A report on the ecological surveys undertaken at Middleton and Elizabeth Reefs, February 2006
A survey of the Elizabeth and Middleton Reefs Marine National Nature Reserve was carried out during February 2006 over the period 9th -16th (Middleton) and 17th-18th (Elizabeth). The primary focus was on Middleton Reef and the provision of estimates of live coral cover, benthic invertebrate abundances including Acanthaster, Drupella and echinoids, black cod and reef shark abundances and a number of fish species endemic to the reserve. In addition abundances of holothurians were estimated.
A total of 397 formal samples were collected to estimate abundances of the above categories of organisms with the majority (351) being carried out at Middleton. Mean live coral cover at Middleton Reef was low with an overall mean of 11.4 per cent. This contrasts with estimates from reefs to the north that vary between 18-30 per cent live cover with some exceeding 40 per cent and also with the estimate of 25 per cent cover from Elizabeth Reef. Low coral cover is a consistent feature of Middleton Reef as the 2006 estimates were similar to those obtained in an earlier survey in 1994 using the same sites as in the present survey. The highest coral cover occurred on exposed reef fronts and crests and in the enclosed lagoon.
No evidence of recent coral bleaching or aggregations of the coral feeding gastropod Drupella were observed. A single small aggregation of Acanthaster on the south-east reef front was located counted and mapped. There was only limited evidence of recent Acanthaster feeding at the aggregation site and no evidence at any other locality examined. The 2006 findings on Acanthaster match those from the 1994 survey in which an aggregation of similar size was located and censused in the western lagoon.
Holothurians dominated by Holothuria atra were common only in lagoonal habitats. Their local distribution was strongly correlated with the presence of sand and sediment.
A total of 28 long-swim transects were done at Middleton Reef to census black cod (Epinephelus daemelii). These returned a mean estimate of 2.9 individuals per hectare with habitat-specific densities ranging from 1.2 to 4.1 per hectare. The highest abundances were in sheltered and lagoonal environments while the lowest were on exposed reef fronts. Estimates of the abundance of the Galapagos shark Carcharhinus galapagensis varied by habitat with very high numbers, 12 per hectare being recorded from enclosed lagoonal environments. Both black cod and Galapagos shark abundances where greater than those achieved by equivalent species in protected lower latitude reef environments.
Abundance estimates of large labrid fishes including the doubleheader, an endemic wrasse, Coris bulbifrons and excavating and scraping parrot fishes, revealed high densities of these functionally important species. Abundances of Coris bulbifrons ranged from two to five per 1000 m2 with the greatest abundances consistently recorded from sheltered and lagoonal habitats. Grazing and excavating parrotfishes achieved their greatest abundances on exposed reef fronts and crests in association with high coral cover. There is evidence that the large excavating parrotfish Chlorurus microrhinos feeds on established colonies of living coral and may be a significant source of coral removal. The most characteristic element of the Middleton Reef fish are the large schools of algal browsing herbivores including the sawtail surgeon fish Prionurus maculatus and the chub Kyphosus pacificus. These species were also characteristic of the outer reef slopes and crests achieving densities more than double those of parrot fishes. Algal grazing invertebrates primarily echinoids did not achieve high abundances at any locality.
Species lists of taxa encountered during the sampling were compiled and a comprehensive list of reef fish species developed. A total list comprised 322 species of reef fishes, of which 51 were new records for the Elizabeth/Middleton Reef system.