BULLETIN OF MARINE SCIENCE, 80(3): 473–495, 2007 nearsHore Habitat use BY GraY snaPPer (LUTJANUS GRISEUS) anD bluestriPED Grunt (HAEMULON SCIURUS): enVironmental GraDients anD ontoGenetic SHifts Craig H. Faunce and Joseph E. Serafy ABSTRACT Fringing mangrove forests and seagrass beds harbor high densities of juvenile snappers and grunts compared to bare substrates, but their occupancy of these habitats is not homogeneous at ecologically meaningful scales, thus limiting our ability to compare habitat value. Here, density and size information were used to de- termine how gray snapper, Lutjanus griseus (Linnaeus, 1758) and bluestriped grunt, Haemulon sciurus (Shaw, 1803), use vegetated habitats during their ontogeny, and how their use of mangrove forests varied with season across broad spatial scales and physicochemical conditions. Both species exhibited a three-stage ontogenetic strategy: (1) settlement and grow-out (8–10 mo) within seagrass beds, (2) expan- sion to mangrove habitats at 10–12 cm total length, and (3) increasing utilization of inland mangroves during the dry season and with increasing body size. For fishes inhabiting mangroves, multivariate tests revealed that the factors distance from oceanic inlet and water depth were stronger predictors of reef fish utilization than the factors latitude, temperature, or habitat width. These findings highlight that the nursery function of mangrove shorelines is likely limited to the area of immediately accessible habitat, and that more expansive forests may contain a substantial num- ber of larger adult individuals. The importance of coastal vegetated seascapes to the early life history stages of diadromous nekton has been well documented, and has led to the conclusion that many temperate and warm-water marine fishes are “estuarine dependent” G( unter, 1967; McHugh 1967; Day Jr. et al., 1989; Blaber, 2002). However, the spatial separa- tion of juveniles from adults and their utilization of coastal vegetated habitats is not limited to estuarine waters. Day (1951) was among the first to postulate that the fauna observed within a coastal embayment were drawn to “quiet water” conditions rather than estuarine conditions per se. Expanding this idea further, Nagelkerken and van der Velde (2002) offered the idea of “bay habitat dependence”. Despite attempts to provide guidelines for assessing the comparative “importance value” among fish habitats (USDOC, 1996; Beck et al., 2001), the relative utilization of multiple habitats by fishes has been difficult to quantify. Although numerous studies of animal-envi- ronment relationships have been conducted, these studies largely have been limited by a focus on a single habitat type or life-history stage, and/or by sampling at spatial scales that are too small to adequately capture the full suite of habitat use by fishes during their ontogeny (Pittman and McAlpine, 2003). At latitudes that support coral reefs, natural seascapes are vegetated primarily by seagrass beds and coastal mangrove forests. Here, many exploited reef fishes occupy inshore regions as juveniles before migrating offshore to reproduce thereby under- going an ontogenetic pattern of habitat utilization similar to that exhibited by di- adromous fishes in estuarine waters (Ogden, 1997). It has been postulated that the presence of mangroves and seagrass beds serve as extra “waiting room” habitats for juvenile coral reef fishes, and that adopting such a life-history strategy may buffer Bulletin of Marine Science 473 © 2007 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 474 BULLETIN OF MARINE SCIENCE, VOL. 80, NO. 3, 2007 against poor recruitment years (Bardach, 1959; Parrish, 1989). Recently, there have been renewed investigations into whether the presence of seagrass and mangrove habitats in the Caribbean enhance populations of fishes inhabiting adjacent coral reefs (e.g., Nagelkerken et al., 2002; Dorenbosch et al., 2004; Halpern, 2004; Mumby et al., 2004). In all of these studies, mangrove shorelines have been considered as ho- mogenous habitats. However, mangrove shoreline habitats with different geographic locations, physical structure, and associated physicochemical environments may not contribute equally to future adult populations of reef fishes. Despite these manage- ment implications, few field studies have investigated the factors responsible for the utilization of mangrove shoreline habitats by reef fishes during their ontogeny, and those few have limited analyses to simple correlations performed at the entire fish assemblage level (Faunce and Serafy, 2006). The preceding is important because there is growing evidence that the processes or factors underpinning observed patterns of habitat utilization are species- and scale-dependent. For