-1·100. Artificial selectivity was
also calculated in the same way as in Chapter V-1.
A mesh selectivity master curve (logistic model) was calculated by a least square
method following Chen et al. (1992): Sm = 1·[1+exp-k(L/p-m)]-1, where Sm is mesh selectivity,
L is TL class (mm), p is mesh size, k and m are constant numbers of logistic curve. The 25,
50, and 75% mesh selected TLs were deduced from the formulae as follows: L25 =p(km-ln3)k-1, L50 = pm, and L75 = p(km+ln3)k-1, respectively.

Survivability of the fish passing through the codend
Ten rearing experiments were conducted from October 1994 to October 1995 using
whitespotted conger caught in the cover net to determine their survivability after passing
through the codend. The fish were transported alive to the laboratory, where they were reared
for seven days by feeding frozen-stored sardine in a 0.5 kl round-shaped, black-colored FRP
tank. Three PVC pipes (75 mm in nominal diameter, 30-50 cm long) were disposed in thewater. A total of 10 kl sand-filtered seawater was supplied into the tank a day. All surviving
fish were measured for TL (mm) and BW (g) after each rearing experiment.

Results

Mesh and artificial selectivities
The small-sized fish (<250 mm TL) were caught in the codend with a 23 mm mesh
aperture from August to December, but these fish were mostly discarded. The landed fishmeasured more than 300 and 250 mm TL from May to September and October to next April,respectively (Fig. V-2-1).
The mesh selectivity increased from April to July but dropped in August (Fig. V-2-2).
While the fish caught in the codend were mostly over 250 mm TL from February to July(Fig. V-2-1), the artificial selectivity was nearly 100% (Fig. V-2-2), and almost all the fishcaught were not discarded. In association with the appearance of the small-sized fish (<250mm TL) from August to December, the artificial selectivity abruptly decreased in August and119ranged from 79 to 29% (Figs. V-2-1, V-2-2).
The parameters of the logistic curve were estimated as m=12.1 and k=0.99. The
calculated 25, 50, and 75% mesh selected TLs are shown in Table V-2-1. In the codend witha mesh aperture of 23 mm (12 setsu), 25% of whitespotted conger of 256 mm TL can be
captured. Also, 50 and 75% of the fish can remain at 282 and 308 mm TL, respectively, inthe cod end. Most of the fish over 300 mm TL can escape from the codend with a 29 mmmesh aperture (10 setsu).

Survivability of the fish passing through the codend
Whitespotted conger used for the rearing experiments were those that escaped from 23
mm apertures of the codend. They measured 178-377 (mean: 257, N=276) mm TL and 7-79
(22, N=276) g BW (Table V-2-2). Mean water temperature ranged from 8.5-27.8°C (Table
V-2-2), which was the same as the sea surface temperature of the fishing ground. No
mortality was observed during the experiments.

Discussion

If the mesh size of the usual codend can be enlarged as one of the options for the stock
management to protect small whitespotted conger, their length frequency distribution as wellas the mesh and artificial selectivities should be considered. Although the landed fish fromthe northeastern Harima Nada Sea are usually more than 250 mm TL (see Chapter V-1), thefish smaller than this size are caught in the codend from August to December (Fig. V-2-1),
when the artificial selectivity is low (Fig. V-2-2), and thus on-board fish selection for
discarding bothers the fishermen. In this study, the codend was made of a net with a 23 mmmesh aperture (12 setsu), and its 50% mesh selected TL was estimated 282 mm (Table
V-2-1). Based on the mesh selected TLs calculated, observed monthly changes in length
frequency distribution of the fish caught in the codend, and mesh and artificial selectivities(Figs. V-2-1, V-2-2, Table V-2-1), it may be possible to enlarge the codend mesh size from23 mm (12 setsu) to 26 mm (11 setsu) during May to September because the smallest size
for landing is more than 300 mm TL. Another issue is while the fish less than 300 mm TL
are landed from October to next April. In this case, we need discuss the use of a larger meshsize for the codend to protect such smaller fish (i.e., “Which is more profitable to protect the120

