Small Coral Research Articles

Size matters in competition between corals and macroalgae

Corals and macroalgae compete for space, but the influence of species and size on the competitive outcome is poorly understood. Using a manipulative experiment, we evaluated the effect of macroalgal competition on the growth rate of corals with an emphasis on the colony size, species identity and the intensity of competition. Coral-macroalgal competition was studied among 3 Caribbean coral species (Porites astreoides, Agaricia agaricites and Colpophyllia natans) and 2 macroalgal species (Lobophora variegata and Halimeda opuntia) for 1 yr. Two coral colony sizes were used and, for the smaller size class, 2 levels of intensity for macroalgal competition (25 and 100% contact with the coral perimeter). Coral size had the greatest effect on competitive outcome; 2 species of large corals under competition grew as much as controls and a third species did not lose tissue. All small colonies lost between 18 and 22% of their original area after a year of competing with macroalgae, and the competitive outcome was insensitive to algal species. Coral colony size is a critical factor for the competitive outcome with algae and the intensity of contact between competitors is not important in smaller corals. In general, our results support the theory that algal blooms can inhibit coral population dynamics by causing a bottleneck in the survivorship of smaller size classes.

Water scarcity in the Spermonde Archipelago, Sulawesi, Indonesia: Past, present and future

Abstract Water scarcity is a serious issue on many small coral islands, including the Spermonde Archipelago in South Sulawesi, Indonesia. An important driver behind it is population growth. Additionally, the predicted impacts of climate change will further accelerate the problem. While a lot of scientific attention has been paid to water scarcity in the Pacific, little is known about the situation in Indonesia. The paper presents the results of the first scientific study on water scarcity in an Indonesian Archipelago. It combines an analysis of historical and contemporary documents with a study about local perceptions on water scarcity. 160 people on four case study islands were interviewed about water availability and quality in the past and at present. The results show that although water scarcity is not a new issue, it has increased from the 1960s onwards. Possible anthropogenic and natural drivers behind this development are discussed, and management implications presented. It is argued that management measures need to be island-specific and require local institutions for water management.

O povo que mede forças com a morte: os ilhéus de Tuvalu, no Pacífico Sul, e a subida das águas do mar

Localizado no Pacífico Sul, a cerca de 1.100 km ao norte das Fiji, o arquipélago Tuvalu é constituído por nove ilhas de coral. Soma meros 26 km² de superfície, onde residem cerca de 11.000 ilhéus, no geral, pescadores ou agricultores de subsistência, dedicados ao cultivo das nativas pulaka e taro, assim como fruta-pão e coco. O povo pertence ao grupo étnico polinésio, sendo a sua cultura e o tipo físico bastante homogêneos de ilha para ilha. Em fevereiro de 2010, o Instituto de Investigação Científica Tropical realizou uma missão científica ao atol de Funafuti, conhecido mundialmente pela sua vulnerabilidade às mudanças climáticas, devido à subida do nível das águas do mar. Os objetivos do projeto etnogeográfico foram dois: avaliar a percepção que os ilhéus têm das mudanças climáticas e, coerentemente, avaliar a sua percepção da morte. A amostra recolhida no atol foi composta por cinquenta e oito entrevistas semiestruturadas. Os resultados demonstraram que os ilhéus, majoritariamente cristãos, não temem a subida das águas do mar, assim como não temem a morte, devido ao fato de possuírem uma confiança inapelável na Divina Providência.

The influence of canopy structure and tidal level on fish assemblages in tropical Southeast Asian seagrass meadows

Seagrass meadows support abundant and diverse fish assemblages, but there are very few studies on the relation between seagrass beds with distinctly different plant canopies and their associated fish fauna. In the present study, fish assemblages were investigated by underwater visual census at intertidal and subtidal sites with varying seagrass species composition, shoot density, biomass, and leaf area index (LAI) on two small coral islands in the Spermonde Archipelago, Indonesia. We investigated (1) whether fish assemblages in distinctly different seagrass beds differ regarding community parameters, and (2) whether seagrass parameters affect fish abundances. Overall, more than 120 fish taxa were found. Bray–Curtis cluster analysis and non-Metric Multidimensional Scaling ordination (nMDS) showed site-specific similarities for fish assemblage structure with a distinct separation into subtidal and intertidal sites. Species accumulation curves for gamma diversity, single study sites, and the two most diverse fish families (Labridae, Pomacentridae), suggesting that species numbers are likely to increase with diel sampling. Total fish abundance and abundance for six out of the nine most common species varied distinctly among the study sites. The study indicates that seagrass beds with differing canopy parameters support distinct fish assemblages that differ with respect to species richness, dominant species, and the abundance of total and most common fish species. These differences are likely due to different canopy structures and water depth.

