Environ Biol Fish (2011) 91:429–448 DOI 10.1007/s10641-011-9802-6
Fish communities on staghorn coral: effects of habitat characteristics and resident farmerfishes Michele K. Johnson & Sally J. Holbrook & Russell J. Schmitt & Andrew J. Brooks
Received: 28 June 2010 / Accepted: 23 March 2011 / Published online: 7 April 2011 # Springer Science+Business Media B.V. 2011
Abstract Branching corals, like many in the genus Acropora, provide structurally complex habitats for reef fishes and other organisms. Fluctuations in the abundance, distribution and characteristics of thicketforming staghorn Acroporids may contribute to changes in the abundance and species composition of reef fishes due to changes in the availability of shelter habitat and food. Farming damselfishes of the genus Stegastes can occur in high abundances in staghorn corals and actively defend food and nest space against organisms that threaten these resources. Here we assess the value of staghorn as habitat for fishes in the central South Pacific, and how the presence of territorial farming damselfishes may influence the assemblage of fishes that associate with staghorn corals. Surveys of 185 Acropora pulchra patches located in the lagoons surrounding the island of Moorea, French Polynesia revealed 85 species of fish from 25 families. Total fish abundance and species richness values ranged from no fish on a patch to a high of 275 individuals and 26 species. M. K. Johnson : S. J. Holbrook (*) : R. J. Schmitt Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106-9610, USA e-mail:
[email protected] M. K. Johnson : S. J. Holbrook : R. J. Schmitt : A. J. Brooks Coastal Research Center, Marine Science Institute, University of California, Santa Barbara, CA 93106-6150, USA
Patch area was the most important characteristic in explaining variation in attributes of the fish assemblage, with other characteristics explaining little of the species composition or trophic structure. Behavioral observations revealed that farming damselfishes were most aggressive toward corallivores, herbivores, and egg predators, while they ignored most carnivores and omnivores. Despite this pattern, we observed positive covariance between Stegastes and the group of fishes that elicited the strongest aggressive response when the effect of patch area was removed, suggesting these fishes remain drawn to the resources produced or enhanced by Stegastes on A. pulchra. Keywords Fish communities . Staghorn coral . Acropora . Farmerfish . Stegastes . Fish behavior . Behavioral interactions
Introduction Hermatypic corals form the foundation of coral reef ecosystems and provide critical habitat for hundreds of species of coral associated organisms. Acroporid and other branching corals are especially significant in their capacity to provide structurally complex refuge space for a large number of reef fishes (Holbrook et al. 2002a). Microhabitats such as holes, crevices, and open interior areas between branches are important aspects of habitat structure and can account for much of the variation in species richness and total abundance (Holbrook et al.
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2002a, b; Brooks et al. 2007). In general, abundance and species richness of fishes scale with the overall size of the coral colony or patch (Lomolino 2001; Holbrook et al. 2002a, b, 2008), and fluctuations in abundance of branching corals may be accompanied by changes in the abundance and species composition of fishes due to changes in availability of shelter, food and/or nest sites (Syms and Jones 2000; Berumen and Pratchett 2006). Studies that have explored relationships between coral habitats and the associated fish assemblages have reported a range in the extent to which heterogeneity in the fish community could be explained by variation in attributes of the habitat (Ormond et al. 1996; Munday et al. 1997; Ault and Johnson 1998; Syms and Jones 2000; Holbrook et al. 2000, 2002a; McClanahan and Arthur 2001). Explorations that focused on coral habitats comprised of a single species of coral tended to find that variation in microhabitat characteristics and the physical setting of the patch explained a large fraction of the observed variation in the fish assemblage. In these studies, the focus on the abundance of various microhabitats within patches made up of single species of corals eliminated potential heterogeneity arising from differing amounts of complex combinations of structural morphologies in patches composed of multiple coral species (e.g., Holbrook et al. 2002a). For example, variation in physical characteristics (colony size, availability of microhabitats) of the patch-forming coral Porites rus in French Polynesia accounted for a majority of the variation in species richness and total abundance of fishes among colonies, and for a given set of physical attributes, species composition of the fish assemblage was influenced greatly by the location of the P. rus colony within the lagoon [fringing reef or mid-lagoon; Holbrook et al. (2002a)]. It remains unknown whether the relationships between colony microhabitat features or the location of a colony and attributes of the associated fish communities are similar for other major habitat-providing species of corals, including staghorn Acroporids. Several species of staghorn Acropora have long, cylindrical branches and can form dense thickets that can cover tens to hundreds of meters of bottom. Thickets of staghorn coral, which are widely distributed geographically (Wallace 1999), harbor numerous species of fishes, invertebrates and algae (Wilkes et al. 2008). One group of fishes frequently found in high abundance within staghorn coral are farming damselfishes (Pomacentridae) in the genus Stegastes, which
Environ Biol Fish (2011) 91:429–448
cultivate and defend filamentous turfs of algae within territories (Mahoney 1981; Robertson 1984; Hata and Kato 2004; Precht et al. 2010). Farming damselfishes generally are territorial (Low 1971), and intraspecific competition for food and nest sites as well as nest defense have been implicated as underlying causes for this behavior (Ebersole 1977; Haley and Muller 2002). Aggressive behavior by farming damselfishes typically is directed at herbivorous fishes, egg predators and grazing sea urchins (Ebersole 1977; Hata and Kato 2004). Territorial behavior by species of Stegastes in the Caribbean can decrease recruitment of surgeonfish and butterflyfish within territories (Shulman et al. 1983), while in the Indo-Pacific recruitment of branching corals can be enhanced in the territories of farming damselfishes (Glynn and Colgan 1988; Done et al. 1991; Gleason 1996). As yet, however, we have an incomplete understanding of the value of staghorn coral as habitat for fishes or how the presence of territorial farming damselfishes may influence the associated assemblage of fishes. Here we explore the relationship between the staghorn coral Acropora pulchra and the associated fish assemblage at Moorea, French Polynesia. We (1) examine how various aspects of the fish community as a whole varied among patches of A. pulchra as a function of structural and other attributes of the coral, (2) focus on variation in population size of farming damselfishes (Stegastes spp.) inhabiting staghorn coral, and (3) explore whether these territorial Stegastes spp. influence the composition and/or abundance of the other fishes in the community.
