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A Comparison of Damselfish Densities on Live Staghorn Coral (Acropora cervicornis) and Coral Rubble in Dry Tortugas National Park
Allison A. Wilkes, Melissa M. Cook, Anthony L. DiGirolamo, John Eme, Jeff M. Grim, Bernadette C. Hohmann, Sara L. Conner, Cheryl J. McGill, Christopher M. Pomory, and Wayne A. Bennett

Southeastern Naturalist, Volume 7, Number 3 (2008): 483–492

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2008 SOUTHEASTERN NATURALIST 7(3):483–492 A Comparison of Damselfish Densities on Live Staghorn Coral (Acropora cervicornis) and Coral Rubble in Dry Tortugas National Park Allison A. Wilkes1,*, Melissa M. Cook1, Anthony L. DiGirolamo2, John Eme3, Jeff M. Grim4, Bernadette C. Hohmann5, Sara L. Conner1, Cheryl J. McGill6, Christopher M. Pomory1, and Wayne A. Bennett1 Abstract - Over the past 30 years, cold events and disease have reduced much of the live Acropora cervicornis (Staghorn Coral) in Dry Tortugas National Park (DTNP), FL to fields of coral rubble. It is unclear how the resulting loss of threedimensional reef structure has affected density and distribution of reef-dependent damselfishes. We compared densities of Stegastes adustus (Dusky Damselfish), Stegastes leucostictus (Beaugregory Damselfish), Microspathodon chrysurus (Yellowtail Damselfish), Stegastes planifrons (Three-spot Damselfish) and Stegastes variabilis (Cocoa Damselfish) inhabiting DTNP’s last live Staghorn Coral formation with densities from surrounding coral rubble. Live Staghorn Coral supported a 65% higher damselfish density compared to coral rubble. Density of Dusky, Cocoa, Beaugregory, Yellowtail and Three-spot Damselfish on coral rubble (0.11, 0.58, 0.74, 0.02, and 0.06 fish/m2, respectively) was less than that found on living Staghorn Coral colonies (2.03, 0.45, 0.25, 0.50, and 0.96 fish/m2, respectively). Dusky Damselfish dominated the live Staghorn Coral site, while Cocoa and Beaugregory Damselfish dominated the coral rubble site. Juvenile density was ten times greater on coral rubble than on live Staghorn Coral, whereas adults had highest densities on live Staghorn Coral. Introduction Approximately 113 km west of Key West, FL, the Dry Tortugas National Park (DTNP) is the site of one of the most expansive and pristine coral reefs in the Florida reef tract (Fig.1) and the continental United States. Although extensive branching coral formations were reported within the DTNP area from the early 1880s through the mid-1970s (Davis 1982), stands of branching coral within the Park have been in decline for the past 30 years. A severe cold front during the winter of 1976–77 destroyed nearly 96% of DTNP’s Staghorn Coral within two meters of the surface (Bohnsack 1983), and outbreaks of white band disease during the 1980s further degraded already damaged reefs in the Florida reef track (Williams 1997) and throughout the Caribbean (Aronson and Precht 2001, Vargus-Angel et al. 2003). While cycles of natural destruction and rebuilding are typical on healthy reefs 1Department of Biology, University of West Florida, Pensacola, FL 32514. 2Florida Fish and Wildlife, Jacksonville, FL 32221. 3University of California, Irvine, CA, 92627. 4Ohio University, Athens, GA 45701. 5Mote Marine Lab, Sarasota, FL 34236. 6US Environmental Protection Agency, Gulf Breeze, FL 32561. *Corresponding author - 484 Southeastern Naturalist Vol.7, No. 3 (Davis 1982, Smith and Buddemeier 1992), cold events followed closely by disease outbreaks have overtaxed the regenerative abilities of Staghorn Corals in the Park (Davis 1982, Porter et al. 1982, Reaka-Kudlat et al. 1996). Based on personal observation and using site locations provided by other researchers and DTNP officials, our systematic search revealed that of the Park’s once extensive living Staghorn Coral formation, only a single large 65-m2 remnant of Staghorn Coral remains (24°37'13"N, 82°52'10"W). Whereas destructive effects of environmental disasters on reefs are immediately noticeable, long-term repercussions of coral loss on reef-dependent fishes are less obvious and often overlooked (Booth and Beretta 2002, Jaap 2001). Damselfishes (Family: Pomacentridae) are an important component of the coral reef ichthyofauna. Commonly regarded as keystone species, damselfishes directly influence reef ecology and diversity (Hixon and Brostoff 1983, Williams 1980) and are considered good indicators of reef assemblage health (Aronson and Precht 1997, Emery 1973, Lieske and Meyers 1999, Longley and Hilderbrand 1941). Shifts from complex branching coral habitat to relatively uniform rubble have