From Citizen Science to Policy Development on the Coral Reefs of Jamaica

This paper explores the application of citizen science to help generation of scientific data and capacity-building, and so underpin scientific ideas and policy development in the area of coral reef management, on the coral reefs of Jamaica. From 2000 to 2008, ninety Earthwatch volunteers were trained in coral reef data acquisition and analysis and made over 6,000 measurements on fringing reef sites along the north coast of Jamaica. Their work showed that while recruitment of small corals is returning after the major bleaching event of 2005, larger corals are not necessarily so resilient and so need careful management if the reefs are to survive such major extreme events. These findings were used in the development of an action plan for Jamaican coral reefs, presented to the Jamaican National Environmental Protection Agency. It was agreed that a number of themes and tactics need to be implemented in order to facilitate coral reef conservation in the Caribbean. The use of volunteers and citizen scientists from both developed and developing countries can help in forging links which can assist in data collection and analysis and, ultimately, in ecosystem management and policy development.

The evolution of mid Paleocene-early Eocene coral communities: How to survive during rapid global warming

Today, diverse communities of zooxanthellate corals thrive, but do not build reef, under a wide range of environmental conditions. In these settings they inhabit natural bottom communities, sometimes forming patch-reefs, coral carpets and knobs. Episodes in the fossil record, characterized by limited coral-reef development but widespread occurrence of coral-bearing carbonates, may represent the fossil analogs of these non-reef building, zooxanthellate coral communities. If so, the study of these corals could have valuable implications for paleoenvironmental reconstructions. Here we focus on the evolution of early Paleogene corals as a fossil example of coral communities mainly composed by zooxanthellate corals (or likely zooxanthellate), commonly occurring within carbonate biofacies and with relatively high diversity but with a limited bioconstructional potential as testified by the reduced record of coral reefs. We correlate changes of bioconstructional potential and community compositions of these fossil corals with the main ecological/environmental conditions at that time. The early Paleogene greenhouse climate was characterized by relatively short pulses of warming with the most prominent occurring at the Paleocene-Eocene boundary (PETM event), associated with high weathering rates, nutrient fluxes, and pCO2 levels. A synthesis of coral occurrences integrated with our data from the Adriatic Carbonate Platform (SW Slovenia) and the Minervois region (SW France), provides evidence for temporal changes in the reef-building capacity of corals associated with a shift in community composition toward forms adapted to tolerate deteriorating sea-water conditions. During the middle Paleocene coral–algal patch reefs and barrier reefs occurred from shallow-water settings, locally with reef-crest structures. A first shift can be traced from middle Paleocene to late Paleocene, with small coral–algal patch reefs and coral-bearing mounds development in shallow to intermediate water depths. In these mounds corals were highly subordinated as bioconstructors to other groups tolerant to higher levels of trophic resources (calcareous red algae, encrusting foraminifera, microbes, and sponges). A second shift occurred at the onset of the early Eocene with a further reduction of coral framework-building capacity. These coral communities mainly formed knobs in shallow-water, turbid settings associated with abundant foraminiferal deposits. We suggest that environmental conditions other than high temperature determined a combination of interrelated stressors that limited the coral-reef construction. A continuous enhancement of sediment load/nutrients combined with geochemical changes of ocean waters likely displaced corals as the main bioconstructors during the late Paleocene-early Eocene times. Nonetheless, these conditions did not affect the capacity of some corals to colonize the substrate, maintain biodiversity, and act as locally important carbonate-sediment producers, suggesting broad environmental tolerance limits of various species of corals. The implications of this study include clues as to how both ancient and modern zooxanthellate corals could respond to changing climate.