smaller fish or to catch them now?”). It seems also possible to enlarge the codend mesh size
from 23 mm (12 setsu) to 26 mm (11 setsu) because of the following reasons: 1) no mortality
was observed in the fish collected from the cover net during the rearing experiments (thisstudy; Tanda and Nishikawa 2001), which suggests that the escaped fish can survive; 2) thefish grow from 250 to at least 300 mm TL within the year (see Chapters IV-1, IV-2-1); 3)
among the three market sizes [small (<30 cm TL), medium (30-35 cm TL), and large (>35cm TL)], the wholesale price of the medium-sized fish is two times higher than that of thesmall-sized fish in the northeastern Harima Nada Sea (Anon. 1996); and 4) if the fishermen
release the small-sized fish (<300 mm TL), their catch sales are estimated to fall down to
93% of the same year’s catch sales but expected to increase to 111% in 3 years (Anon. 1996).
Therefore, it is considered that enlarging the codend mesh size will reduce sorting operationand discards but increase catch sales. Also, 300 mm TL is generally recommended as theminimum size limit for commercial whitespotted conger. Nonetheless, the fishermen neveragree to use a 29 mm (10 setsu) mesh size for the codend because such codend can allow
50% of individuals of 353 mm TL to escape (i.e., most of the commercial-sized fish will belost).
Even if the same mesh size is used for the codend, a square-shaped aperture can allow
larger fish to escape than a diamond-shaped aperture (Chen et al. 1992). In Osaka Bay, afishing experiment was conducted for the mesh selectivity of whitespotted conger by using a
diamond-shaped mesh for the codend (Nishikawa et al. 1994). In contrast, in this study
conducted in the northeastern Harima Nada Sea, I used a square-shaped mesh and its meshsize was 23.3 mm. A 50% mesh selected TL of this mesh size was estimated 310 mm fromthe mesh selectivity master curve presented by Nishikawa et al. (1994). This value is larger
than that (282 mm) in this study. It is thus likely that the mesh selectivity is affected by thedifference in fishing method and the amount and content of catch in the codend. Since it may
also fluctuate by the fishing location, the estimated parameters are probably not valid forother regions.
In conclusion, I conducted a fishing survey in the northeastern Harima Nada Sea to
examine the mesh and artificial selectivities using a usual codend with a 23 mm mesh
aperture. A 50% mesh selected TL was estimated as 282 mm. Based on the monthly changes
in length frequency distribution and the mesh and artificial selectivities, it appeared possible
to enlarge the codend mesh size from 23 mm (12 setsu) to 26 mm (11 setsu). Since the mesh121

selectivity was considered to be closely related to various fishing conditions (e.g., the fishingmethod and area), the parameters of the mesh selectivity obtained in this study might not bevalid for other regions. Therefore, the mesh selectivity of whitespotted conger needs to be
examined by using the fishing method in the commercial fishing ground.

122

30
31 Oct. 1994, N=303

19 May, N=138

16 Dec., N=316

23 June, N=101

3.Feb. 1995, N=58

13 July, N=124

24 Feb. N=29

25 Aug. N=24

23 Mar. N=48

21 Sep., N=61

11 Apr., N=80

20 Oct., N=81

20

10

0
30

20

10

0
30

20

Frequency (%)

10

0
30

20

10

0
30

20

10

0
30

20

10

0
150

200

250

300

350

400

450

500 150

200

250

300

350

400

450

500

Total length class (mm)Fig. V-2-1. Monthly changes in length frequency distribution of whitespotted conger caught
in the codend of a small trawl fishing boat in the northeastern Harima Nada Sea from
October 1994 to October 1995. Open bars, fish to be discarded; closed bars, fish to be landed.
N, sample size.

123

100
Artificial selectivity

Selectivity (%)

80

60

40
Mesh selectivity

20
Oct.

Dec.
Nov.

1994

Feb.
Jan.
1995

Apr.
Mar.

June
May

Aug.
July

Oct.
Sep.

Fig. V-2-2. Seasonal changes in mesh selectivity (open circles with dashed line) and artificial
selectivity (closed circles with solid line) of whitespotted conger caught in the northeastern
Harima Nada Sea from October 1994 to October 1995.

