Fish yield of Kilifi coral reef in Kenya

The fish yield on Kilifi reef which is about 4 km² was estimated for three years: 1982–1984. It was found that the yield on the reef ranged from 5.07 t km^–2 to 12.9 t km^–2 with a mean of 8.8 t km^–2 year^–1. The major groups of fish caught were mostly Siganidae, Scaridae, Plectorhychidae, Scombridae, Lutjanidae, Serranidae, Carangidae, Sphyreanidae and Ceasiodidae. There were more fish caught during the northeast monsoon when the sea was calm than during the southeast monsoon when the sea was rough.     

Varying responses of herbivorous and invertebrate-feeding fishes to macroalgal reduction on a coral reef

 The consequences of macroalgal overgrowth on reef fishes and means to reverse this condition have been little explored. An experimental reduction of macroalgae was conducted at a site in the Watamu Marine National Park in Kenya, where a documented increase in macroalgal cover has occurred over the last nine years. In four experimental 10 m by 10 m plots, macroalgae were greatly reduced (fleshy algal cover reduced by 84%) by scrubbing and shearing, while four similar plots acted as controls. The numerical abundance in all fish groups except wrasses and macroalgal-feeding parrotfishes (species in the genera Calotomus and Leptoscarus) increased in experimental algal reduction plots. Algal (Sargassum) and seagrass (Thalassia) assays, susceptible to scraping and excavating parrotfishes, were bitten more frequently in the algal reduction plots one month after the manipulation. Further, surgeonfish (Acanthurus leucosternon and A. nigrofuscus) foraging intensity increased in these algal reduction plots. The abundance of triggerfishes increased significantly in experimental plots relative to control plots, but densities remained low, and an index of sea urchin predation using tethered juvenile and adult Echinometra mathaei showed no differences between treatments following macroalgal reduction. Dominance of reefs by macrofleshy algae appears to reduce the abundance of fishes, mostly herbivores and their rates of herbivory, but also other groups such as predators of invertebrates (triggerfishes, butterflyfishes and angelfishes). Accepted: 2 February 1999     

Bitten down to size: Fish predation determines growth form of the Caribbean coral reef sponge Mycale laevis

Interactions between organisms add complexity to ecosystem function, particularly on coral reefs. The Caribbean orange icing sponge Mycale laevis is semi-cryptic, often growing under coral colonies or between coral branches. This association is reportedly a mutualism, with the sponge deterring boring sponges from invading the coral skeleton and the coral providing an expanding surface for sponge growth. But is there an alternative explanation for the proximity of sponge and coral? We examined the importance of fish predation on the growth of the sponge. While the semi-cryptic growth form of M. laevis predominates on reefs off the Florida Keys and the Bahamas Islands, M. laevis grows with a non-cryptic, erect morphology off Bocas del Toro, Panama. Surveys revealed that sponge-eating fishes were rare or absent at Bocas del Toro compared to sites in the Florida Keys. Because past studies were inconsistent about the palatability of M. laevis to fish predators, we conducted feeding experiments with sponges from all three sites. Crude organic extracts of M. laevis from all three sites were palatable to generalist fish predators in aquarium assays, and field feeding assays and caging experiments conducted in the Florida Keys confirmed that spongivorous fishes readily ate exposed fragments of M. laevis. Our results suggest that M. laevis is restricted to its semi-cryptic growth form by spongivorous predators, with corals providing a physical refuge from predation. This alternative explanation supports the broader hypothesis that Caribbean reef sponges can be categorized on the basis of chemical defense into defended, palatable, and preferred species, the last of which are restricted to refugia.     