Effects of Flow and Temperature on Growth and Photophysiology of Scleractinian Corals in Moorea, French Polynesia

To test for threshold effects in the response of coral physiology to increasing seawater flow, field and laboratory experiments were conducted in Moorea. First, the growth of juvenile massive Porites spp. and branching P. irregularis was compared among habitats differing in water motion. Growth of massive Porites spp. responded to flow in a pattern consistent with a threshold effect, whereas growth of P. irregularis increased linearly with flow. Second, a recirculating flume was used to test the effect of flow on photophysiology (ΔF/F(m)', effective photochemical efficiency) for massive Porites spp.; ΔF/F(m)' displayed a threshold response at 23 cm s(-1) and 28 °C, but not at 31 °C. Finally, intra-colony variation in the response of ΔF/F(m)' to flow and temperature was explored to evaluate the functional significance of colony shape in small corals. ΔF/F(m)' on the top and upstream surfaces of massive Porites spp. responded with a threshold effect of flow at 28 °C (but not 31 °C), but ΔF/F(m)' on downstream surfaces was unresponsive to flow. ΔF/F(m)' for P. irregularis was less responsive to flow than for massive Porites spp., suggesting that the photophysiological response of corals to varying flow speeds may differ between species and morphologies. Together, these results emphasize that flow can have diverse effects on the physiology of corals, with the outcome depending on flow speed, temperature, location on the colony, and perhaps morphology.

Size-structure patterns of juvenile hard corals in the Maldives

In April 2006 and May 2007 abundance data of small coral colonies were collected in the central atolls of the Maldives (N3°35.9–4°26.6 E72°47.3–73°57.5) in order to evaluate variability in hard coral recruitment and post-settlement success. Visual quadrats were randomly placed in two reef typologies (oceanic reef and lagoonal reef) at three different depths. Colonies were conventionally defined as ‘recruits’ when smaller than 5 cm in diameter, whereas the term ‘juveniles’ was reserved for colonies ranging between 5 and 15 cm. Clear differences in the relative importance of the two size-classes across the three different depths and the two reef typologies were found. A size-structure index (SsI%), based on abundance data, was calculated in order to evaluate the percentage of ‘recruits’ out of the total juvenile hard corals. SsI% values differed between reef typologies and according to depth zones with a higher percentage of ‘recruits’ found in shallow oceanic reefs. Physical and ecological differences between the two reef typologies according to depth are presumed to cause different success in the settlement of new colonies and their survival during growth. Coupled with the usual estimates of hard coral cover, SsI% may represent a useful tool for monitoring, and be effective for the quick assessment of coral reef recovery after disturbances.

Beijing's South China Sea Debate

The three million square kilometres of the South China Sea are of particular strategic importance. Sovereignty over a plethora of small islands, atolls, rocks and coral reefs, including the two mai...

Hydrochemical analysis to evaluate the seawater ingress in a small coral island of India

The sustainable development of the limited groundwater resources in the tropical island requires a thorough understanding of detail hydrogeological regime including the hydrochemical behavior of groundwater. Detail analysis of chemical data of groundwater helps in assessing the different groundwater zone affected by formation as well as sea water. Groundwater and saline water interaction is better understood using groundwater major ion chemistry over an island aquifer. Multivariate methods to analyze the geochemical data are used to understand geochemical evolution of groundwater. The methods are successfully used to group the data to evaluate influence of various environs in the study area. Various classification methods such as piper, correlation method, and salinity hazard measurements are also employed to critical study of geochemical characteristics of groundwater to identify vulnerable parts of the aquifer. These approaches have been used to successfully evaluate the aquifer zones of a tiny island off the west coast of India. The most part of island is found to be safe for drinking, however some parts of island are identified that are affected by sea water ingress and dissolution of formation minerals. The analysis has successfully leaded to identification of that part of aquifer on the island which needs immediate attention for restoration and avoids further deterioration.

Emigration behaviour during sea-to-land transition of the coconut crab Birgus latro: Effects of gastropod shells, substrata, shelters and humidity

Populations of the coconut crab, Birgus latro, have been severely depleted on most inhabited islands throughout the Indo-Pacific because of over-harvesting and environmental degradation. Coconut crabs emigrate from the sea as megalopae carrying gastropod shells. Better knowledge of emigration behaviour can assist in conservation of coconut crab populations through understanding and protecting the habitats indispensable for completing their life cycle. We examined the behaviour of coconut crabs during sea-to-land transition by culturing megalopae and juveniles in containers that hold sea-water and hard substrate simulating an intertidal microhabitat in nature. The test environmental factors were availability of gastropod shells and substrata (small and coarse coral sands) including shelters on land, and humidity (relative humidity, 76% and 98%). Megalopae matured their nature for acquiring shells and emigration from the sea in around 10 days. Humidity and shell availability affected animal behaviour. Low humidity decreased the proportion of animals on land, and burrowing behaviour was stimulated in low humidity and shell-less conditions. Animals in low humidity had a low survival rate. Shelters and shells stimulated the proportion of animals on land at low humidity, and animals concentrated under the shelters on land. Shells, burrows and shelters should act to protect animals from desiccation by keeping microclimates around animals on high humidity. Thus, coconut crab megalopae and juveniles preferred a humid environment, and high-humidity conditions stimulated emigration from the sea. Our findings highlight the importance of a humid environment around the seashore in sustaining the recruits for coconut crab populations.

