1. Introduction

The coastal areas have been well known to support a greater diversity and abundance of fish, and larger numbers of juveniles on a world-wide scale. Many fish species use coastal areas as feeding and nursery grounds, including many economically important fishes(Jenkins and Wheatley, 1998; Lazzari et al., 1999; Guidetti, 2000; Paperno et al.,2001; Kwak and Klumpp, 2004; Kwak and Huh, 2007). Especially higher diversities of habitat for fishes and shellfishes have been in the southern sea, and fisheries production have been developed until now. 

Gwangyang Bay located on the middle part of the southern sea of Korean peninsula which was characterized by semi-enclosed estuarine embayment, being encompassed by Yeosu and Namhae Island. The major source of freshwater input into the bay is Seomjin River at thenorthern end of the bay, and a few streams including Sueocheon and Dongcheon provide additional freshwater and nutrients input to the bay(Huh and Kwak, 1997a; Huh et al., 1998). A variety of coastal fisheries and farming have been prevalence in the most part of Gwangyang Bay, however, environmental disturbances have predicted every year due to the construction of Gwangyang Steel Mill Company(POSCO) in 1980’s, Hadong Power Plant and industrial complex since 1993. To date the studies of fish assemblage in Gwangyang Bay have been confined to describe species composition and seasonal variation at particular periods around Dae Island(Choo, 1997; Huh et al., 1998), western part of Gwangyang Bay(Cha and Park, 1997;  Oh, 2003), and eelgrass bed(Huh and Kwak, 1997a). Far less is known about the studies on variation of fish assemblages with long-term periods despite of higher occurrence of human impacts in Gwangyang Bay. 

The objective of this study was to examine the seasonal variation in species composition and abundance of fishes inhabiting around Dae Island in Gwangyang Bay, Korea and to compare fish assemblages between our study(2009∼2010) and previous studies(1991∼1992 and 1995∼1996) in order to determine the change in fish assemblage with environmental disturbance. It may be useful as a good information for ensure fish resources sustainability in the coastal areas. 

2. Materials and Methods

The coastal water off Dae Island in Gwangyang Bay has been highly productive and characterized by fisheries and farming area nearby eelgrass bed, intertidal flat, and Hadong Power Plant(Fig. 1). Fish samples were collected monthly by 5-m trawl(1.9-cm mesh wing and body). Four 30-min tows in each sampling time were carried out from March 2009 to February 2010. Specimens were preserved immediately in 10% formalin after capture and later transferred to 70% isopropanol. These samples were identified according to Masuda et al.(1984), Yoon(2002), and Nakabo et al.(1993), and weighed to the nearest gram in wet weight. Specimens were measured to the nearest mm(Total length TL). Surface water temperature(by thermometer) and salinity(by salinometer) were monitored monthly on each sampling occasion. 

Fig. 1. Location of the study area(the black area).

The fish data was analysed to obtain the following community variables. Diversity H’(Shannon and Weaver, 1949) was calculated as: 

 where n is the number of individuals of each i species in a sample and N is the total number of individuals. Association of fish species, Pianka’s similarity index (Pianka, 1973), A_ij  was calculated as:

where A_ij is the similarity of species j on species i; p_ih is the proportion of individuals of a species i in a particular month h; p_ih is the proportion of individuals of a species j in a particular month h. Values for the similarity index may vary between 0, if no similarity occurs, and 1 for complete similarity. The Pianka’s similarity index was subjected to an average linkage cluster analysis. 

 For comparison of fish assemblages between our study (2009∼2010) and previous studies(1990∼1991(Huh et al.,1998) and 1995∼1996(Choo, 1997)) at our study sites, the total number of individuals of fish species was described as percentage. Abundances of fishes were transformed with per unit area(m²) at each sampling periods. 

3. Results

3.1 Temperature and salinity

Temperature at the study site ranged from 9.6℃ to 24.3 ℃ and varied significantly with months. The peak of temperature was around August and September 2009, a decline in October 2009 and a minimum during winter (January 2010)(Fig. 2). Salinity ranged from 22.3 psu to 33.7 psu and did not vary significantly between months with displayed a similar pattern except in July 2009 when it dropped(about 22.3 psu)(Fig. 2). 

Fig. 2. Monthly variations of temperature and salinity in the coastal waters off Dae Island in Gwangyang Bay.

