Seagrass beds have long been recognized as important habitats for various benthic invertebrates due to high primary productivity and organic materials they support (Thayer et al., 1975). These habitats also provide feeding grounds, shelter or nursery ground for early stage of fishes (Klumpp et al., 1989; Huh and Kwak, 1997c; Jenkins and Wheatley, 1998; Guidetti, 2000; Hemminga and Duarte, 2000). In many shallow seagrass habitats, tidal and diel effects are important factors that control fish assemblages (Sogard et al., 1989), and these effects may have one of the greatest influences on the pattern of biological inter-habitat connectivity (Sheaves, 2005). Several studies have reported diel and tidal variations in seagrass fish assemblage in the Wadden Sea (Polte and Asmus, 2006) and Wakatobi Marine National Park of Indonesia (Unsworth et al., 2007). The number of individuals and biomass of fishes have shown significant variation with tidal and diel cycles in the intertidal seagrass beds worldwide which can be facilitated by small-scale movement of fishes (Guidetti and Bussotti, 2000; Nagelkerken et al., 2002; Kopp et al., 2007). Morrison et al. (2002) reported that small-scale movement patterns over short temporal and spatial scales in shallow intertidal zone may be directly related to feeding dynamics, habitat usage and avoidance of potential predators. Kopp et al. (2007) also showed that diel assemblage shift in the seagrass beds was due to nocturnal trophic incursions of fishes into seagrass habitats from nearby coral reef. Such movements may be strongly structured by tidal and day/night cycles. Thus, such studies are essential for their future management because they elucidate the ecological mechanisms pertinent to their effective management (Parrish, 1989; Sheaves, 2005; Larkum et al., 2006).

Zostera marina (eelgrass) is the most common seagrass species in temperate waters and forms seagrass beds in shallow coastal areas that increases habitat complexity and provides living space for various marine animals (Klumpp et al., 1989; Hemminga and Duarte, 2000; Nagelkerken et al., 2002). Several studies have been made on seagrass fish assemblage in southern Korean waters (e.g., Lee et al., 2000; Baeck et al., 2005). In the study area, there have been some studies on seasonal variation in abundance of fish assemblage (Im, 2004), comparison of fish assemblages between eelgrass bed and unvegetated habitat (Kwak et al., 2006), and feeding habits and partitioning food resource of abundant fish species (Kwak and Huh, 2004; Kwak et al., 2003, 2004, 2005). However, little is known about short term variations in fish assemblage and factors influencing such variations in intertidal seagrass habitats of Korea.

The present study focused on changes in species composition and abundance of fish assemblage with respect to day/night, spring/neap and ebb/flood tidal cycles to determine how tidal phase and diel rhythm affect fish assemblage structure. Results from this study will contribute to understanding short-term dynamics of the fish assemblage in seagrass habitats locally and globally.

In the present study, species composition and abundance of fish assemblage varied with tidal (spring and neap tide) and diel (day and night) cycles, but there was no effect of ebb/flood tidal cycle. The interplay of these two factors influenced fish assemblage compositions. The tide determines the habitat accessibility for tidal migrating fish species, while the diel cycle controls the visual conditions and diel changes in the prey organisms (Krumme et al., 2004). The influence of the tidal and diel cycle on the movements of tidal migrating fish is well established (Gibson, 1998), and Krumme et al. (2004) reported that evidence for distinct assemblages resulted from such movements. An nMDS ordination plot and two-way ANOSIM result showed that the fish assemblage structure differed significantly with tidal and diel cycles. The differences between day and night, however, were higher during spring tide than neap tide. This was probably due to strong currents during spring tide leading to a more heterogeneous assemblage structure.

Although there were no significant differences in abundance and diversity between spring and neap tide (three-way ANOVAs, P>0.05), the number of species was significantly higher during spring than neap tide (P<0.05). Fish assemblage structures were also significantly different between spring and neap tide. These spring/neap tide changes in species composition may be due to the differences in the amount of the intertidal zone that can be accessed at spring tide compared to neap tide (Wilson and Sheaves, 2001). During spring tide, the tidal level shifted from 0.2 m to 2 m, and the upper parts of the intertidal zone can be accessed by various fish species at high tide. During neap tide, however, the tide does not rise as high as spring tide, and consequently, fish are forced completely into the lower zone for longer periods of time. In addition, a high tidal range may be great enough to allow large predators to readily access shallow water habitats such as seagrass at high tide, altering fish assemblage structure and changing behavioral interactions (Blaber, 1986; Sheaves, 2005). Thus, more fish species are likely able to use shallow eelgrass beds during spring tide.

