Gbr Reef Fishes Patterns in space

Some 1625 species, including trawl fishes, are recorded from the GBR, of which 1468 are coral reef species. This is a relatively high value and may be explained by two critical influences.

First, there is the geographical location of the reef and its setting in terms of the hydrodynamic environment. The position of the GBR relative to the Indonesian and Philippine archipelagoes to the north and the close proximity to the western Pacific reefs has resulted in a diverse fauna of reef fishes. Colonisation by larval reef fish from sources to the north and west are reflected in the strong biogeographical affinities of the GBR fish fauna with widespread tropical Indo-West Pacific elements.

Second, there is the size, configuration and habitat structure of the reef. The GBR is exceptional in terms of

Figure 28.1 A satellite image of the north-east Australian coast showing the GBR and the characteristic structure from the coast to the outer barrier with inner, mid and outer shelf reefs across a longitudinal gradient. Reefs of the Queensland Plateau represent habitats from which reef fishes could recolonise the GBR during periods of rising sea level. A, Coral Sea reefs of the Queensland Plateau; B, Structure of the GBR: 1, Outer reefs exposed to oceanic influences; 2, Mid shelf reefs; 3, Inner reefs and islands merging with mid shelf reefs. (Image by NASA.)

Figure 28.1 A satellite image of the north-east Australian coast showing the GBR and the characteristic structure from the coast to the outer barrier with inner, mid and outer shelf reefs across a longitudinal gradient. Reefs of the Queensland Plateau represent habitats from which reef fishes could recolonise the GBR during periods of rising sea level. A, Coral Sea reefs of the Queensland Plateau; B, Structure of the GBR: 1, Outer reefs exposed to oceanic influences; 2, Mid shelf reefs; 3, Inner reefs and islands merging with mid shelf reefs. (Image by NASA.)

its size (350 000 km2), extending along the east Australian coastline from 10°S to 24°S, a distance of 1200 km (Fig. 28.1). The main structure of the reef terminates just south of the tropic of Capricorn. Beyond this there are a number of isolated coral reefs (Elizabeth and Middleton Reefs and Lord Howe Island) extending as far south as 31°30'S. These support faunas of coral reef fishes and also representatives of subtropical and temperate fish groups. A recent survey of Elizabeth and Middleton identified 322 species of reef associated fishes including 26 groupers, 25 butterfly fishes, 21 benthic feeding dam-selfishes, 22 parrotfishes and 21 surgeonfishes. Some species are endemic to these southern regions. Similar data from the northern end of the reef (Lizard Island) identifies a major shift in diversity, with approximately 900 species of reef associated fishes recorded. For the same groups the diversities were: groupers 41, butterfly fishes 38, benthic feeding damselfishes 48, parrotfishes 25 and surgeonfishes 31. However, the southern reefs also harbour species with warm temperate water affinities including morwongs (Cheilodactylidae), ludericks or sea chubs (Kyphosus, Girella), large pomacentrids (Parma), wrasses (Pseudolabrus) and surgeonfishes (Pri-onurus). These species contribute to the overall diversity of the reef, providing a small but distinctive southern element to the reef fauna (Fig. 28.2).

There are also strong longitudinal trends in reef fish diversity. Comparison of equivalent habitats from inner coastal reefs and mid-shelf and outer barrier reefs at the central region of the GBR revealed the following

Figure 28.2 A, The large surgeon fish Prionurus maculatus schooling on the Middleton reef crest. B, The kyphosid Kyphosus pacificus that frequently schools with P. maculatus on the Middleton reef crest. Both species use fermentation to digest their primary food source: macroscopic algae. These species extend into the southern GBR. C, The girellid fish Girella cyanea schooling with K. pacificus. Girella cyanea feeds on a higher proportion of animal material than the kyphosid. (Photos: A. M. Ayling, Sea Research.)

Figure 28.2 A, The large surgeon fish Prionurus maculatus schooling on the Middleton reef crest. B, The kyphosid Kyphosus pacificus that frequently schools with P. maculatus on the Middleton reef crest. Both species use fermentation to digest their primary food source: macroscopic algae. These species extend into the southern GBR. C, The girellid fish Girella cyanea schooling with K. pacificus. Girella cyanea feeds on a higher proportion of animal material than the kyphosid. (Photos: A. M. Ayling, Sea Research.)

Figure 28.3 A, Inner reef feeding group of grazing fishes. The group consists of two species of parrotfishes Scarus ghobban, often found in non-reef environments and a predominantly inshore species S. rivulatus. Macroscopic algae is usually present at such sites. B, A mixed school of grazing fishes on a mid-shelf reef. These diverse groups of grazing fishes are characteristic of mid-shelf reefs. The example consists of eight species of parrotfish and four species of surgeon-fish. C, The exposed outer barrier reef crest. This group is dominated by the large browsing surgeonfish Naso tonganus and plankton feeding Acanthurus mata. (Photos: A, B, A. Lewis, Tevene'I Marine; C, JCU.)

Figure 28.3 A, Inner reef feeding group of grazing fishes. The group consists of two species of parrotfishes Scarus ghobban, often found in non-reef environments and a predominantly inshore species S. rivulatus. Macroscopic algae is usually present at such sites. B, A mixed school of grazing fishes on a mid-shelf reef. These diverse groups of grazing fishes are characteristic of mid-shelf reefs. The example consists of eight species of parrotfish and four species of surgeon-fish. C, The exposed outer barrier reef crest. This group is dominated by the large browsing surgeonfish Naso tonganus and plankton feeding Acanthurus mata. (Photos: A, B, A. Lewis, Tevene'I Marine; C, JCU.)

differences in species diversities: butterfly fishes: inner 7, outer 15; damselfishes: inner 10, outer 26; parrot-fishes: inner 8, outer 20; surgeonfishes: inner 4, outer 15. An important aspect of this distribution is that some inshore species have restricted distributions that reflect some of the unusual habitat features of inshore reefs (Fig. 28.3). An important summary point is that the GBR maintains a very high diversity of fishes for two reasons. The first is its sheer size, comprising 2000 reefs spread over 350 000 km2. This provides a massive target with a high probability of suitable habitats for larval fishes that may disperse from other regions to the north and east. The second is the variety of habitats, including inshore coastal reefs extending out to reefs exposed to fully oceanic conditions.

The continuous nature of the reef, its latitudinal distribution from tropical to subtropical environments and the strong longitudinal gradient of habitat structure from the coasts to the Coral Sea are factors that underlie the notable diversity of reef fish species. In addition, the unique evolutionary history of the Australian coastal fauna has made an important contribution to the diversity of the present day GBR fish assemblage.

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