Small fish and crustaceans demonstrate a preference for particular small-scale habitats when mangrove forests are not accessible Ross Johnston ⁎ , Marcus Sheaves 1 School of Marine and Tropical Biology, James Cook University, Townsville, Queensland, 4815, Australia Received 20 April 2006; received in revised form 23 May 2007; accepted 29 May 2007 Abstract Availability of large areas of complex habitat (particularly mangrove forest) is an important reason why fish use estuaries as nursery areas. However for aquatic species, access to much of the complex habitat in tidal systems is restricted to short periods of time. Consequently, aquatic species must spend considerable time in ex-forest habitats; habitats available when mangrove forests are not accessible. The objective of this study was to determine the extent to which the availability of particular small-scale, ex-forest habitats influenced the distribution of small fish (b 100 mm FL) and crustaceans. Substantially higher numbers of small fish and crustaceans were recorded from muddy substrata than from sandy substrata. Over muddy substrata, bank architecture such as drains returning water to sub- tidal areas and water depths b 0.375 m strongly influenced the distributions of abundant species, whereas areas of low current velocity and hydrodynamic features had only minor influence on distributions. Similarly, water depths b 0.375 m and bank architecture, such as drains, strongly influenced the distribution of the abundant crustacean taxa. Current velocity and hydrodynamic features had little influence on the distribution of crustaceans. Most of the abundant taxa showed a positive response to small-scale ex-forest habitats that either provided longer access time to complex intertidal habitat and/or shallow water. However it was unclear whether those habitat preferences also provided feeding opportunity, refuge from predation or energetic advantage. © 2007 Elsevier B.V. All rights reserved. Keywords: Crustacean distribution; Depth; Drains; Fish distribution; Habitat; Tropical estuaries 1. Introduction Availability of large areas of complex habitat, in particular mangrove forest, has been used to explain why estuaries function as nursery areas (Robertson and Blaber, 1992). However, although it is apparent that fish and crustaceans use mangrove forests extensively (Thayer et al., 1987; Vance et al., 1996) many questions remain about the way fish use intertidal and sub-habitats and about the biological connectivity between those habitats. Biological connectivity between sub-tidal and inter-tidal habitats clearly exists because fish use mangrove forests when they are available (Thayer et al., 1987; Laegdsgaard and Johnson, 1995; Vance et al., 1996) and, with a few specialist exceptions, cannot remain in the high intertidal once the tide has receded. Tidally-driven migrations (Gibson, 2003; Krumme et al., 2004) are one mechanism that facilitates connectivity between intertidal and sub-tidal habitats, but previous literature provides little information about the habitat- related distribution of small fish when intertidal habitats such as mangrove forests are not available (Laegdsgaard and Johnson, 1995). Mangrove forests are only accessible to aquatic species for short periods; access to forests is restricted Journal of Experimental Marine Biology and Ecology 353 (2007) 164 – 179 www.elsevier.com/locate/jembe ⁎ Corresponding author. Tel.: +61 7 4781 4626; fax: +61 7 4725 1570. E-mail addresses: ross.johnston@jcu.edu.au (R. Johnston), marcus.sheaves@jcu.edu.au (M. Sheaves). 1 Tel.: +61 7 4781 4144; fax: +61 7 4725 1570. 0022-0981/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2007.05.039