almost certainly altered damselfish distribution and ecology in DTNP, but it is unclear how and to what degree populations may have been affected. Previous research has shown damselfish populations differ with substrate complexity (Almany 2004, Holbrook et al. 2000, Lirman 1994, Nemeth 1998), coral cover (Clapp 2005, Sale 1972), competition (Jones 1987), and community structure (Abrey 2005; Itzkowitz 1977, 1985). Life-history studies have described damselfish habitat preferences (Myrberg 1971, Wellington 1992, Williams 1979), but have not examined damselfish distribution after preferred areas have been altered or destroyed. Likewise, reports of damselfish distribution and density on Florida reefs are limited Figure 1. Map of Dry Tortugas National Park showing live Staghorn Coral (Acropora cervicornis) (S) and coral rubble (R) study sites within the Park. 2008 A.A. Wilkes et al. 485 (Emery 1973, Wallman et al. 2004) and have focused primarily on ecology of damselfishes inhabiting low-energy patch-reef formations that were largely unaffected by cold and disease. Loss of branching coral habitat has been a chronic problem throughout the Caribbean. A better understanding of reef fish distribution on rubble fields will provide insights into aspects of reef ecology following disturbance. Subsequently, we quantified adult and juvenile damselfish densities and species assemblages on live Staghorn and adjacent coral rubble habitats in DTNP. Materials and Methods We determined damselfish densities on the largest remaining live Staghorn Coral formation and a nearby coral rubble habitat within DTNP during the first week of May 2004. Live Staghorn Coral and coral rubble study sites were located at 24°37'13"N, 82°52'10"W near Garden Key. Coral rubble was comprised of dead and broken Staghorn located approximately 30 m from the live Staghorn patch. All sampling was conducted during daylight hours. Damselfish densities were quantified on 20-m transect lines marked at 1-m intervals and carefully placed on live Staghorn Coral and coral rubble areas (1-2 m depth). Owing to unfavorable weather conditions, fewer transect were sampled on rubble sites. Eight non-overlapping transects were sampled on live Staghorn Coral, and four transects were sampled on coral rubble. Each transect was distanced far enough apart to prevent fish territory overlapping. Transects were sampled by four teams of two snorkelers on the surface moving at a slow rate, not exceeding 1 m/min. Damselfish within one-half meter of either side of the transect line were recorded. Using distinctive differences in coloration pattern, relative body shape, and size characteristics as described by Smith (2002), damselfish were identified to species and classified as either juvenile or adult. Data were categorized as: 1) total number of damselfish regardless of species or life stage, 2) total number of adult and juvenile fish regardless of species and within each species, and 3) total number of each species regardless of life stage. Counts of the four snorkel teams were averaged to produce a single value for each transect for each category. Counts per transect were divided by total transect area and reported as fish/ m2. Total damselfish density (all species and life stages combined) on live Staghorn Coral and coral rubble was compared using one-way ANOVA on ranked data. Total density was used for comparisons in order to compensate for zeroes in individual species data. Comparisons of adult densities (all species combined) between habitat types, as well as juvenile densities (all species combined) between habitat types were made using one-way ANOVA on ranked data. Comparisons of adult versus juvenile densities (all species combined) within both habitat types were made using a blocked ANOVA on ranked data with transects as blocks. Comparisons of densities by species of damselfish (adults and juveniles combined) within both habitat types were 486 Southeastern Naturalist Vol.7, No. 3 made using a blocked ANOVA on ranked data. Tukey’s multiple comparison procedure was used following significant ANOVA. Based on the number of statistical tests performed using the same data set, α = 0.007 was used as the significance level to adjust for multiplicity. Mean density of juvenile and adult damselfish by species and by habitat type are reported, but were not statistically compared. Results Damselfish densities differed markedly between live Staghorn Coral and coral rubble sites. Total damselfish density was significantly higher, approximately 65%, (one-way ANOVA: F1,10 = 20.43, P = 0.0011) on live Staghorn Coral (mean = 4.18 ± 3.611 