124

Table V-2-1. Calculated mesh selected size of whitespottedconger at various mesh sizes of the codends
23 ± 0.6 1

26 ± 0.3

29 ± 0.4

12 2

11

10

25 % selective TL

256

282

321

50 % selective TL

282

311

353

75 % selective TL

308

339

386

Mesh size (mm)Mean ± SD, N =100; 2 nominal mesh size, "setsu " or "fushi ";TL, total length (mm).
1

125

126

12 ± 3.3

30

20.4

Body weight (g)NMean WT (ºC)

12.3

30

22 ± 7.7

256 ± 28.1

8.5

28

24 ± 7.3

264 ± 27.7

10.5

30

23 ± 6.6

256 ± 25.8

23 Mar.

12.1

30

27 ± 7.7267 ± 27.6

11 Apr.

13 July

25 Aug.

21.0

29

32 ± 8.923.6

842 ± 18.2

27.8

29

17 ± 5.8294 ± 24.1 312 ± 32.7 245 ± 19.5

23 JuneAll the fish were survived throughout the experimental period. 1 Mean ± SD; N , number of fish examined.

215 ± 20.7

24 Feb.

16 Dec.

31 Oct.

1

1995

1994

Total length (mm)Date of catch

Table V-2-2. Results of 7-day rearing experiment of whitespotted conger caught in the cover net

24.3

30

18 ± 5.8248 ± 24.2

21 Sep.

22.0

32

20 ± 9.4

257 ± 32.1

20 Oct.

VI. Biological aspects of whitespotted conger in the Akashi Strait

The Akashi Strait is located between Honshu (the main island of Japan) and Awaji
Island in the eastern Seto Inland Sea (Fig. VI-1). The distance between both islands is only 4km, and the maximum water depth of the strait is about 120 m; the strait also has a strong
tidal current (Kaseno 1976). There are various fisheries using different types of fishing gear,such as small trawls, longlines, boat seines, gillnets, angling, etc., whereas the fishery using
pots is not commonly operated because of the strong current (Uchihashi 1976).
Although large-sized whitespotted conger are generally known to be commerciallycaught in the Akashi Strait, little information is available on the biology of the species there.
Because those large-sized fish are considered to be a spawning stock (see Chapter IV-2-3), it
is very important to study the ecology of whitespotted conger in the Akashi Strait in terms ofstock management and spawning stock conservation.
Since 1982, a monitoring survey using pots has been conducted in the Akashi Strait inorder to clarify annual changes in length frequency distribution and catch per unit effort(CPUE) of whitespotted conger. Although this survey must be operated during the neap tide,
it has the advantage of stable catchability, regardless of fishing grounds, and is considered tobe appropriate to conduct the stock assessment in the Akashi Strait.
In this chapter, biological aspects of whitespotted conger in the Akashi Strait arepresented. Seasonal changes in gonadosomatic index, relative condition factor, and age
composition are also discussed on the assumption that the large-sized females conduct aspawning migration in the eastern Seto Inland Sea including this strait.

Materials and Methods

Fish collection
The study area is located in the western Akashi Strait (Fig. VI-1). Water depth of thesurvey area ranges from 20-50 m. The bottom consisted of gravel and cobbles. In 1982 to2009, a fishing survey has been conducted monthly from 1982 to 1999 and 2-5 times a year
from 2000 to 2009, using commercially available pots (see Fig. III-3). Sampling was done
from evening to night. Deeply frozen sardines were thawed, cut in pieces, and put as bait in
the pots. Pots were usually put down to the sea before 1 hour of sunset and allowed the fish127

to trap for 4 hours. The effective number of pots in each fishing survey ranged from 60-110.
The CPUE was expressed as mean number of the fish caught for 4 dipping hours per pot.
Oceanographic data, such as water temperature (WT), salinity, and depth, wereobtained with STD (JFE Advantech Co., Ltd., Nishinomiya, Japan) before fishing.

Fish examination
The fish caught were transferred to the laboratory, where they were measured for total
length (TL, mm) and body weight (BW, g). In 1996 to 2009, they were sexed by theobservation of the gonad (see Table IV-2-2), and the gonad was weighed (GW, g). For age
determination, otoliths were removed and stored in small sealable plastic bags until analysis.
Burnt otoliths were observed by UV light technique (Katayama et al. 2002) and, before
observation, their serially-transverse sections were etched with 0.2N HCl for several tens ofseconds. As the spawning season is estimated to be from August through February (Tanaka
et al. 1987; Lee and Byun 1996; Mochioka 2001; Kurogi et al. 2002), age is expressedchronologically, assuming that the fish hatch every January. Gonadosomatic index (GSI) andrelative condition factor (CF) were calculated as follows: GSI (%) = GW·BW -1·100, CF =BW·TL-3·106, respectively.

Statistical analysis
Results were presented as mean and standard deviation (SD). Significant differenceswere determined by one-way analysis of variance (ANOVA), Tukey-Kramer method,
Pearson’s correlation coefficient test, or Mann-Whitney’s U test.

