Aerial images can detect 3D small patch reefs that are potential habitats for anemonefish Amphiprion frenatus
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Patchy habitats often enhance species coexistence and, consequently, abundance of each species. The present study examined two indicators of potential habitats for anemonefish Amphiprion frenatus: total area of dark-colored patch reefs that are detectable on an aerial image with image analysis software, and total area of tall patch reefs (>1.5 m in height) that are detectable on stereoscopic aerial images with a stereoscope. Relationships between patch reef area and anemonefish abundance, as estimated by number of host anemone Entacmaea quadricolor, were investigated at Shiraho Reef, Ishigaki Island, Japan. Total number of anemones was not correlated with total area of tall patch reefs but was highly correlated with total area of dark-colored patch reefs. Underwater observations confirmed that upper surface of tall patch reefs often involved bare substratum, whereas dark-colored patch reefs did not include this substratum. Because reef area approximately reflected reef volume within shallow back reefs, total area of dark-colored patch reefs that are detectable by aerial photography will reflect total volume of reef fish habitat. The present study suggests that aerial photography with a drone can provide a useful predictor of reef fish abundance.
KeywordsCoral reef fish Drone Habitat structure Seascape structure
Patchy habitats often enhance species coexistence and, consequently, abundance of each species (Tilman and Kareiva 1997; Turner et al. 2001). Coral reef fish communities, which are among the most diverse animal communities in nature, occur on various sized patch reefs (e.g., Lowe-McConnell 1987; Sale 1991; Spalding et al. 2001). While species richness and abundance of individuals can be high in patchy habitats (Tilman and Kareiva 1997; Turner et al. 2001; Hattori 2002), several authors suggest that total area of habitat patches is the crucial determinant of species richness and abundance of each species (Lomolino 1994; Tjörve 2003; Fahrig 2013).
A back reef is an essential component of a mature fringing reef and harbors many coral reef fish. Back reefs are usually less than 3 m deep and include various sized patchy reefs that are formed by coral heads and outcrops dispersed throughout sandy bottom (Chave and Eckert 1974; Lowe-McConnell 1987; Spalding et al. 2001). Aerial photography at low altitude using unmanned aerial vehicles (UAV) or drones have been reported to provide detailed landscape or seascape images (Paine and Kiser 2012; Anderson and Gaston 2013). Aerial images of shallow back reefs can differentiate among the sandy bottom, submerged patch reefs and emerged patch reefs (see Hattori and Kobayashi 2007, 2009). However, aerial images usually provide two-dimensional information on habitat patches. A study that used high-resolution aerial color images as detailed field maps to assess species-area relationships of damselfish assemblages over 84 small patch reefs (>0.75 m2) within a back reef located at Ishigaki Island, Okinawa, Japan, found that a large patch reef harbored fewer species than small patch reefs of equivalent area (Hattori and Shibuno 2010). Subsequently, interspecific competition among territorial herbivores was found to be less intense on a tall reef (>1.5 m in height) than on a flat reef because, on the former, inferior competitors were able to use the vertical side of the reef (Hattori and Shibuno 2013). Harborne et al. (2012) also reported that reefs with high rugosity (>0.5 m difference in height) were found to harbor more species than reefs with low rugosity (≤0.5 m difference in height). Relatively low species density on large reefs may be attributed to tendency of larger reefs to be flat within shallow back reefs (Hattori and Shibuno 2010, 2013, 2015). Although high species richness and abundance of coral reef fishes are often related to high coral cover on coral reefs, many reef fishes inhabit structural habitats even in the absence of high coral cover (Pratchett et al. 2016). Structural complexity of seafloor influences species richness and abundance of each species (Pittman and Olds 2015). Total amount of patch reefs with high rugosity must be important for reef fish habitats.
Assessment of habitat quality of shallow back reefs is necessary to prioritize protection measures given threats from land reclamation, such as pier, airport and waterfront construction (Spalding et al. 2001; Tsuchiya et al. 2004; Hattori and Kobayashi 2007, 2009). Total amount of patch reefs with high rugosity may be an indicator of habitat quality. For example, total volume of 3D patch reefs (i.e., reef area × reef height) has been reported predictive of species richness of damselfish assemblages (Hattori and Shibuno 2015). In shallow back reefs, however, it is impractical for field ecologists or research divers to measure total volume of patch reefs, or structural complexity of seafloor, except for research using small quadrats (e.g., 5 m × 5 m), because of complex seascapes and shallowness. The present study examined whether two indicators from aerial images on a hectare scale (2.9 ha) can be used for habitat quality assessment of coral reef fish: (1) total area of dark-colored patch reefs that are detectable on an ordinary aerial photograph with image analysis software, and (2) total area of tall patch reefs (>1.5 m in height) that are detectable on stereoscopic aerial images with a stereoscope.