example, Martino and Able (2003) suggested that patterns in fish assemblage structure across large spatial scales (i.e., > 10 km) are due to individ- ual species responses to dominant environmental gradients, whereas smaller-scale (1 km) patterns are due to habitat selection, competition, and/or predator avoidance strategies. In addition, recent studies in the Netherlands Antilles have reported pat- terns in the ontogenetic utilization of seagrass and mangrove habitats by reef fishes as a function of geographic scale (Nagelkerken et al., 2000b; Cocheret de la Morinière et al., 2002). However, their results apply to a 3 km wide bay. Determining whether similar patterns of ontogenetic utilization occur across larger spatial scales in other regions of the western Atlantic is important, especially if reef fish can move season- ally over distances more than a few kilometers (Gillanders et al., 2003; Pittman and McAlpine, 2003). An expansive (ca. 1000 km2) series of lagoons and embayments are found in south- eastern Florida bordered to the west by the Florida mainland and to the east by the Florida Keys. In general, the influence of fresh water increases to the west-southwest, whereas oceanic and tidal influences are greater to the east and northeast. Across this diverse hydrographic seascape, the bays of southeastern Florida are primarily vegetated by the same seagrass (i.e., Thalassia testudinumBanks ex. König, 1805) and mangrove (i.e., Rhizophora mangle Linnaeus) species. Therefore, the region provides the opportunity to examine how reef fish utilization of seagrass and mangrove habitats changes with ontogeny and how utilization of a single habitat type (i.e., mangroves) varies across broad spatial scales and physicochemical conditions. This study aims to address these issues for two reef fishes common to the region, gray snapper,L utjanus griseus (Linnaeus, 1758) and bluestriped grunt, Haemulon sciurus (Shaw, 1803). Methods Data Collection.—The southeastern Florida coast was defined as the area be- tween Key Biscayne to the northeast (80°09´N, 25°43.8´W), Russell Key to the southwest (25°4.2´N, 80°38.4´W), and Tavernier Creek to the southeast (25°06´N, 80°32.4´W). The mangrove-lined shorelines of southeastern Florida were divided into five spatially-dis- tinct strata. In order of their general position from west to east these strata include: (1) the mainland (ML); (2) islands within bays (IS); (3) landbridges (LB) that connect por- tions of the ML and the Florida Keys; (4) the western (i.e., leeward) shorelines of the Keys FAUNCE AND SERAFY: NEARSHORE ONTOGENIC HABITAT SHIFTS BY TWO REEF FISHES 475 (LK); and (5) the eastern (i.e., windward) shorelines of the Keys (WK; Fig. 1A). These strata were sampled using an iterative stratified random design during the wet (June–Au- gust) and dry (January–March) seasons of 2000 and 2001 following Cochran (1977) and detailed by Ault et al. (1999a; Fig. 1B). Sample locations, including five alternates, were selected each season by first dividing each stratum into sequentially numbered 30 m latitudinal sections, and using a random number generator to select the latitude of each sample. Samples consisted of (30 × 2 m) 60 m2 visual belt transects wherein a trained underwater observer identified, enumerated, and estimated the minimum, average, and maximum total length (TL) of each fish species to the nearest 2.5 cm. Only one individ- ual conducted all fish surveys, thus eliminating inter-observer bias (St. John, 1990).P rior to the study, the observer’s ability to identify and rapidly enumerate fishes accurately was tested according to three criteria. First, the observer must have correctly identified all of the species common to the region (as defined from table 1 of Serafy et al., 2003) from digital photographs, including ontogenetic color variations where appropriate (e.g., parrotfishes). The observer must also have correctly estimated the identification of all species and number of fish within 10% from a 30 s digital video clip of a 30 m mangrove transect. Finally, following Bell et al. (1985), accurate and precise length assessment of fishes underwater was achieved by having the observer repeatedly estimate the sizes of plastic pipes of various lengths. To ensure fishes could be effectively observed within each transect, horizontal visibil- ity was determined prior to surveys using a vertically mounted secchi disk and a mea- suring line. If visibility was < 2 m, the survey location was abandoned and an alternative location was visited. If no suitable alternative was available, locations were re-visited on a later date. Because fish observations in mangroves may be influenced by tidal stage and light levels, we attempted to control for this factor by conducting