Materials and methods All fieldwork was conducted in the lagoons of Moorea, French Polynesia (17º30′ S, 149º50′ W), a high volcanic island 25 km northwest of Tahiti in the Society Archipelago, where the semi-diurnal tidal range rarely exceeds 40 cm (Adjeroud 1997). Surveys were carried out in the northern portion of the Temae lagoon on the east side of the island, and along the north shore between Opunohu Bay and the Ava Iti Pass. On Moorea, a barrier reef surrounds shallow lagoons that are occupied by coral patches, rubble and sand. A shallow fringing reef with dense coral cover occurs in the lagoons on the north shore of the island but was largely absent in our study areas in the Temae lagoon.
Environ Biol Fish (2011) 91:429–448
Patterns of spatial variation of staghorn coral patches and their associated fish assemblages To examine the variation in structural and other attributes of staghorn coral (Acropora pulchra), we surveyed 185 patches, ranging from 0.5 m depth along the fringing reef, to 2.5 m in the mid-lagoon, during the Austral winter of 2008. These were found by divers through systematic searches of the north shore and Temae lagoons. The GPS position of each patch was recorded, and distances from the shore and reef crest were obtained using the software package Google Earth (2009) (Version 5.1). Patches were classified by their position in the lagoon (fringing reef or mid-lagoon). Measurements of several physical characteristics were recorded in a manner consistent with those used in previous studies to examine the relationship between colonies of Porites rus and associated fish assemblages (Holbrook et al. 2002a; Brooks et al. 2007). Measurements included length (measured along the longest dimension), width (measured along the longest diameter perpendicular to the length) and height (from the base of the coral to the top of the tallest branch, perpendicular to the substrate). Patch area and volume were estimated using the equations for an ellipse (πlw/4) and elliptical cylinder (πlwh/4) respectively. Water depth (distance from the sea surface to the base of a patch) and the height of the water column above each colony (distance from the sea surface to the tallest point of the coral) were measured. Non-destructive, visual fish censuses were conducted to estimate the fish assemblage associated with each patch of staghorn coral. All non-cryptic (visible to a scuba diver without any disturbance to the habitat) fishes were counted and identified to species. First, a diver conducted an initial count of all fishes in the patch and in the water column immediately above while slowing circling at a distance of approximately 3 m. A fish was considered associated with the staghorn if it was observed swimming into the patch or interacting with the coral or other organisms living within it. Individuals resting on nearby sand or those that were merely passing by were not included in the counts. The surveyor then moved directly above the patch to examine the interior for fishes living within the staghorn structure. Surveys lasted 5 to 30 min, depending on the size of the patch, and with the total time spent searching per unit volume of the patch
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remaining approximately constant. We determined the species richness, total abundance, community composition and density of fishes on each staghorn patch from these estimates of abundance. Species were assigned to feeding categories (carnivore, corallivore, egg predator, herbivore, and omnivore) based on published dietary descriptions (e.g., Randall 2005; Pratchett 2005, 2007). We explored relationships between characteristics of the fishes that occur on staghorn coral and physical characteristics of the patches (area, height, water depth, height of the water column above, distance to shore, distance to reef crest, average length of branches that were live coral, and average percent coral per branch), using stepwise multiple regressions (SAS Proc REG; SAS Statistical Software, Version 9.1, 2005). The predictive measures area and height, along with the response variables fish abundance and species richness, were log10 transformed to linearize the relationships between the physical characteristics of the patches and fish abundance and species richness. Preliminary analyses indicated that patch area and volume were significantly and highly correlated (P