Influence of recruit condition on food competition and predation risk in a coral reef fish

Settlement rate is considered to be a major determinant of the population structure of coral reef fishes. In this study, the effects of larval physiological condition on survival, predation risk and competitive ability are assessed for a small damselfish, Pomacentrus moluccensis. New settlers were collected and fed for 5 days to produce high and low condition (measured as lipid) treatment fish. In a field experiment, pairs (one high and one low condition fish) were transplanted to corals. Persistence over 2 weeks was much higher (100% vs. 25%) in high condition fish. In mixed groups in the laboratory, high condition fish were both aggressively dominant and consumed more of a limiting prey source than low condition fish. In addition, low condition fish were shown to be at much higher risk of predation. All of the low condition fish but only 33% of high condition fish in mixed groups were consumed by fish predators, and in a separate experiment, 73% of feeding strikes by predators were directed at low condition fish. Quality of new settlers can have an important influence on subsequent juvenile survival. The mechanisms for this effect are likely to include a combination of effects of condition on food competition and predation risk.     

Effects of corallivory and coral colony density on coral growth and survival

A suite of processes drive variation in coral populations in space and time, yet our understanding of how variation in coral density affects coral performance is limited. Theory predicts that reductions in density can send coral populations into a predator pit, where concentrated corallivory maintains corals at low densities. In reality, how variation in coral density alters corallivory rates is poorly resolved. Here, we experimentally quantified the effects of corallivory and coral density on growth and survival of small colonies of the staghorn coral Acropora pulchra. Our findings suggest that coral density and corallivory have strong but independent effects on coral performance. In the presence of corallivores, corals suffered high but density-independent mortality. When corallivores were excluded, however, vertical extension rates of colonies increased with increasing densities. While we found no evidence for a predator pit, our results suggest that spatio-temporal variation in corallivore and coral densities can fundamentally alter population dynamics via strong effects on juvenile corals.

     

Nitrification on a coral reef.

We report that the algal pavement just behind the reef crest at Enewetak Atoll produces nitrate at measurable rates. In situ and in vitro incubations with N-Serve indicate that the autotrophic pathway involving two separate organisms is effective in this oxidation of ammonia to nitrate. Significant nitrification is indicated throughout the reef environment; Nitrobacter agilis has specifically been identified as at least one of the organisms responsible for the terminal oxidation of nitrite to nitrate.     

Fish feeding guilds along a gradient of bay biotopes and coral reef depth zones

The study of fish feeding guild structure is a useful method to compare fish communities of complex marine ecosystems. Guild structure was determined in four coral reef depth zones, viz. the fringing reef at depths of 2, 5, 10, and 15 m, as well as in seven shallow-water biotopes within a single bay, viz. notches in fossil reef rock, mangroves, fossil reef boulders, seagrass beds, algal beds at a depth of 2 m, algal beds at a depth of 5 m, and the channel. The study was done in an inland bay on the Caribbean island of Curaçao, using a visual census technique. Total fish densities within the different feeding guilds varied considerably between the biotopes, and were generally higher in the reef biotopes and on the boulders than in the remaining bay biotopes. Cluster analysis revealed that the greatest dissimilarity in guild structures in terms of fish densities was that between the algal beds and all other biotopes, followed by that between the reef depth zones and other bay biotopes (notches, mangroves, seagrass beds, channel). The species composition of the guilds also differed considerably among the various biotopes. Species richness within the various guilds showed much smaller differences between the biotopes, but was generally somewhat higher in the reef biotopes. Cluster analysis of guild structures in terms of species richness revealed little dissimilarity among the various biotopes. The coral reef was dominated by omnivores and zooplanktivores, whereas the bay was dominated by zoobenthivores and herbivores. Differences in guild structure between the bay and the adjacent reef indicate differences in food availability.     