Results

Total length distribution
A total of 14,370 whitespotted conger were collected in this survey, 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, N=14,354) g. The males and females
measured 390-506 (446, N=17) and 314-950 (575, N=773) mm TL, respectively. Of the 829
fish examined for the sex, the vast majority (N=773, 93%) were females, but the remaining
17 and 39 fish were males and unsexed, respectively.

128

There were one or two major modes in length frequency distribution (Fig. VI-2). In
October, the fish less than 400 mm TL newly appeared. The proportion of these fish
gradually increased in the following months. Further, the percentage of the large-sized fish(>500 mm TL) was about 40% in January then increased gradually and reached 76% in
December (Fig. VI-3). The overall monthly mean TL (MTL) gradually increased during aperiod from February to December (Fig. VI-4). There was a significant difference in MTL
between four seasons (P<0.001, ANOVA): The MTL in the fall (October-December) was thehighest (P<0.001, Tukey-Kramer test) (Fig. VI-5).

CPUE
While the CPUE showed a considerably large annual and seasonal fluctuations, it
showed a decreasing trend from 1982 to 1999 but a gradual recovery after 2000 (Fig. VI-6).
It was high from May to August and November to January (the highest in June) but low from
September to October and February to April (the lowest in March) (Fig. VI-7). This monthly
change in mean CPUE well coincided with that of WT. It was also usually high when WT
ranged from 10-25°C but low when WT was very high (over 25°C) or very low (under
10°C) (Figs. VI-7, VI-8).
The annual CPUE of whitespotted conger caught by the pot survey was correlated withthe annual commercial catch of the species in the eastern Seto Inland Sea off Hyogo
Prefecture (Fig. VI-9). There was a positive, significant correlation between the annual
CPUE and catch from 1984 to 1999 (P=0.002, r=0.7), but no significant correlative
relationship was detected from 2000 to 2008 (P=0.27) (Fig. VI-10).

Gonadosomatic index
As stated above, the majority of the fish examined for sex were females. The overall
GSI ranged from 0.02-5.06 (mean: 1.59, N=245) in females and from 0.14-0.44 (0.25, N=6)
in males. Further, GSI ranged from 0.18-5.06 (mean: 1.65, N=233) in the large-sized females(>500 mm TL), while it ranged from 0.02-0.86 (0.46, N=12) in the small-sized ones. Most of
the GSI of the large-sized females were less than 4, and the mean monthly GSI were 1.5 inApril-December and 2.7 in January-February, respectively (Fig. VI-11). Despite the small
number (N=4) of the large-sized females examined in February, the GSI of this month wereclearly separated into two groups (high and low).

129

There was a significant difference in GSI of the large-sized females (>500 mm TL)
between four seasons (P<0.001, ANOVA). The GSI was the highest in January-February(P<0.001, Tukey-Kramer test), decreased markedly in April-June but increased gradually in
July-December (Fig. VI-12).

Condition factor
The overall CF ranged from 1.3-2.8 (mean: 1.8, N=772) in females and from 1.5-2.2(mean: 1.8, N=17) in males. In the large- and small-sized females, CF ranged from 1.3-2.8(1.9, N=599) and from 1.3-2.2 (1.7, N=173), respectively. The male CF ranged from 1.5-2.2(1.8, N=16) in the small-sized fish and was 1.6 in one large-sized specimen (506 mm TL).
Like GSI, there was a significant difference in CF of the large-sized females between
four seasons (P<0.001, ANOVA): it was the highest in January-February (P<0.001,
Tukey-Kramer test) but decreased in April-June. There was no significant difference in CFof the small-sized females between four seasons (Fig. VI-13).

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. The 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. VI-14). Since
there was a wide variation in TL for each age, the TL of the fish overlapped from age-1 to 4.

Discussion

Based on a long term fishing survey conducted from 1982 to 2009, this study has
shown for the first time biological information on whitespotted conger, especially large-sized
fish, caught in the Akashi Strait. Unlike the fish in Osaka Bay and the northeastern Harima
Nada Sea (see Chapter V-1), the fish caught in this study were usually over 450 mm TL, andmany of them measured more than 500 mm MTL between July and December (Figs. VI-2,
VI-4, VI-5). In addition, almost all the fish examined were females. These results clearly
indicate that whitespotted conger inhabiting the Akashi Strait are large-sized females. No
evidence of their spawning, however, was obtained during this survey.
After recruitment of the fish less than 400 mm TL in October, they grew and were130