Isabelle Côté

Isabelle Côté

Shortspine thornyhead and rockfish (Scorpaenidae) distribution in response to substratum, biogenic structures and trawling

Learmonth Bank in northern British Columbia sustains an active trawl fishery that returns large bycatches of deep-sea sponges and corals. To examine effects of biogenic structures on the distribution of fish, we examined nearly 30 km of high-definition imagery from a remotely oper- ated vehicle and documented locations of 2770 scorpaenid fish. The 2 local genera have similar abun- dances, averaging about 1.2 individuals 100 m -2 , but have different spatial abundance patterns: shortspine thornyhead Sebastolobus alascanus are randomly distributed on featureless substrata and their abundance increases with depth. Rockfish Sebastes spp. associate with higher seafloor relief nonrandomly and select for sponges and corals over the inert substrata alone; 95% of the rockfish occurred on 27% of the seafloor surveyed. Sponges (Demospongia and Hexactinellida) were abun- dant on the bedrock and boulders of the bank and adjacent moraine and covered 30 to 55% of the seafloor compared with 1% of the sediment and aggregates of the surrounding basin. The majority of rockfish (80%) occurred with sponges ≥50 cm in height, and even beds of short sponges attracted 4 times as many rockfish than did substrata with no large epifauna. While over half of primnoid corals over 30 cm tall had associated rockfish, less than 2% of the seafloor had large coral, and small coral had no associated rockfish. On the adjacent seafloor with past trawling activity, Primnoa pacifica was 13 times less abundant, and large corals and sponges were rare. Thornyhead abundance doubled but rockfish had a 3-fold reduction in numbers. Our study indicates that degradation of biogenic struc- tures is a long-term detriment to rockfish species and, although the mechanism remains unclear, our data suggest it occurs through the destruction of a habitat that is more effective for shelter than rough inert seafloor.

Phenotypic plasticity in sex allocation for a simultaneously hermaphroditic coral reef fish

Phenotypic plasticity can facilitate reproductive strategies that maximize mating success in variable environments and lead to differences in sex allocation among populations. For simultaneous hermaphrodites with sperm competition, including Serranus tortugarum a small coral reef fish, proportional male allocation (testis in total gonad) is often greater where local density or mating group size is higher. We tested whether S. tortugarum reduced male allocation when transplanted from a higher density site to a lower density site. After 4 months, transplants mirrored the sex-allocation patterns of the resident population on their new reef. Transplants had significantly lower male allocation than representatives from their source population, largely as a result of reduced testis mass relative to body size.

Conventional tagging and acoustic telemetry of a small surgeonfish, Zebrasoma flavescens, in a structurally complex coral reef environment

Passive acoustic telemetry and conventional tag/re-sight techniques were used to study daily movement patterns of adult yellow tang, Zebrasoma flavescens, over a period of months. Range testing and visual observations revealed the limitations of using small acoustic transmitters to monitor movements of small coral reef fish in a topographically complex and noisy coral reef environment. Visual observations of conventionally tagged and albino fish suggest individuals return each day to forage over the same few hundred m2 of shallow, turf algae dominated boulder and reef flat habitat for periods of at least weeks to months. Acoustic telemetry data suggest lower frequency of repeated use of daytime foraging, nighttime refuge and sunset spawning sites. However, integration of observation and acoustic telemetry data revealed that many fish were not detected while they were within the empirically tested range of the receivers. These observations indicate that data from passive acoustic telemetry can underestimate the frequency and duration of repeated use of specific areas. Yellow tang adults made daily crepuscular migrations of up to 600 m between foraging and spawning or sheltering sites at consistent times relative to sunset and sunrise. While there was high individual variability in migration distance, almost all individuals moved in the same direction (from south to north) at sunset. This study provided valuable information for evaluating ongoing fishery management efforts using marine protected areas in Hawaii.