Density-associated recruitment mediates coral population dynamics on a coral reef

Theory suggests that density-associated processes can modulate community resilience following declines in population size. Here, we demonstrate density-associated processes in two scleractinian populations on the outer reef of Moorea, French Polynesia, that are rapidly increasing in size following the effects of two catastrophic disturbances. Between 2006 and 2010, predation by the corallivorous crown-of-thorns sea star reduced coral cover by 93 %; in 2010, the dead coral skeletons were removed by a cyclone, and in 2011 and 2012, high coral recruitment initiated population recovery. Coral recruitment was associated with coral cover, but the relationship differed between two coral genera that are almost exclusively broadcast spawners in Moorea. Acroporids recruited at low densities, and the density of recruits was positively associated with cover of Acropora, whereas pocilloporids recruited at high densities, and densities of their recruits were negatively associated with cover of Pocillopora. Together, our results suggest that associations between adult cover and density of both juveniles and recruits can mediate rapid coral community recovery after large disturbances. The difference between taxa in sign of the relationships between recruit density and coral cover indicate that they reflect contrasting mechanisms with the potential to mediate temporal shifts in taxonomic composition of coral communities.     

Selective predation for low body condition at the larval-juvenile transition of a coral reef fish

Mortality is known to be high during the transition from larval to juvenile life stages in organisms that have complex life histories. We are only just beginning to understand the processes that influence which individuals survive this period of high mortality, and which traits may be beneficial. Here we document a field experiment that examines the selectivity of predation immediately following settlement to the juvenile population in a common tropical fish, Pomacentrus amboinensis (Pomacentridae). Newly metamorphosed fish were tagged and randomly placed onto replicated patches of natural habitat cleared of resident fishes. After exposure to transient predators for 3 days, fish were recollected and the attributes of survivors from patch reefs that sustained high mortality were compared to individuals from patch reefs that experienced low mortality. Seven characteristics of individuals, which were indicative of previous and present body condition, were compared between groups. Predation was found to be selective for fish that grew slowly in the latter third of their larval phase, were low in total lipids, and had a high standardized weight (Fultons K). Traits developed in the larval phase can strongly influence the survival of individuals over this critical transition period for organisms with complex life cycles.     

Photoinhibition of photosynthesis on a coral reef

Photoinhibition of macroalgae in the epilithic algal community (KAC) of coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. F_v/F_m (variable to maximum fluorescence, darkened samples) of shallow macroalgae declined by 50% on fine summer and winter days, recovering in late afternoon. Within a species, thalli from low-light habitats were more photoinhibited (2h at 1400μimol m^?2 s^?1) than those from high-light habitats. The sensitivity of Lobophora variegata (Phacophyta) and Chlorodesmis fastigiata (Chlorophyta) increased with depth (1 versus 20 m). However, shallow Halimeda tuna (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas. Photoinhibition and recovery were depth- and species-specific. Shallow Lobophora and Chlorodesmis maintained a greater degree of Q _A oxidation during photoinhibition. In deep thalli, reduced effective quantum yield of open photosystem II centres reflected lower proportions and excitation capture efficiencies of open centres. In Lobophora, zeaxanthin formation accompanied non-photochemical fluorescence quenching (NPQ), but in Chlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may compensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity.     

Predators reduce abundance and species richness of coral reef fish recruits via non-selective predation

Predators have important effects on coral reef fish populations, but their effects on community structure have only recently been investigated and are not yet well understood. Here, the effect of predation on the diversity and abundance of young coral reef fishes was experimentally examined in Moorea, French Polynesia. Effects of predators were quantified by monitoring recruitment of fishes onto standardized patch reefs in predator-exclosure cages or uncaged reefs. At the end of the 54-day experiment, recruits were 74% less abundant on reefs exposed to predators than on caged ones, and species richness was 42% lower on reefs exposed to predators. Effects of predators varied somewhat among families, however, rarefaction analysis indicated that predators foraged non-selectively among species. These results indicate that predation can alter diversity of reef fish communities by indiscriminately reducing the abundance of fishes soon after settlement, thereby reducing the number of species present on reefs.     