replaced by the newly appearing small-sized fish in next October (Fig. VI-2). The large-sizedfish (>500 mm TL), on the other hand, were more abundant than small-sized fish from Julyto December. The percentage of the fish more than 500 mm TL dropped in January andthereafter increased gradually (Fig. VI-3). In addition, both the monthly GSI and CF of thelarge-sized females peaked in January-February, dropped in April-June, but then gradually
increased (Figs. VI-11, VI-12, VI-13). In January-February, most of the GSI in thelarge-sized females caught in the Akashi Strait remained less than 4 (Fig. VI-11), which wassignificantly lower (P=0.001, U test) than those of tag-released and recaptured females in
Osaka Bay and the Kii Channel (see Chapter IV-2-3). Moreover, the females caught in the
Akashi Strait were found to be less than age-5 (Fig. VI-14). These results can support the
new hypothesis proposed in Chapter IV-2-3 that large-sized female whitespotted conger(>500mm TL) move from the Akashi Strait to other regions for spawning during January toAugust, and other fish remaining in the strait continue to grow. Especially, the females withhigh GSI (>4) caught in January-February may be just about to begin their spawningmigration. Thus, the females more than 500 mm TL which acquired high GSI and CF byage-5 might launch spawning migration. Consequently, this strait is considered to be an
important region for the growth and maturation of the species. The migration of males,
however, is still yet unknown because of the small sample size in this study.
The monthly change in mean CPUE of whitespotted conger was closely related to thatin WT, and high CPUEs were observed when WT ranged from 10-25°C (Figs. VI-7, VI-8).
This is probably because the feeding of the species is dependent on water temperature (seeChapter IV-1).
In 1984 to 1999, the annual change in CPUE was significantly correlated with theannual commercial catch of the species in the eastern Seto Inland Sea off Hyogo Prefecture,
but the correlative relationship disappeared from 2000 to 2008 (Figs. VI-9, VI-10). This
non-correlative relationship during the latter period may be due to the reduction of thefrequency of the survey because it was conducted almost monthly intervals from 1984 to
1999, but it was done only 2-5 times a year from 2000 to 2008. Thus, it is desirable toconduct the monthly pot survey in order to get knowledge for the stock assessment ofwhitespotted conger in the eastern Seto Inland Sea off Hyogo Prefecture. It should be also
noted that we need to analyze the relationship between catch data from the survey andcommercial fisheries. More information on the age composition and other biological aspects

131

of the species in the Akashi Strait is necessary to clarify the trigger to launch its spawningmigration.
To summarize, length frequency distribution of whitespotted conger caught in the
Akashi Strait by the pot survey showed the presence of two size groups. The large-sized
females were dominant in this strait. The Akashi Strait is likely to be an important region for
the growth and maturation of whitespotted conger before they conduct the spawningmigration. The species may stay there up to four years. If monthly pot surveys could be
conducted, it may be possible to provide more reliable information on the abundance ofwhitespotted conger in the Akashi Strait.

132

135ﾟ
34ﾟ40'AkashiHarima Nada Sea

Sea of Japan

Akashi Strait

Osaka Bay

Awaji Is.

Pacific Ocean

5 km

Fig. VI-1. The study area (hatched) for the pot survey in the Akashi Strait, the eastern Seto
Inland Sea.

133

10
8

Jan., N=1,024

July, N=2,121

Feb., N=413

Aug., N=1,152

Mar., N=203

Sep., N=689

Apr., N=496

Oct., N=1,129

May, N=1,469

Nov., N=1,459

June, N=2,845

Dec., N=1,370

6
4
2
0
10
8
6
4
2
0
10
8
6

Frequency (%)

4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
10
8
6
4
2
0
200 300 400 500 600 700 800 900 1000200 300 400 500 600 700 800 900 1000

Total length class (mm)Fig. VI-2. Monthly changes in length frequency distribution of whitespotted conger caught
in the Akashi Strait through the pot survey. Monthly data taken from 1982 to 2009 are
combined. N, sample size.

134Percentage of the fish (>500 mm TL)

100
80
60
40
20
0
Jan.

Mar.
Feb.

May

Apr.

July

June

Sep.

Aug.

Nov.

Oct.

Dec.

Fig. VI-3. Monthly changes in proportion of large-sized whitespotted conger (>500 mm TL)
caught in the Akashi Strait through the pot survey. Monthly data taken from 1982 to 2009 are
combined.