3.2 Species composition

A total of 8,057 fish belonging to 74 species in 38 families were collected around Dae Island in Gwangyang Bay(Table 1). Numerically dominant fish were Leiognathus nuchalis(62.9%), Argyrosomus argentatus(7.2%), Acentrogobius pellidebilis(5.0%), Sillago japonica(3.3%), Apogon lineatus (3.3%), Thryssa hamiltoni(2.4%), Repomucenus richardsonii (1.5%), Pholis nebulosa(1.5%), Pleuronichthys cornutus(1.3 %), Hexagrammos otakii(1.1%) and Pleuronectes yokohamae (1.1%), together accounting for 89.5% of the catch. The numerically dominant fish species made up 58.9% of  biomass. For biomass, L. nuchalis(27.3%), Okamejei kenojei(14.2%), Liparis tanakai(7.6%), H. otakii(7.5%), P.nebulosa(5.8%) and A. argentatus(5.4%) were higher than those of other fish species. 

Table 1. Total number of individuals and biomass of fish species collected using a beam trawl in the coastal waters off Dae Island in Gwangyang Bay from March 2009 to February 2010

The size distributions of principal fishes were differed (Fig. 3). The size range of L. nuchalis was 3.3∼11.3 cmTL with peak numbers in 5.0∼7.9 cmTL. The number of A. argentatus was higher in 3.0∼4.9 cmTL which were ranged from 1.3 to 24.2 cmTL. The A. pellidebilis occurred in size ranged from 3.1∼10.2 cmTL, however, S. japonicus in 3.3∼19.4 cmTL with high numbers in 6.0∼7.9 cmTL. The peak numbers of A. lineatus was from 5.0 to 6.9 cmTL, and those of T. hamiltoni was about 10 cmTL.

Fig. 3. Size–distribution of principal fishes collected using a beam trawl in the coastal waters off Dae Island in Gwangyang Bay from March 2009 to February 2010.

3.3 Seasonal variation in abundance of fish

 The number of fish species(9∼30 species) varied with months. Fish species was abundant in April 2009 and October 2009(Fig. 4(a)). Number of individuals remarkably varied with months(Fig. 4(b)). Fish numbers were most abundant May 2009 when L. nuchalis and A. argentatus which were ranged in 3.2∼8.6 cmTL and 6.4∼15.3 cmTL were dominant. Fish numbers was the lowest in February 2010. The fish biomass differed substantially between different months(Fig. 4(c)). Highest biomass was also in May 2009 when many large common fish species such as L. nuchalis, A. argentatus, and S. japonicus were present. The range of diversity index(H’) was 0.71∼2.31, and higher value was in October 2009 and February 2010(Fig. 4(d)).

Fig. 4. Monthly variations in number of species (a), number of individual (b), biomass (c), and species diversity index (d) of fish collected using a beam trawl in the coastal waters off Dae Island in Gwangyang Bay from March 2009 to February 2010.

The dendrogram shows six clusters which identify the fish species, and one fish species(Fig. 5). The first group was composed of P. nebulosa, H. otakii, L. yokohamae, Pholis fangi, Amblychaeturichthys hexanema, A. lineatus, P. cornutus, A. pellidebilis, R, richardsonii, A. argentatus and T. hamiltoni with occurring predominantly over study periods. The second group contains Repomucenus lunatus, Cryptocentrus fillifer, Cynoglossus joyneri, C. robustus, Upeneus japonicus and Platycephalus indicus. This group was high numbers during summer, especially August 2009 and September 2009, while numbers of them were few in other periods. The third group was composed of Conger myriaster, Takifugu niphobles, Repomucenus valencienneii, R. kenojei and Repomucenus sp. which were peak numbers from late summer to fall. The fourth group was consisted of S. japonicus, Acanthopagrus schlegeli, Engraulis japonicus, Acanthogobius flavimanus, Parapercis sexfasciatus with occurrence from late fall to winter. The fifth group was composed of Paralichthys olivaceus, Hemitripterus villosus, Tridentiger trigonocephalus, L.tanakai, and Zoarces gilli with occurrence from winter to March 2010, however, the sixth group was Johnius belengerii, Muraenesox cinereus and Trachurus japonocus which were only present March, April and May 2009. The last one species was L. nuchalis with higher numbers at each sampling occasion during study periods.