SE fish/m2) compared with coral rubble (mean = 1.50 ± 1.155 SE fish/m2). Differences were also found in damselfish density relative to life stage between and within the two habitat sites. Adult damselfish density was significantly higher (one-way ANOVA: F1,10 = 46.24, P < 0.0001) on live Staghorn Coral (mean = 4.13 ± 3.089 SE fish/m2) compared with coral rubble (mean = 0.62 ± 0.327 SE fish/m2). Conversely, juvenile damselfish density was significantly higher (one-way ANOVA: F1,10 = 45, P < 0.0001) on coral rubble (mean = 0.88 ± 0.961 SE fish/m2) compared with live Staghorn Coral (mean = 0.04 ± 0.135 SE fish/ m2). Adult damselfish density was significantly higher than juvenile damselfish density on live Staghorn Coral (blocked ANOVA: F1,7 = 23.97, P = 0.0018), but no significant difference was found on coral rubble (blocked ANOVA: F1,3 = 18, P = 0.024). Species composition differed within the two habitat sites as well. On live Staghorn Coral (Fig. 2A), the density of Stegastes adustus (Troschel) (Dusky Damselfish) was significantly higher (blocked ANOVA: F4,28 = 8.32, P = 0.0001) than Microspathodon chrysurus (Cuvier) (Yellowtail Damselfish), S. leucostictus (Muller and Troschel) (Beaugregory Damselfish), S. variabilis (Castelnau) (Cocoa Damselfish) and Stegastes planifrons (Curier) (Three-spot Damselfish) (similar densities). On coral rubble (Fig. 2B), Beaugregory and Cocoa Damselfish (similar densities) had significantly higher densities (blocked ANOVA: F4,12 = 104.5, P < 0.0001) than Yellowtail, Dusky, and Three-spot Damselfish (similar densities). The pattern of species density by habitat and life stage was variable. Juvenile density on live Staghorn Coral was highest for Beaugregory Damselfish and lowest for Cocoa Damselfish, but adult density was highest for Dusky Damselfish and lowest for Beaugregory Damselfish (Table 1). Juvenile density on coral rubble was highest for Beaugregory Damselfish and lowest in Yellowtail and Three-spot Damselfish, but adult density was highest for Cocoa and Beaugregory Damselfish and lowest in Yellowtail Damselfish (Table 1). No juvenile Dusky Damselfish were found on either habitat type. 2008 A.A. Wilkes et al. 487 Discussion Complex reef topography of branching corals like Acropora are thought to be a major factor affecting reef fish distribution and abundance, and any stressor that modifies coral morphology will likely have significant and unpredictable impacts on associated reef fish populations (Lirman 1999). Figure 2. Damselfish density by species ( ± SE) on live Staghorn Coral (A) and coral rubble (B) in the Dry Tortugas National Park. Groups marked with different letters are significantly different based on Tukey’s multiple comparison test. Table 1. Damselfish density by species and life stage ( ± SE) on live Staghorn Coral (Acropora cervicornis) and coral rubble within the Dry Tortugas National Park, May 2004. Live Staghorn Coral Coral rubble Adult Juvenile Adult Juvenile (fish/m2) (fish/m2) (fish/m2) (fish/m2) Species (common name) N = 8 N = 8 N = 4 N = 4 Stegastes adustus 2.01 ± 1.078 Not observed 0.11 ± 0.390 Not observed (Dusky Damselfish) Stegastes variabilis 0.45 ± 0.711 0.002 ± 0.016 0.23 ± 0.291 0.35 ± 0.428 (Cocoa Damselfish) Stegastes leucostictus 0.24 ± 0.381 0.02 ± 0.082 0.23 ± 0.135 0.51 ± 0.570 (Beaugregory Damselfish) Microspathodon chrysurus 0.50 ± 0.774 0.01 ± 0.047 0.01 ± 0.125 0.01 ± 0.059 (Yellowtail Damselfish) Stegastes planifrons 0.94 ± 0.163 0.01 ± 0.047 0.05 ± 0.138 0.01 ± 0.060 (Three-spot Damselfish) 488 Southeastern Naturalist Vol.7, No. 3 Within DTNP, loss of branching coral has resulted in an increase in rubble habitat, which heavily favors juvenile fishes, while the more dimensionally complex habitats offered by live Staghorn Coral are dominated by adults. The higher adult numbers (three times higher than nearby rubble sites) may indicate that branching habitat is more amiable to adult fish; however, inflated adult densities resulting from a relative scarcity in branching corals would also explain our findings. While it is unknown how important resources may differ between rubble and live branching coral habitats, greater fish densities in complex reef habitat have been linked to inherently higher numbers of desirable territories for feeding, shelter, or reproduction (Almany 2004, Holbrook et al. 2000) Despite the fact that pre-impact data are not available, our study, although limited in scope, suggests that reductions in damselfish density are the likely outcome in reefs where expanses of live branching coral are in decline and are being replaced by relatively low-dimensional