135

800

Total length (mm)

700
600
500
400
300
200
100
0
Jan.

Mar.

Feb.

May

Apr.

July

June

Sep.

Aug.

Nov.

Oct.

Dec.

Fig. VI-4. Monthly changes in mean total length of whitespotted conger caught in the
Akashi Strait through the pot survey. Monthly data taken from 1982 to 2009 are combined.
Vertical bars represent standard deviation.

136

800

1,640

3,962

4,810

3,958

Total length (mm)

700
600
500
400
300
200
100
0
Jan.-Mar. Apr.-June July-Sep. Oct.-Dec.

Fig. VI-5. Seasonal changes in mean total length of whitespotted conger caught in the
Akashi Strait through the pot survey. Seasonal data taken from 1982 to 2009 are combined.
Vertical bars represent standard deviation. Sample size is given for each sampling season.

137

4

CPUE

3

2

1

0
1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

Fig. VI-6. Seasonal changes in CPUE (mean number of the fish caught in a pot per 4
dipping hours) through the pot survey in the Akashi Strait from June 1982 to November
2009.

138

30

2.0

25
20
15

1.0

WT

Mean CPUE

1.5

10
0.5
5
0

0.0
Jan.

Mar.

Feb.

May

Apr.

July

June

Sep.

Aug.

Nov.

Oct.

Dec.

Fig. VI-7. Monthly changes in mean CPUE and bottom water temperature during the pot
survey in the Akashi Strait. Monthly data taken from 1982 to 2009 are combined. Closed
circles, CPUE; open circles, bottom water temperature (WT, °C).

139

4

CPUE

3

2

1

0
5

10

15

20

25

30

WT

Fig. VI-8. Relationship between bottom water temperature (WT, °C) and CPUE during the
pot survey in the Akashi Strait from 1982 to 2009.

140

2,500

2.0

2,000

1.5

1,500

1.0

1,000

0.5

500

Catch (mt)

CPUE

2.5

0
0.0
1984 1987 1990 1993 1996 1999 2002 2005 2008

Fig. VI-9. Annual changes in CPUE and commercial catch of whitespotted conger in the
Akashi Strait and the eastern Seto Inland Sea off Hyogo Prefecture, respectively, from 1984
to 2008. CPUE data were collected through the pot survey in the Akashi Strait. Closed
circles, CPUE; open circles, catch (metric tons, mt).

141

3000

Catch (mt)

2500
2000
1500
1000
500
0
0.0

0.5

1.0

1.5

2.0

2.5

CPUE

Fig. VI-10. Relationship between annual CPUE and catch of whitespotted conger in the
Akashi Strait and the eastern Seto Inland Sea off Hyogo Prefecture, respectively, from 1984
to 2008. CPUE data were collected through the pot survey in the Akashi Strait. Closed
circles, data from 1984 to 1999; open circles, data from 2000 to 2008.

142

Gonadosomatic index (%)

10
8
6
4
2
0
Jan.

Sep. Nov.
July
Mar. May
Aug. Oct. Dec.
Feb.
Apr. June

Fig. VI-11. Monthly changes in gonadosomatic index of female whitespotted conger caught
in the Akashi Strait through the pot survey. Monthly data taken from 1996 to 2009 are
combined. Closed circles, actual values from large-sized fish (>500 mm TL, N=233); open
circles, mean values from large-sized fish; open triangles, actual values from small-sized fish
(N=12).

143

Gonadosomatic index (%)

4

31

32

27

143

3

2

1

0
Jan.-Feb. Apr.-June July-Sep. Oct.-Dec.

Fig. VI-12. Seasonal changes in mean gonadosomatic index of large-sized female
whitespotted conger (>500 mm TL) caught in the Akashi Strait through the pot survey.
Seasonal data taken from 1996 to 2009 are combined. Vertical bars indicate standard
deviation. Sample size is given for each sampling season. No data in March are available.

144

2.5

Condition factor

65

2.0

24

335

122

77

66

54

29

1.5

1.0
Jan.-Feb. Apr.-June July-Sep. Oct.-Dec.

Fig.VI-13. Seasonal changes in mean condition factor of female whitespotted conger caught
in the Akashi Strait through the pot survey. Shaded bars, large-sized fish (>500 mm TL);
open bars, small-sized fish. Seasonal data taken from 1996 to 2009 are combined. Verticalbars indicate standard deviation. Sample size is given for each sampling season. No data in
March are available.

