Ayling and Choat., 2008: 

Summary

"We also recorded the abundance of large teleost reef fishes during all counts. There were similar patterns in the density of the common coral trout (Plectropomus leopardus), with 1.5 times as many coral trout on green reefs as on blue reefs but 2.5 times as many on pink reefs compared with green reefs.

Both pink reefs and green reefs are no-take areas and, given a similar zoning history, should support similar densities of sharks and coral trout assuming the reefs are similar in ecology, structure, shelf position and latitude.

We suggest that these observed density differences in reef sharks and targeted fishes between the supposed no-take green reefs and the no-go pink reefs are likely to reflect real differences in fishing effort rather than differences in shark and fish behaviour between the different zones. We also think that differences in reef type, habitat structure and shelf position for the Townsville survey reefs would be more likely to increase shark and target fish densities on the green and blue reefs relative to the pink reefs rather than to be the driving force behind the observed density differences."

Coral trout density

"Both species of coral trout are major targets in the Great Barrier Reef line fishery (Mapstone et al. 2004). The common coral trout (Plectropomus leopardus) was significantly more abundant on pink reefs than on green or blue reefs when all size classes were included (table 4, 5, figure 7). Densities on blue and green reefs were not significantly different. However, when only those individuals over the minimum legal length were considered (described here as 'adults'), densities in all three zones were significantly different (figure 8). There were 1.5 times as many adult common coral trout on green reefs than on blue reefs and 2.5 times as many on pink reefs than on green reefs. The proportion of adult individuals in the population was different in the different zones. On pink reefs 73 per cent of common coral trout were adults, on green reefs 61 per cent were adults but on blue reefs only 36 per cent were adults. There were significantly higher densities of common coral trout on the back of green reefs compared to the front but habitat differences were not significant on blue and pink reefs. As a result there was a significant habitat x zone interaction for common coral trout densities (table 4). There were also significant differences in common coral trout densities between the different survey reefs within the three zones (table 4).

The bluespot coral trout (Plectropomus laevis) was significantly more abundant on pink and green reefs than on blue reefs but abundance was similar on both pink and green reefs (table 4, 5, figure 9, 10). Patterns were similar for overall densities and for adult densities of this large coral trout species. Densities of adult bluespot coral trout on pink reefs were almost three times higher than on blue reefs."

 

 

 

Coral trout size

"The mean length of common coral trout on blue reefs was significantly lower than on green and pink reefs (table 6, t test, p<0.05). Although the mean length of bluespot 13 coral trout individuals recorded on pink reefs was nominally higher than those from blue reefs (table 6), these differences were not significant."

 

Density of other target fish groups

"The redthroat emperor (Lethrinus miniatus) is another important target species in the Great Barrier Reef reef line fishery (Mapstone et al. 2004). Densities of this species were similar on blue and green reefs but were significantly higher on pink reefs where numbers were about 2.5 times greater (table 4, 5, figure 11). Although reef dwelling lutjanids are not major targets in the reef line fishery, many species are caught incidentally in the fishery and retained by fishers. Overall densities of lutjanids were also significantly higher on pink reefs than on green or blue reefs (table 4, figure 12). There were no significant patterns in Maori wrasse Cheilinus undulatus abundance on the survey reefs (table 4). This species was previously caught and retained in the reef line fishery but has been totally protected for the past few years."

Density of non-target fish groups

"Densities of fish groups that are not targeted by fishers may provide information on ecological differences between the survey reefs in the different zones. If the reefs have similar physical and biological attributes then densities of non-target species should have similar densities in the three zones unless there are indirect effects of fishing on prey species. There were no significant zone effects on densities of sweetlips (Haemulidae), batfishes (Ephippidae) or parrotfishes (Scaridae) (table 4, 5)."

Comparisons with other surveys

"Comprehensive coral trout counts were made by A.M. Ayling on six offshore Townsville reefs, including Glow and Dip Reefs, as part of the Effects of Line Fishing (ELF) Experiment (Mapstone et al. 2004). These counts used six sites of five 50 x 5 m transects on each reef to search a total area of 7500 sq m (0.75 ha) on each reef compared with 6 Ha per reef for the present survey. Densities of adult common coral trout recorded in the 2004 and 2005 ELF surveys were very similar to those recorded during the present survey (figure 17) despite the very different count methodologies.

Densities of common coral trout on the Great Barrier Reef vary markedly depending on the shelf position of the reef (Ayling and Ayling 1988). This species is rare or absent on turbid inshore and coastal reefs but increases in abundance toward the midshelf, with peak densities found on reefs between 0.4 and 0.8 of the distance across the shelf. Densities then fall rapidly toward the outer edge of the shelf and this species is again rare to absent on the front of reefs on the outer edge of the continental shelf (figure 18). To evaluate whether coral trout densities recorded during the present survey were affected by the relative shelf position of the reefs, the present data were compared with a comprehensive cross-shelf survey of reefs in the Central Section of the Great Barrier Reef Marine Park carried out in 1984 that included many of the present survey reefs (Ayling and Ayling 1986). Coral trout densities on six of the survey reefs were within the expected density range for their shelf position but densities on the two blue reefs Centipede and Grub were well below what would be expected (figure 18)."

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Citation and/or URL

Ayling A.M. and Choat J.H. 2008, Abundance patterns of reef sharks and predatory fishes on differently zoned reefs in the offshore Townsville region: Final Report to the Great Barrier Reef Marine Park Authority. Research Publication No. 91.  

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Spatial Coverage

Townsville region and comparison with northern GBRMP and reefs outside the GBRMP 

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Temporal Coverage

One-off survey 2008, comparative information ranging back to 1984 in some instances

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Update Frequency

Not applicable 

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Other Information