The worldwide decline in coral cover has serious implications for the health of coral reefs. But what is the future of reef fish assemblages? Marine reserves can protect fish from exploitation, but do they protect fish biodiversity in degrading environments? The answer appears to be no, as indicated by our 8-year study in Papua New Guinea. A devastating decline in coral cover caused a parallel decline in fish biodiversity, both in marine reserves and in areas open to fishing. Over 75% of reef fish species declined in abundance, and 50% declined to less than half of their original numbers. The greater the dependence species have on living coral as juvenile recruitment sites, the greater the observed decline in abundance. Several rare coral-specialists became locally extinct. We suggest that fish biodiversity is threatened wherever permanent reef degradation occurs and warn that marine reserves will not always be sufficient to ensure their survival.M any ecologists have expressed concern over the worldwide decline in coral cover due to global warming and associated coral bleaching, overfishing, and coastal pollution (1-5). Coral reefs support a high diversity of fishes that may ultimately depend on corals for their survival; however, the impact of long-term reef degradation on fish populations is unknown. Most attention to the protection of marine fish populations has focused on the benefits of controlling exploitation by establishing ''no-take'' marine reserves (6-8). However, comprehensive strategies for protecting marine biodiversity also require an understanding of how species respond to degradation of their habitats.In the past, there has been a dichotomy of opinion over how closely fish communities are linked to their habitat, with some information indicating a high degree of variability that is independent of habitat change (9-14) and other data showing that coral-specialists clearly suffer when coral cover is reduced (13-17). Here we ask the following questions. If coral reefs continue along a path of degradation, what will be the fate of fish communities as a whole? Will marine reserves provide fish communities with any resilience to the effects of habitat loss? MethodsIn 1996, we observed the beginning of what progressed into a long-term decline in coral cover in four marine reserves in the Tamane Puli Conservation Area, Kimbe Bay, Papua New Guinea (150°06ЈE, 5°25ЈS). To predict the potential response of fish assemblages to declining coral in this area, we began by estimating the proportion of reef fish species that only fed on coral tissue or those that only lived in association with branching corals. We surveyed all species in 20 different families of fishes associated with coral reefs in the region (18). Those species dependent on live coral as food or living space were distinguished from the rest, based both on our own observations and published accounts of diet and habitat associations (19,20).The cover of branching scleractinian corals was estimated from annual surveys of eight reefs between 1996 a...

Many coral reef fishes exhibit distinct ontogenetic shifts in habitat use while some species settle directly in adult habitats, but there is not any general explanation to account for these differences in settlement strategies among coral reef fishes. This study compared distribution patterns and habitat associations of juvenile (young of the year) butterflyfishes to those of adult conspecifics. Three species, Chaetodon auriga, Chaetodon melannotus, and Chaetodon vagabundus, all of which have limited reliance on coral for food, exhibited marked differences in habitat association of juvenile versus adult individuals. Juveniles of these species were consistently found in shallow-water habitats, whereas adult conspecifics were widely distributed throughout a range of habitats. Juveniles of seven other species (Chaetodon aureofasciatus, Chaetodon baronessa, Chaetodon citrinellus, Chaetodon lunulatus, Chaetodon plebeius,Chaetodon rainfordi, and Chaetodon trifascialis), all of which feed predominantly on live corals, settled directly into habitat occupied by adult conspecifics. Butterflyfishes with strong reliance on corals appear to be constrained to settle in habitats that provide access to essential prey resources, precluding their use of distinct juvenile habitats.More generalist butterflyfishes, however, appear to utilise distinct juvenile habitats and exhibit marked differences in the distribution of juveniles versus adults.