Fig. 5. Dendrogram illustrating the species associations of fishes collected using a beam trawl in the coastal waters off Dae Island in Gwangyang Bay from March 2009 to February 2010.

3.4 Comparison between our study(2009∼2010) and previous studies(1990∼1991 and 1995∼1996)

The percentage of total number of individuals was differed with each sampling periods(Table 2). For example, the percentage numbers of L. nuchalis increased significantly(62.29% in 2009∼2010, 27.16% in 1995∼1996 and 26.08% in 1990∼1991) with sampling periods. The A. argentatus (7.13%), R. richardsonii(1.45%), P. cornutus(1.27%), R.lunatus(0.90%) were also increased, and A. lineatus(3.22%) and T. hamiltoni(2.39%) almost occurred only in 2009∼2010. 

Table 2. Comparison in percentage numbers of fish species between our study and previous studies in the coastal waters off Dae Island in Gwangyang Bay

On the other hand, those of L. tanakai and J. benegerii were significant decreased(0.54% and 0.34% in 2009∼2010, 26.59% and 3.37% in 1995∼1996, and 15.49% and 11.56% in 1990∼1991). The A. pellidebilis(4.94%), P. nebulosa (1.44%), L. yokohamae(1.06%), P. fangi(0.66%), R. valenciennei (0.22%), T. Kammalensis(0.16%) and C. myriaster(0.11%) were also decreased gradually in 2009∼2010.

For total number of individuals and biomass per m²  (Table 3), the number of individuals and biomass were higher(0.068 ± 0.061, 0.733 ± 0.359) in 2009∼2010 than those of in 1995∼1996(0.027 ± 0.019, 0.240 ± 0.165) and 1990∼1991(0.060 ± 0.052, 0.729 ± 0.531). 

Table 3. Comparison in average and standard deviation of total number of individuals and biomass per m2 of fish species between our study and previous studies in the coastal waters off Dae Island in Gwangyang Bay

4. Discussion

A total of 74 fish species was recorded from the coastal waters off Dae Island in Gwangyang Bay and L. nuchalis, A. argentatus, A. pellidebilis, S. japonica, A. lineatus, T.hamiltoni, R. richardsoni, P. nebulosa, P. cornutus, H.otakii and L. yokohamae were numerically dominant. Most of fish species have been known as coastal fish species around Korea, and some of them are of commercial and recreational importance. For example, H. otakii, P. cornutus and L. japonicus are valued as live fish in the southern sea, Korea, and P. nebulosa, A. argentatus and T. hamiltoni harvested as a food fish(Yoon, 2002; Kim and Kang, 1993). Broad-scale surveys of fish communities from other regions nearby our study sites suggest a similar community structure. L. nuchalis was also dominated the fish community in the western part of Gwangyang Bay(Cha and Park, 1997; Oh, 2003), A. pellidebilis, S. japonica, H. otakii and L.yokohamae in an eelgrass bed of Gwangyang bay and coastal waters off Namhae Island with genera Acentrogobius, Thryssa and Repomucenus groups(Huh and Kwak, 1997a; Kwak et al., 2008). Fish collected from the coastal waters off Dae Island in Gwangyang Bay appeared to be dominated by small fishes and juveniles of most species. This indicated that the coastal waters off Dae Island in Gwangyang Bay function as nursery areas. Such conclusions are in general agreement with previous studies of fish assemblage at our study sites(Choo, 1997; Huh et al., 1998; Huh and Kwak, 1998a). For example, the size distribution of common fish species were similar to those of fish species nearby our study area. 

The predomination of L. nucahlis exhibited distinct occurrence pattern except March 2009, January 2010 and February 2010 at our study sites. The range of percent numbers of L. nuchalis were from July 2009(56.1%) to May 2009(86.9%) during study periods, and those of A.pellidebilis and L. tanakai were higher in March 2009, and H. otakii and P. nebulosa in January and February 2010. These results have shown different patterns with other studies of fish assemblage. For example, dominant fish species occurred their own distinct seasonal patterns and different time of peak abundances(Choo, 1997; Cha and Park, 1997; Huh et al., 1998; Huh and Kwak, 1998a; Huh and Kwak, 1998b; Oh, 2003; Kwak et al., 2008). Thus fish assemblage of our study area might be influenced by environmental characteristics. 