The structure of species composition varied with changes in day-night and tide heights. Abundance and biomass of fishes showed a tendency to increase as tide level increased with a peak at high tide during night. Increasing fish abundance with increasing tidal level is probably the result of fish migrating from deeper water into shallower habitats as a function of small changes in tidal height (Unsworth et al., 2007). Furthermore, overall abundance of fishes was higher at night than day. Nagelkerken et al. (2002) reported that fish abundance in coastal habitat including seagrass beds was higher at night. Results for seagrass bed fish assemblages in Philippine and Indonesia also supported this (Hemminga and Duarte, 2000; Unsworth et al., 2007). These diel differences in species composition and abundance might relate to diurnal changes in fish behaviors associated with diel variation in light intensity. This can be explained by some form of visual gear avoidance or escapement during trawl survey (Walsh, 1991), because fish can easily escape from the path of the survey vessel before the sampling gear arrived during the day. The day/night differences of prey availability are another possible factor explaining diel variation of fish abundance in a seagrass bed. In the study area, most of common species mainly preyed on seagrass-associated crustaceans and polychaetes which are abundant in seagrass beds of southern Korean waters (Huh and Kwak, 1997a, b, 1998a, b, 1999). Probably the feeding shift of these fish species toward the small-sized crustaceans at night can be attributed to these prey items becoming more available for predation, as a result of their nocturnal movement from substrate into the water column (Robertson and Howard, 1978; Robertson and Klumpp, 1983). In addition, epiphytic crustaceans have been found to increase in abundance during the nocturnal hours in shallow seagrass habitats (Guest et al., 2003; Unsworth et al., 2007). Other studies have shown similar patterns of variable food availability in diel variation of fish communities of seagrass beds. For example, movements of the planktivorous Atherina presbyter as well as piscivorous Belone belone to the intertidal seagrass beds with rising tide at night can be satisfactorily explained by increased food supply (Polte and Asmus, 2006), and higher abundances of common fish species found in mixed meadows of Cymodocea nodosa and Zostera noltii are explained by seagrass beds at night acting as feeding and spawning grounds (Guidetti and Bussotti, 2000). Hence, we suggest that the day-night differences of food availability are responsible for the diel variation of fish abundance through predator-prey interactions in this eelgrass bed.

Most of the common fish species excluding N. nuchalis were recorded at higher numbers at night, and these fish species have also occurred in unvegetated habitat near the eelgrass bed in the study area (Kwak et al., 2006). In contrast, N. nuchalis was higher in abundance during daytime of spring tide. Modde and Ross (1981) reported higher abundance of clupeoid fishes during day, because the shoaling behavior of these fishes may allow them to occur in daylight since in this formation they are protected from visual predators, such as adult fish or shore birds. Similarly to clupeoid fishes, N. nuchalis aggregates in schools and can occur in high numbers at the daytime of spring tide. There was also a tendency for larger fishes to dominate the catch at night, whereas smaller conspecifics were more abundant at daytime. In the study area, sizes of common fish species were also larger in adjacent unvegetated habitat than eelgrass bed (Kwak et al., 2006). Thus, we concluded that larger individuals of common species migrate from nearby coastal habitat to eelgrass bed at night on a diel basis.

This study gives important insights into the short-term changes in species composition and abundance of fish assemblage in relation to diel and tidal cycle in an eelgrass bed of southern Korean waters. Results indicated that fish assemblage structure varied with diel and tidal cycles, and thus these changes are probably due to the movements of common fish species. Such results are essential for understanding fish diversity in seagrass habitats, and are an important baseline for studies on management and conservation efforts of marine coastal habitats in southern Korean waters.

The authors thank Hyun Gi Choo (Korea Ocean & Fisheries Institute) for his assistance in sample collection. We also thank Daniel Welsh (University of Illinois) for reading the manuscript and for his valuable suggestions.