fields of reef rubble. Luckhurst and Luckhurst (1978) suggested that an increase in habitat complexity also leads to an increase in reef fish species richness. However, we observed the same damselfish species present on both coral rubble and live Staghorn Coral sites. While no damselfish species found inhabiting live Staghorn Coral were completely absent from the coral rubble habitat, damselfish community structure displayed marked shifts. Dusky Damselfish dominated on live Staghorn Coral, whereas Cocoa and Beaugregory were dominant on coral rubble. Some damselfish species may require habitat complexity provided by branching corals (Robertson 1984), whereas others, such as Cocoa Damselfish, are better suited to exploit a wider range of habitat types and display no specific coral preference (McGehee 1995, Wallman et al. 2004). Fangue et al. (2001) found Cocoa Damselfish thriving in hyperthermic and hypoxic tidepools on the northwest corner of Loggerhead Key in DTNP. Damselfish species able to tolerate the widest range of environmental conditions may dominate following a habitat disturbance. Additionally, damselfish population relative to life stage differed signifi- cantly between living Staghorn Coral and coral rubble. The number of adult damselfish on live Staghorn Coral decreased by 85% relative to coral rubble habitats, while the number of juvenile damselfish increased by 95%. Higher adult density on live Staghorn Coral may be related to the increase in threedimensional habitat that would provide predator refuge dimensions more conducive to adult body sizes that require larger shelter spaces (Almany 2004, Holbrook et al. 2000, Nemeth 1998). Numerous smaller hiding spaces along with relatively low adult densities may allow juvenile damselfishes to more effectively exploit rubble habitats. Lirman (1994) reported that juvenile Three-spot Damselfish on the Honduras Barrier Reef off Roatan Island preferred dead coral. Similarly, our data showed juvenile damselfish reached their highest density on coral rubble. Implications of damselfish life-stage changes between reef and rubble are not immediately clear. Coral rubble areas are always present to some degree 2008 A.A. Wilkes et al. 489 near branching coral reefs and probably serve as important nursery areas to juvenile and sub-adult fishes before they recruit to the reef. Damselfish larvae have a short dispersal time, typically two to five weeks, and fish are not believed to recruit from distant reefs (Floeter and Gasparini 2000), but rather settle in nearby areas (Paris and Cowen 2004, Sweatman 1985, Victor 1986). Lower recruitment and associated reduction in population size may also result in potential loss of reproductive variability as well as decreased physiological or behavioral plasticity. If branching coral loss continues to exceed the reef rebuilding rate, the decrease in adult numbers may affect damselfish recruitment throughout DTNP, although some species may be less affected than others. Wallman et al. (2004) reported patch reefs within DTNP supported populations of adult Dusky, Cocoa, and Yellowtail Damselfish. These species may recover more quickly on new Staghorn formations due to rapid recolonization from nearby patch reefs. Given the important ecological role of damselfish as a keystone species, it is likely that changing damselfish populations associated with loss of branching coral will alter reef structure and fauna in DTNP. If Staghorn Coral should begin to recover, it is unclear if damselfish and other reef fish populations will revert to configurations seen prior to coral loss of the 1970s, or assume some new equilibrium. Coral reefs are sensitive habitats that have not always recovered from environmental stressors. For example, Caribbean reefs have never fully recovered from ecological consequences associated with the Diadema antillarum population crash of 1983–84 (Chiappone et al. 2002). There can be little doubt that coral reef ecology is shaped by relationships between reef residents and corals that provide the structural habitat underpinnings. Damselfishes are a notably important member for fish reef fauna because they are known to directly shape structure and fish distribution through their feeding and guarding behaviors (Almany 2004, Hixon and Brostoff 1983). Our findings suggest that loss of branching coral habitats from cold and disease (Davis 1982) have affected damselfish assemblages in DTNP, and these changes could have broad implications to reef ecology for not only the Florida reef track, but throughout the Caribbean. 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