Optimization of pumping rate and recharge through numerical modeling with special reference to small coral island aquifer

The groundwater is the only source of availability of fresh water in tiny coral islands. In the past decades, there has been growing demand for fresh water to meet the need of domestic besides other purposes. The aquifer system on these islands is fragile besides being subjected to various stresses like high subsurface discharge, increased abstraction, improper disposal of waste water and tidal waves of ocean all of which subject the aquifer prone to sea water intrusion and thus reduction and deterioration the water quality. Therefore, understanding the aquifer’s behavior and then work out a sustainable option for fresh water is essential. The paper concerns optimizing of pumping and artificial recharge paces to reduce the effects of various stresses over tiny and fragile lens-shaped coral island aquifer system. The density driven ground water flow was simulated using SEAWAT (MODFLOW and MT3D based computer program) model. Detailed hydrogeological investigations were carried out to determine the quantity of freshwater that could be pumped to avoid the seawater intrusion into the aquifer through modeling. Initial heads, physical parameters and boundary conditions of the study area have been defined in the model based on field data, geophysical measurements and interpretations and hydrogeological studies. The model was calibrated by obtaining a match of computed and observed values of the water table, as hydraulic head is much more sensitive to pumping rates than any other stress. A few sentences about: flow model were utilized to derive optimal pumping rate; the effect of artificial recharge through the model, has also proved that the salt-water intrusion could be stopped by raising the water level through temporarily storing the artificially recharged water post construction of subsurface dam near the coast.

Resurrection of Porites hawaiiensis Vaughan, 1907; a Hawaiian coral obscured by small size, cryptic habitat, and confused taxonomy

The purpose of this note is to propose recognition of Porites hawaiiensis Vaughan, 1907, (Figure 1A–D) a species currently regarded as a junior synonym of Porites rus (Forskål 1775), as a valid species, based on molecular and morphological characteristics. Vaughan (1907 p. 217, pl 91 figs 2, 2a) described Porites (Synaraea) hawaiiensis from a specimen collected from Kalihi Harbor on the island of O‘ahu, Hawai‘i (Figure 1 C). Porites (Synarea) hawaiiensis was also reported from the Marshall Islands by Wells (1954 p. 455, pl 170 figs 6,7). Porites hawaiiensis was subsequently thought to be a junior synonym of Porites (Synaraea) convexa Verrill, 1864, due to the small calices that are characteristic of the subgenus Synaraea (Maragos 1977). Later both species were made synonyms of P. (Synaraea) rus, Forskål 1775 (Veron & Pichon 1982; Cairns 1991). Vaughan, 1907 described the calices of P. hawaiiensis as “densely spinulose” with “coenchyma” equaling, or exceeding the 0.5 mm diameter of the calices, and a pitted star shaped space between the pali (Figure 1C,D). In the absence of living specimens, the Vaughan, 1907 type specimen was difficult to distinguish from newly settled P. rus colonies, but upon closer examination in the field, Maragos et al. (2004) recognized small coral colonies that appeared to match the description of P. hawaiiensis. This species can readily be distinguished from Porites rus and other Porites by very small colony size (<10cm), mottled yellow and green-brown coloration, encrusting form, and thicket of spiny denticles between distantly spaced corallites (Figure 1A–D). Genetic data from Forsman et al. 2009 confirmed that this small ‘patch coral’ is distinct from P. rus (n = 3 of each species; uncorrected pair-wise distance; mtCOI = 0.5% ± 0.2 SE; mtCR = 0.7%, and nuclear ITS = 14.2 % ± 1.3 SE), and is also distinct from all other Hawaiian congeners. The genetic data further indicated that ‘Synaraea’ was surprisingly closely related to other Poritids and may not warrant sub-genus status (Forsman et al. 2009). Fenner (2005) referred to this same small ‘patch coral’ as Porites cf. bernardi, however; P. bernardi Vaughan, 1907 type specimens were coralliths (Figure 1E) with calices similar in size to those of most other Porites (Figure 1 F). The geographic range of Porites hawaiiensis is unknown, although it is abundant throughout the Northwest and Main Hawaiian Islands, and has been reported at depths from 1 to 55m (30 fathoms) in the Marshall Islands (Wells 1954). This species can be easily overlooked; it tends to grow in cryptic habitats (cracks, crevices, and interstitial spaces), and at first glance, the small patches of colonies (0.5–10cm) can be confused with crustose coralline algae, or new recruits of other Porites species. This species is remarkable because of its small adult colony size; a curious life history characteristic since many Porites in the Pacific can be among the largest and longest-lived scleractinain corals (Brown et al. 2009). We propose that this small ‘patch coral’ is a distinct species, and that P. hawaiiensis is the most appropriate name.