Hattori and Shibuno (2010) investigated species richness and abundance of individuals of damselfishes among 84 small patch reefs and found that abundance of anemonefish Amphiprion frenatus was most positively correlated to damselfish species richness. In addition, Hattori and Kobayashi (2007) reported that total area of dark-colored patch reefs that were detected on aerial images reflects total number of A. frenatus, although they used analog type aerial photographs and did not distinguish tall patch reefs (>1.5 m in height) from the dark-colored patch reefs. The objective of the present study was to examine whether total area of tall patch reefs (>1.5 m in height) that are detectable on stereoscopic aerial images, as well as total area of dark-colored patch reefs that are detectable on ordinary aerial images, can be used for habitat quality assessment of the anemonefish. Total area of dark-colored patch reefs can be compared with the results of Hattori and Kobayashi (2007). Underwater observations on patch reefs were also conducted to confirm relationships between anemonefish abundance and total area of patch reefs.
Materials and methods
The giant sea anemone Entacmaea quadricolor is an indicator species for abundance of Amphiprion frenatus, because this anemonefish inhabits E. quadricolor only (Allen 1975; Fautin and Allen 1992; Hattori 1991, 2005; Kobayashi and Hattori 2006; Hattori and Kobayashi 2007). To determine relationships between total area of patch reefs and anemonefish abundance, total number of E. quadricolor in each quadrat was assessed on the seascape map. That is, one part of the sequence of aerial images (C-19-1607) was enlarged, printed, waterproofed and used to map anemones in June and July 2014. While searching for anemones and plotting their location on seascape maps, environmental features were determined. Original data of Hattori and Kobayashi (2007), which were determined in 2002 using aerial images taken in 1995, were reanalyzed and compared with results of present study. All data were analyzed with R 3.0.1 statistical software (R Development Core Team 2013).
Total area of tall patch reefs was highly correlated with total area of dark-colored patch reefs (r = 0.83, t = 4.04, P = 0.004, n = 9), and regression line shown in Fig. 2b could be used to predict total area of tall patch reefs based on total area of dark-colored patch reefs. However, total numbers of anemones in Quadrats 4 and 7, where total area of tall patch reefs largely deviated from regression line in Fig. 2b, were very close to total numbers of anemones predicted from total area of dark-colored patch reefs and regression line in Fig. 2a. Field observations confirmed that Quadrat 4 contained tall patch reefs but many dark-colored patch reefs on seafloor (see Fig. 1). Direct observations also confirmed the abundance in Quadrat 7 of shallow platforms with bare substratum (i.e., reef pavement around lowest tides level, yellowish colors on aerial images of Fig. 1). Upper surface of a large and tall reef was very close to the water surface, involving bare substratum (e.g., a microatoll such as a large massive Porites, and a shallow platform or reef pavement). In contrast, field observations revealed that bare substratum was not present at sites corresponding to dark-colored patch reefs on aerial images.
The present study demonstrated that total area of dark-colored patch reefs (>0.5 m in reef height), as measured on aerial images, can be an indicator of anemonefish abundance, although it requires that areas of seagrass beds be removed from aerial images, based on field observations. Although number of anemones was found to show a stronger correlation with the product of total area and total perimeter of dark-colored patch reefs than with total area alone (Hattori and Kobayashi 2007), the perimeter of dark-colored patch reefs could not be determined in this study (see “Materials and methods”). After measuring height of 84 patch reefs within the present study site (2.9 ha), Hattori and Shibuno (2015) suggested that total volume (∑ reef basement area × reef height) of dark-colored patch reefs is an indicator of total species richness of damselfish assemblages within shallow back reefs. Accordingly, we expected that total area of tall patch reefs (>1.5 m in height) that were detected on a sequence of two aerial images with a stereoscope would indicate anemonefish abundance. However, upper surface of tall patch reefs, in which sessile organisms are exposed to air at lowest tides, often consisted of bare substratum, and total area of tall patch reefs did not relate with number of anemones (i.e., number of anemonefish). In contrast, dark-colored patch reefs that were detected on aerial images with image analysis software did not involve such bare substratum. Thus, all patch reefs taller than 0.5 m without bare substratum could be detected on aerial images without a stereoscope. In addition, comparison of total patch reef area between the present and the past aerial images could give some explanation about change in number of anemones. These indicate that unmanned aerial vehicles (UAV) or drones without stereoscopic aerial photography will be able to provide reliable information on patch reef habitats over a wide range.
Mature fringing reefs are often characterized by shallow platforms, or reef pavement with bare substratum, such as a wide platform on the reef flat (Spalding et al. 2001). On aerial images, the reef flat on a mature fringing reef appears as very large patch reefs, which will not harbor high species richness of reef fishes because living space for fish rarely include bare substratum (Fig. 5b). Because it does not contain bare substrata, total area of dark-colored patch reefs reflects total amount of reef fish habitats, which are measurable on ordinary aerial images (Fig. 5). In conclusion, aerial photography with a drone can provide a useful predictor of reef fish abundance within back reefs.
The author is grateful to D. Price, and three anonymous reviewers for their valuable advice regarding the manuscript. This work was supported by JSPS KAKENHI Grant numbers JP15K07218 and JP23570022.
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