Seasonal variation in both species composition and abundance of fish species appear to be considerable for fish assemblages. The number of individuals and biomass of  fish species in the study area were higher in May 2009 when higher temperature was in temperate area, whereas those of were the lowest value in January and February 2010. Compared with previous studies of fish assemblage at our study sites, higher numbers and biomass of fish species were in spring but those of were lower in winter regardless of sampling periods(Choo, 1997; Huh et al., 1998). The higher number of eelgrass fishes was also corresponded with higher temperature in an eelgrass bed around Dae Island(Huh and Kwak, 1997a). Several other studies have demonstrated a positive correlation between temperature and fish abundances(Huh and Kwak, 1998a; Huh and Kwak, 1998b; Lazzari et al., 1999; Paperno et al., 2001; Huh and An, 2002; An and Huh, 2002; An and Huh, 2003; Oh, 2003; Hwang, 2007; Kwak et al., 2006; Kwak et al., 2008; Kwak and Huh, 2007). Prey availability may also be an important factor influencing fish abundance at our study sites. Most of dominant species(e. g., L. nuchalis, S. japonica, H. otakii , and L. yokohamae ) fed mainly on small-sized zooplankton and crustaceans such as amphipods and isopods in Gwangyang Bay(Huh and Kwak, 1997b; Kwak et al., 2004; Kwak and Huh, 2003; Kwak et al., 2005). Seasonal variation in abundance of these prey animals coincided with those of dominant fishes during the study period(Kwak et al., unpublished; Soh and Suh, 1993). Other  studies of fish assemblage have reported similar correlated seasonal variation in abundance of small sized animals such as crustacean and fish occurrence in the southern sea(Huh and Kwak 1997a; Kwak et al., 2008). 

The fish assemblages have been seemed remarkable to change with long term periods at our study sites. The abundances of L. nuchalis was predominately occurred with more half of total numbers, whereas those of L. tanakai were decreased sharply in 2009∼2010. Especially numbers of L. nuchalis were increased in the southern sea, Korea. For example, 1,564 individuals were around Shinsu Island in 1986∼1987(Kim and Kang, 1991), 2,235 individuals in Nakdong river estuary in 1987∼1988(Huh and Chung, 1999), 916 individuals around Namhae Island, and 39,740 individuals in the western part of Gwangyang Bay 1998∼2002(Huh and Kwak, 1998a; Huh and Kwak, 1998b; Oh, 2003). A significantly greater abundance of L. nuchalis were in the southern sea including our study sites confirmed that this species were likely to be dependent on natural environmental changes such as temperature, sediment characteristics, current and food availability etc.(Huh and Kwak, 1998a; Oh, 2003). It was assumed that human impacts were also one of environmental factors for variation of L.nuchalis numbers. Diana(2004) have demonstrated that higher dominance of one or two fish species have been resulted in the lower diversity of fish assemblages at unstable condition in marine ecosystems. 

On the other hand, abundances of L. tanakai varied with spawning and recruitments periods which were due to temperature and prey availability. Higher number of juvenile L. tanakai were due to largely recruitment after spawned in the coastal area nearby our study sites from March to May in 1990∼1991(Choo, 1997) and 1995∼1996(Huh et al., 1998), while those of L. tanakai were few in 2009∼2010. L.tanakai have been spawned large numbers of egg on the blade of eelgrass with attachments, and higher numbers occurred from February to May in an eelgrass beds around Dae Island as small sized juveniles(Chyung, 1977; Huh and Kwak, 1997a). In Gwangyang Bay, large intertidal flat including delta and eelgrass beds became to reduce gradually with human impact such as the construction of Gwangyang Steel Mill Company(POSCO), Hadong Power Plant and industrial complex since 1993(Yoon, 2006). These results indicated lower numbers of L. tanakai may be corresponded with lower eelgrass biomass nearby our study sites with periods. Other studies were similar patterns which change in species composition and lower diversity have been occurred with higher human development in the coastal area and beach(John, 1997; Diana, 2004). Hence we suggested that changes in species composition and abundances of fish species were influenced by natural environmental factors such as temperature and prey availability, and human impacts indirectly. 

Acknowledgements

We are grateful to Bong Jung, Sung and Jung Yun, Kim of Department of Oceanography, Pukyong University for assistance with sampling and data analysis. We thank Dr.David W Klumpp(AIMS) for his constructive comments in structure of English, and some anonymous referees for their constructive criticism. 

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