Géomorphologie de Tromelin, océan Indien

Tromelin is a small coral reef island (1 km 2) located in the Indian Ocean, approximately 440 km east of Madagascar and 580 km north of La Réunion. Despite the presence of a permanent Météo France weather station on Tromelin since the 1950 s, a detailed geomorhological study of the island has never been undertaken. In this paper, we describe results from the 2008 field season, which enabled us to map and describe seven geomorphological zones on the atoll. (1) Tromelin's bathymetry is characterised by a pronounced submarine slope, which attains depths of 1000 m at only 2.5 km from the island's coastline. This geomorphology is typical of volcanic hotspots. (2) A coral platform borders the emerged crown of the island. This platform is exposed at low tide and has been fashioned in an ancient Pleistocene substratum. The substratum has been eroded by abrasive marine action to its present shape since the stabilisation of relative sea level around 6000 years ago. Erosion of the reef yields most of the biogenic sediment supply to the island. (3) The upper foreshore is characterised by outcrops of beachrock. These formations show a stratfication in slabs and dip gently towards the sea in the same manner as the contemporary clastic sediments. (4) Tromelin's beaches are divided into three sub-units, which reflect the different energy dynamics around the island, in addition to the transfer of sediment from the windward to the leeward side: (i) the southern storm beaches are characterised by coral blocks reworked during episodic high-energy events; they form boulder ramparts. Four perched rampart ridges have been recorded at ∼1 m, ∼3 m, ∼4.5 m and ∼7 m above mean sea level; (ii) transitional beaches are observed between the south and the north, constituting storm blocks and coarse sands and gravels; and (iii) sandy beaches are noted on the northern leeward side, comprising rounded gravels and coarse sands. The northern tip of the island is characterised by a sand spit (∼125 × ∼225 m), whose geometry varies based on the seasonal and meteo-marine contexts. (5) On Tromelin, a number of dune formations are observed. These are best developed in the north of the island due to the south-north translation of clastic sediments by aeolian wind action, and the trapping of sands by the well-developed vegetation. The height of the dunes varies between 10 cm (the southern micro-dunes) to 250 cm in the Northeast of Tromelin. (6) Exceptional waves have deposited storm tracts up to 250 m from the foreshore zone, comprising coarse gravels and coral blocks. The storm tract deposits are most prevalent on the windward side of the island, due not only to the high-energy dynamics, but also to the absence of vegetation cover in this area. (7) The southern depression covers an area of ∼40,000 m 2. At its lowest point it is only 1.2 m above sea level.

Environmental contamination associated with a marine landfill (‘seafill’) beside a coral reef

In Bermuda, bulk waste such as scrap metal, cars, etc., and blocks of cement-stabilized incinerator ash (produced from burning garbage) are disposed of in a foreshore reclamation site, i.e., a seafill. Chemical analyses show that seawater leaching out of the dump regularly exceeds water quality guidelines for Zn and Cu, and that the surrounding sediments are enriched in multiple contaminant classes (metals, polycyclic aromatic hydrocarbons, petroleum hydrocarbons, dioxins and furans, polychlorinated biphenyls and an organochlorine pesticide), i.e., there is a halo of contamination. When compared against biological effects-based sediment quality guidelines (SQGs), numerous sediment samples exceeded the low-range values (where biological effects become possible), and for Hg and Zn exceeded the mid-range value (where they become probable). A few metres away from the edge of the 25acre dump lies a small coral patch reef, proposed here as most contaminated coral reef in the world.

Experimental evidence for density‐dependent reproductive output in a coral reef fish

AbstractWe tested for density‐dependent reproduction in a small coral reef fish using field manipulations of density and observational data. Males of the study species, the bridled goby (Coryphopterus glaucofraenum Gill), defend benthic nest sites, within which they spawn with females, and females can spawn repeatedly over an extended breeding season. In small areas, usually only a single male nested at any one time regardless of how many males were present, so the probability of nesting was inversely proportional to density. Nesting males were almost always the largest in the vicinity, suggesting that, for males whose home ranges overlap, social interactions dictate opportunities to nest. Both the per capita rate at which clutches were laid and the number of eggs produced per clutch declined with increasing density, so the per capita rate of egg production was also density dependent. All three measures of fecundity were better predicted by numerical density (numbers per unit area) than biomass (mass of fish per unit area), and were well described as an inverse function of the number of gobies in the vicinity. A simple hypothesis consistent with these results is that a constant number of females spawn, regardless of density. Alternately, the effect of crowding may depend primarily on the number of interacting individuals and affect all females relatively equally. This density dependence could thus contribute to population regulation at the spatial scale over which populations become reproductively closed.