145

Total length (mm)

1,000
800
600
400
200
0
0

1

2

3

4

5

Age

Fig. VI-14. Total length of female whitespotted conger of various ages caught in the Akashi
Strait through the pot survey from 1996 to 2009.

146

VII. General discussion

This study provides various aspects of the biology of whitespotted conger during partof their life history (from migration to the Harima Nada Sea to probable spawning migration
to a yet unknown spawning ground). The species is one of the commercially importantmarine fishes in the eastern Seto Inland Sea and caught mainly by the small trawl fishery.
Nonetheless, when the research on the species started in 1993, the recognition of its earlystage (leptocephali and newly settled juveniles) by fishermen was very poor: some believedthat the morid Laemonema nana is a juvenile of whitespotted conger. The continuous
research, however, has led the fishermen to a better understanding for the biology of thespecies. Currently, they know that the leptocephali migrating to the eastern Seto Inland Sea
in every early spring are whitespotted conger. This study also provides the fundamental
knowledge on the fishery stock management of whitespotted conger in the eastern Seto
Inland Sea.
In this final chapter, I briefly review the life history of the species in the eastern Seto
Inland Sea and, based on the information I obtained in this study, suggest the way we shouldtake for its fishery stock management.

Overview of the life history of whitespotted conger in the eastern Seto Inland Sea
Whitespotted conger leptocephali commonly emerge in the eastern Kii Channel in
November (Utsumi 2009) and, with metamorphosing, occur in the northeastern Harima
Nada Sea from February to May when sea surface temperature ranges from 8-18°C. After
finishing metamorphosis, they begin the demersal life at 10-20 m in depth on the sand-mud
and gravel bottoms. They initially feed on crustaceans and then shift their prey items topolychaetes and teleosts. Whitespotted conger grow and begin recruiting to the small trawl
fishery at 250 mm in total length (TL) in every fall, some fish reach 400 mm TL by
December. After recruitment, they are continuously caught by the fishery until next fall andaccount largely for the annual catch. The species exhibits various growth rates: some fish
may reach over 60 cm TL within one and a half year. With growth, they tend to move deeper
areas and assemble in such waters as the Akashi Strait. The fish show high diversity ingrowth not only between individuals but also sexes. Age-0 to age-5 fish are found, and age-0
and age-1 fish are mainly caught in commercial fisheries in the eastern Seto Inland Sea.

147

Female whitespotted conger spend up to five years in the eastern Seto Inland Sea andbegin the spawning migration by age-5 from January through August to a yet unknown
spawning ground although the migration of males has not been clarified. The species islikely to have different migratory behavior between sexes.

Fishery stock management approach
Whitespotted conger leptocephali are found as bycatch of the boat seine fisherytargeting Pacific sandlance juveniles in the eastern Seto Inland Sea. Because of the filter
system attached to the fishing net, a large number of the leptocephali are caught as bycatch.
Although the impact of bycatch on the stock abundance of whitespotted conger is not clear
in the region, we should try to minimize such bycatch because Ishida et al. (2003) reported
on a negative impact of bycatch on the stock of the species in the Tohoku region.
Minimizing the bycatch in the boat seine fishery may also reduce unintentional bycatches ofother valuable marine animals as well.
After metamorphosis, whitespotted conger juveniles settle down to the ground andbegin their demersal life. The juveniles show a strong preference for crustaceans as fooditems. Although crustaceans are more abundant on the gravel bottom than sand-mud bottom,
they prefer the sand-mud bottom at depths of 10-20 m. Thus, it is important to conserve thesand-mud bottom as a nursery ground for the juveniles. The gravel bottom where
crustaceans predominate may be significant as a food supplying ground.
Based on my analysis of the age, growth, and fishing status of whitespotted conger inthe eastern Seto Inland Sea, the species begins recruiting to the small trawl fishery at 250
mm TL from September to October. These fish are continuously caught for a year, whichindicates that age-0 and age-1 fish account largely for the annual catch of the species. The
recruited fish are those that migrate to the region as leptocephali from February to May.
These results suggest that the annual catch of the species there depends on the annual
abundance of the migrating leptocephali. It is, however, impossible to control the abundanceof migrating leptocephali. In addition, although the spawning migration from the region toother areas is suggested in this study, there has been so far no evidence that the large-sizedfish occurring in the eastern Seto Inland Sea contain fully mature fish. Therefore, it is quitedifficult at present to get agreement of the fishermen for the reduction of the catch of thoselarge-sized fish as a stock management option. Thus, for the possible stock management of148