Mimicry is a widely documented phenomenon in coral reef fishes, but the underlying relationships between mimics and models are poorly understood. Juveniles of the surgeonfish Acanthurus pyroferus mimic the coloration of different pygmy angelfish Centropyge spp. at different locations throughout the geographic range of the surgeonfish, while adopting a common species-specific coloration as adults. This study examines the ecological and behavioural relationships between A. pyroferus and one of its models, Centropyge vroliki, in Papua New Guinea. Surgeonfish underwent a transition from the juvenile (mimetic) coloration to the adult (non-mimetic) coloration when they reached the maximum size of the angelfish. As typical of mimic-model relationships, mimic surgeonfish were always less abundant than their model. Spatial variation in the abundance of mimics was correlated with models, while the abundance of adults was not. We show that juvenile surgeonfish gain a foraging advantage by mimicking the angelfish. Mimic surgeonfish were always found within 1-2 m of a similar-sized individual of C. vroliki with which they spent c. 10% of their time in close association. When in association with angelfish, juvenile surgeonfish exhibited an increase of c. 10% in the amount of time spent feeding compared to when they were alone. This foraging benefit seems to be explained by reduced aggression by the territorial damselfish Plectroglyphidon lacrymatus, which dominates the reef crest habitat. While adult A. pyroferus and all other surgeonfish were aggressively displaced from damselfish territories, mimic surgeonfish and their models were attacked less frequently and were not always displaced. Stomach contents analysis showed that the diet of C. vroliki differed substantially from P. lacrymatus, while that of A. pyroferus was more similar to the damselfish. We hypothesize that mimics deceive damselfish as to their diet in order to gain access to food supplies in defended areas.

The degree to which species partition resources often depends on the spatial scale of the study. To investigate this, we examined the distribution and abundance patterns of 3 relatively rare pygmy angelfishes (family Pomacanthidae: Centropyge) among reef locations, depths and microhabitats at Lizard Island on the Great Barrier Reef. A strong association among species and between species and their habitat was found at some scales, but not others. On a broad scale, the abundances of the 3 species were highest at the same 4 sites (kilometres apart). These sites were all located at headlands, suggesting that the patterns of abundance may be in response to the topographic features of the island at this scale. Differences among species occurred at intermediate scales (10s to 100s of metres), where species were associated with different depth zones or reef locations. Centropyge vroliki occurred shallower than C. bicolor, while the depth distribution of C. bispinosa overlapped with both of these species. Laterally along the fringing reef, C. vroliki were more abundant in areas where both C. bicolor and C. bispinosa were also abundant, but these 2 latter species were not correlated with each other. The proportions of substratum types present in home patches differed among species, to some extent reflecting the benthic composition of the reef area where each occurred. However on a fine scale (metres), all 3 species appeared to use the same substratum type, which consisted of dead branching coral covered in algae, and occasionally formed multi-species groups. Species abundances were not correlated with this commonly used microhabitat, but rather the availability of the substrata characterizing the reef areas in which they were most abundant. Here, although Centropyge species use the same type of microhabitat, they may be an example of species that partition space on the basis of non-preferred resources. For example, all 3 species used home patches containing high proportions of overgrown corals; however where this habitat was not available, C. bicolor used sand and rubble habitat at the reef base, while C. vroliki used coral habitat on the reef crest. Most importantly, this study emphasizes that a multi-scale approach is necessary to determine appropriate scales for examining species associations and resource partitioning in reef fishes.

On coral reefs, sites that receive consistently high levels of recruitment relevant to other sites ("re cruitment hotspots") may be crucial to the persistence of populations. However, few studies of coral recruitment have the necessary replication in space and time required to detect recruitment hotspots. The aim of this study was to detect recruitment hotspots at One Tree Reef (southern Great Barrier Reef) and to explore associations between hotspots, hydrodynamics, adult abundance and reef benthos. Recruitment hotspots were detected on the reef slope and in the lagoon. Almost all hotspots were located on the leeward side of the reef, however, there was little congruence among hotspots for different families of coral recruits. Recruitment hotspots in some taxa in some habitats were correlated with water flow or adult abun dance. A clear recruitment hotspot for two families (Pocilloporidae and Poritidae) in the lagoon had medium levels of water flow (-4 cm s −1 ), but there was no re lationship with water flow on the slope. In experimental aquaria, Acropora nasuta settlement was six to 10 times greater under low (2.1 cm s −1 ) compared to medium water flow (4.6 cm s −1 ). Abundance of pocilloporid and poritid, but not acroporid, recruits at each site was often correlated with adult cover indicating either aggregative settlement or limited dispersal. Recruitment hotspots are likely to be both sources and sinks for some taxa, and therefore identifying and protecting hotspots should be a high priority in marine reserve design.

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