J. Mar. Biol. Ass. U.K. (2007), 87, 1287–1296 Printed in the United Kingdom Journal of the Marine Biological Association of the United Kingdom (2007) doi: 10.1017/S002531540705638X Tidal fish connectivity of reef and sea grass habitats in the Indo-Pacific The present study considered the influence of the tide on shallow water fish assemblages within the Wakatobi Marine National Park, Indonesia. Timed underwater visual observations were made across a gradient of intertidal to subtidal habitats from near-shore to reef crest at different tidal heights. Transient fish were found to dominate shallow water fish assemblages and the assemblage composition varied with tidal state. Fish assemblages were more diverse and abundant at higher tides in both coral and sea grass habitats, however, this was more pronounced within sea grass habitats. A tidal reduction from ≈2.0m to ≈0.8m (above chart datum) corresponded to a 30% reduction in fish abundance, while species richness also significantly decreased from 13.5 to 10.8 species per standardized timed observation. Fifty fish groups were reported from sea grass habitats with the most abundant being from the Engraulidae family and Lethrinus harak, which form important local subsistence fisheries. This research confirms the importance of tidal changes in structuring the fish fauna of Indonesian sea grass habitats and underlines the connectivity that exists between these habitats and nearby coral reefs. INTRODUCTION Sea grass and mangrove habitats play a fundamental role in coral reef fish life history (Nagelkerken et al., 2002; Dorenbosch et al., 2007; Unsworth et al., 2007). It is important to identify/understand the mechanisms that connect these shallow water habitats, and specifically, few studies have considered how this habitat connectivity impacts sea grass fish assemblages. In light of recent concern about the degradation of these habitats (Fortes, 1991; Orth et al., 2006) it is vital for their future management that the ecological mechanisms pertinent to their effective management are more clearly understood (Parrish, 1989; Sheaves, 2005; Larkum et al., 2006). Tidal state is an important factor controlling fish assemblages in many shallow water environments (Sogard et al., 1989; Thompson & Mapstone, 2002), and it may have one of the greatest influences on the pattern of biological inter-habitat connectivity (Sheaves, 2005). For example, a high tidal range may be great enough to allow large predators to readily access shallow water habitats such as sea grass at high tide, altering fish assemblage structure and changing behavioural interactions (Blaber, 1986; Sheaves, 2005). Fish must develop effective strategies to maximize the resources of inter-tidal habitats in relation to tidal water movements, thereby optimizing the trade-off between predation risk and food availability (Sogard, 1992; Allouche, 2002; Sheaves, 2005). An ebbing tide commonly results in a reduction in fish abundance and diversity (Sogard et al., 1989; Nagelkerken et al., 2000) as a consequence of environmental or predatory pressures (Young et al., 1997; Nagelkerken et al., 2000). In tropical coastal habitats tidal fish movements to and from sea grass beds may include migration to other habitats including coral reefs (Robertson, 1980), mangroves (Nagelkerken et al., 2000) or various deeper water environments (Pessanha & Araujo, 2003), with many of these movements being for foraging (Robertson, 1980; Robblee & Zieman, 1984). As the water level begins to fall with the ebbing of the tide the movement of fish may exist as a gradual process rather than an instantaneous mass migration when the tide turns. Previous research investigating the impact of tide on faunal assemblages has mainly documented the difference between extremes of the tide (Penn, 1975; Sogard et al., 1989; Pessanha & Araujo, 2003) or as a function of the ebbing or flooding of the tide (Bretsch & Allen, 2006). Research has not considered whether small, subtle changes in tidal level have an effect, specifically within sea grass habitats. It is not clear whether fish utilize these habitats differently at different stages of the tide or whether there is a high degree of species-specific strategies with respect to tidal height. Despite shallow Indo-Pacific intertidal areas often being continuous sea grass, they can effectively be split into zones based on the relative amount of time they become exposed by the tide, with inshore habitats being exposed for longer periods. In many habitats changing water depth and aerial exposure causes changes in the associated floral and faunal assemblages (Chapman, 1977) and has been found to influence sea grass bed fauna in the Caribbean (Robertson, 1980; Sogard et al., 1989). Although research has commonly considered the distinct habitats of coral reef, sea grass and ‘bombie/patch reef’ (Kochzius, 1999; Nakamura & Sano, 2004), there has been little characterization of the fish assemblages that inhabit these inter-tidal depth and habitat gradients (Nakamura & Sano, 2004). Richard K.F. Unsworth*, James J. Bell † and David J. Smith* ‡ *Coral Reef Research Unit, Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, UK. †School of Biological Sciences, Victoria, University of Wellington, PO Box 600, Wellington, New Zealand. ‡ Corresponding author, e-mail: djsmitc@essex.ac.uk