the species, it is the best to conserve the small-sized fish until a certain size and to prevent
and/or reduce non-commercial catches because the wholesale price of the medium-sized fish
is two times higher than that of the small-sized fish (Anon. 1996). For the latter purpose, mystudy on the mesh selectivity (Chapter V-2) can provide useful information. As discussed
above, minimizing unintentional bycatches may be also effective.
For future stock management of whitespotted conger in the eastern Seto Inland Sea,
more information is necessary based on the following research: 1) a monitoring survey todetermine the annual abundance of the migrating leptocephali should be addressed. If suchdata are available, it will be quite useful to forecast the annual catch and stock of the species
and also to discuss the factors on the catch decline from the view point of growth overfishingor recruitment overfishing. If the annual abundance of the migrating leptocephali has beenstable, the causes of the catch decline of whitespotted conger may exist after their migration
into the eastern Seto Inland Sea. Alternatively, if the annual abundance of the migrating
leptocephali has been fluctuated, a spawning stock may decrease in a yet unknown spawning
ground, and/or there may be difficulty during their migration to Japanese coastal waters. It isalso helpful to assess the negative impact of bycatches on the stock; 2) an investigation
should be conducted for the sex ratio of the species by age (fish size) and region. In thisstudy, the male- and female-biased sex ratios were observed in the small- and large-sized fish,
respectively. No adequate explanation is possible, but the biased sex ratio may be caused bydifferences in growth, habitat, and movement between sexes. The females may leave forsomewhere out of the survey area (e.g., deeper area) before recruitment to fisheries, and themales may conduct migration to a yet unknown spawning ground earlier than females; 3) a
study on the reproductive biology and farming of the species is needed. Although theovulation can be induced by a hormonal treatment (Horie et al. 2001), and the spermiation
was observed in captivity (Utoh et al. 2004), our knowledge of the reproductive biology isquite limited in terms of fish culture. If we can artificially produce numerous whitespottedconger juveniles and release them to the natural waters, stock enhancement techniques
would be a great tool for the fishery stock management of the species in Japan; and 4) asurvey on the spawning ground of whitespotted conger should be conducted. This may be
the most important research issue for the species in Japanese waters. In this study, I havesuggested that whitespotted conger conduct the spawning migration by age-5 from thenortheastern Harima Nada Sea to the Kii Channel during a period from January to August.

149

Also, based on various surveys by other Japanese scientists, much information has been
currently accumulated on the biology of the species, but nothing is known about the
spawning ground and factors to trigger the launch of spawning migration. Such information
is essential to fully understand the biology of the species and to conduct an efficient fisherystock management in the future.

150

Acknowledgments

My sincere appreciation goes to Prof. Kazuya Nagasawa, Laboratory of Aquaculture,
Graduate School of Biosphere Science, Hiroshima University, for his constructive comments
and valuable suggestions during my study. Without his supervision, this doctoral thesis
would not have been completed.
I am deeply grateful to Prof. Hiroaki Hashimoto, Prof. Kazumasa Uematsu, andAssociate Prof. Tetsuya Umino, Graduate School of Biosphere Science, Hiroshima
University, for their comments on the draft of the thesis.
I am also indebted to Dr. Minoru Tanda, the director of the Hyogo Prefectural
Fisheries Technology Institute (HPFTI), for his encouragement throughout the study. Thanks
also go to Mr. Haruyoshi Nakasuji, the captain of the research vessel Chidori, and the crew
for their help during the survey. I am grateful to Dr. Satoshi Katayama, National ResearchInstitute of Fisheries Science, for his assistance with age determination and usefulsuggestions during my study. I wish to thank Dr. Tetsuya Tamaki and Mr. Tetsuya Ohtani,
HPFTI, for their assistance. Thanks also due to Mr. Keisuke Tanida, HPFTI, for his friendly
support in fish sampling. I extend special thanks to the members of “Research seminar on
the fishery biology of congrid fishes” for their informative topics and useful suggestions
during seminars. I want to thank all the people who were involved in my research.
I would like to express my gratitude to my wife, Nobuko, and my son, Yuki, for theirmoral support and warm encouragement.
Finally, I dedicate this thesis to my parents, Katsuo and Shigeko Gorie. With all of
their love for me, they not only have brought me up but also have provided me with power,
support, and motivation throughout my study.

151

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