Fear of fishers: human predation explains behavioral changes in coral reef fishes

Prey flight decisions in response to predation risk are increasingly being considered in conservation and management decisions in the terrestrial realm, but are rarely considered in marine systems. This field-based study investigated how the behavioral response of coral reef fish families varied along a gradient of subsistence fishing pressure in Papua New Guinea. Specifically, we examined how fishing pressure was related to pre-flight behavior and flight initiation distance (FID), and whether FID was influenced by body size (centimeters total length), group size (including both con- and hetero-specific individuals), or life-history phase. Fishing pressure was positively associated with higher FID, but only in families that were primarily targeted by spear guns. Among these families, there were variable responses in FID; some families showed increased FID monotonically with fishing pressure, while others showed increased FID only at the highest levels of fishing pressure. Body size was more significant in varying FID at higher levels of fishing pressure. Although family-level differences in pre-flight behavior were reported, such behavior showed low concordance with fishing pressure. FID shows promise as a tool by which compliance and effectiveness of management of reef fisheries can be assessed.     

Rapid phase-shift reversal on a Jamaican coral reef

Many Caribbean reefs have experienced a phase-shift in community structure, the principle features being a decline in coral cover and an increase in macroalgal biomass. However, one Jamaican reef—Dairy Bull on the north shore near Discovery Bay—is once again dominated by scleractinian corals and several key species have returned. Living coral cover at 6–8 m depth at Dairy Bull has doubled over the past 9 years and is now ~54%. The absolute cover of Acropora cervicornis was <1% in 1995, but increased to ~11% by January 2004. During this time the cover of macroalgae decreased by 90%, from 45 to 6%. We speculate that long-lived colonies of Montastraea annularis may have facilitated the recovery of this reef by providing structural refugia.     

Spatial and temporal variation in predation on reef fishes by coral trout (Plectropomus leopardus,Serranidae)

The diet of coral trout Plectropomus leopardus (Serranidae) was studied over a two year period at One Tree Island, Great Barrier Reef, Australia. Rapid visual counts demonstrated that P. leopardus were most abundant on the reef slope habitat and inner edge of the enclosed lagoon. Few P. leopardus were found at sites from inner lagoon. It was hypothesized that diet would vary among habitats, times and size classes of coral trout. Ninety-two percent of P. leopardus that contained prey had consumed fish and 87% had only eaten fish. Many types of reef fish were taken by P. leopardus (e.g. Pomacentridae, Scaridae, Blenniidae and Labridae). Most pelagic prey (Clupeidae and Engraulididae) were taken on the reef slope, while some prey were solely or pimarily taken in the lagoon (e.g. Blenniidae and crustaceans). Most pelagic prey were taken on the reef slope in summer by P. leopardus>250 mm (SL). Plectropomus leopardus (<200 mm) from the lagoon had a higher proportion of invertebrates in the diet than fish from the reef slope. Plectropomus leopardus of all sizes ate small fish, while largest fish generally consumed largest prey (especially adult scarids and labrids). I argue that interactions among multiple species of prey and predators need more attention, because piscivores may respond to prey in different ways according to habitat type as well as the number and type of other prey types present. Furthermore, different sizes of fish (e.g. coral trout) may impact assemblages of prey in different ways.     

Spatial and temporal variation in coral predation by parrotfishes on the GBR: evidence from an inshore reef

There have been few studies of coral predation by fishes on the Great Barrier Reef (GBR). However, these studies have indicated that it is an important factor that may shape coral demographics. Here, for the first time, we document the spatial and temporal variation in coral predation by parrotfishes on an inshore reef on the GBR. The densities of parrotfish feeding scars on massive Porites spp. were compared within core and non-core areas of three Chlorurus microrhinos home ranges. The density of parrotfish feeding scars on massive Porites is among the highest recorded on the GBR and elsewhere with a higher abundance of excavating feeding scars within core areas, reflecting the higher occupancy of these areas by C. microrhinos. Furthermore, excavating scars were more abundant in October than in April. This may be related to the higher nutritional quality of coral colonies in October, as coral spawning usually occurs in November at this study location. No spatial or temporal variation was noted in the abundance of feeding scars from scraping parrotfishes. The lack of temporal differences may be a result of the shallow scraping scars which would not be able to reach the gonads within coral polyps. The frequency of parrotfish predation on Porites and the spatial and temporal variation recorded herein highlight the potential importance of parrotfish corallivory on the GBR.