Ayling and Choat., 2008:

"Summary

Robbins et al. (2006) suggested that reef shark populations on the Great Barrier Reef, northern Australia, are in a state of collapse. They reported densities of reef sharks on two no-go Preservation Zone reefs (pink reefs) five to 40 times greater than those on supposed no-take Marine National Park zoned reefs (green reefs). To see if similar shark abundance patterns were present in other Great Barrier Reef regions we counted sharks and other large predatory fishes on two pink reefs in the offshore Townsville region as well as on three nearby green reefs and three fished Habitat Protection zone reefs (blue reefs).

Sharks were counted in six 500 x 20 m transects on the slope of each reef during late March 2008. Densities of whitetip reef sharks (Triaenodon obesus) were twice as high on green reefs as on blue reefs but were twice as high on pink reefs as on green reefs. Similarly, grey reef sharks (Carcharhinus amblyrhynchos) were four times as abundant on green reefs as on blue reefs, but twice as abundant on pink reefs as on green reefs. 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 found numbers of sharks on reefs open to fishing (blue) are heavily depleted compared to more protected (green and pink) reefs. The pink reef – green reef discrepancies found in the present study were not as extreme as those reported by Robbins et al. (2006) but they are still substantial.

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.

Historically, there has been a degree of non-compliance with green zones in the Marine Park. It is still an open question as to whether compliance effectiveness has improved substantially following rezoning of the Marine Park under the Representative Area Program (RAP) in 2004. Effective compliance will be essential for the re-zoning to have a positive effect on shark populations. Reef managers need to acknowledge that green reefs have not been, and are still unlikely to be, totally effective in excluding fishing effort. Strong deterrent messages and educational campaigns are recommended to improve compliance.

Given the conservative life-history characteristics of reef sharks (e.g. slow growth rate, low fecundity) they are likely to be much more vulnerable to low levels of illegal fishing than major fisheries target species such as coral trout. Conservative life histories also mean that effective conservation measures would take several decades to result in tangible increases in reef shark populations (Robbins et al. 2006). We suggest that several future, more comprehensive surveys of reef sharks in other areas of the Great Barrier Reef would be important in helping managers to assess trends in reef shark abundances and the extent of non-compliance on green reefs". 

 

Shark density

"The density of both whitetip reef sharks and grey reef sharks was significantly higher on pink reefs than on green reefs and significantly higher on green reefs than on blue reefs (table 4). Densities for both species were similar. Whitetip reef sharks were approximately twice as abundant on green reefs compared to blue reefs and twice as abundant on pink reefs compared to green reefs (table 5, figure 3). Grey reef sharks were about four times as abundant on green reefs as blue reefs and twice as abundant on pink reefs as green reefs (table 5, figure 4). Grey reef sharks were therefore eight times as abundant in pink reefs (Preservation Zones) as on fished reefs. The habitat, reef and factor interactions were not significant for either shark species (table 4). Only two other sharks apart from these common species were recorded, a silvertip shark (Carcharhinus albimarginatus) on Bowl Reef and a blacktip reef shark (Carcharhinus melanopterus) on Dip Reef. As a result the patterns for total shark numbers were very similar to those for the two common sharks, with almost three times as many sharks on green reefs as on blue reefs and almost twice as many sharks on pink reefs as on green reefs (figure 5). Although there was some variation in total shark numbers within each zone (table 5, figure 6) the reef differences were not significant (table 4)".

 

 

 

 

Comparisons with other surveys

"Surveys of reef shark density on a range of differently zoned reefs in the northern Cairns Section were made by Robbins et al. (2006) using a very similar count technique. They counted sharks along an approximately 400 x 20 m transect compared with the approximately 500 x 20 m transect of the present survey. That study found that densities of whitetip reef sharks on pink reefs were about five times higher than those recorded on blue reefs and almost three times those on green reefs (figure 13). Similarly, grey reef shark densities on pink reefs were more than 10 times those recorded on blue and green reefs (figure 14). On average the abundance of both these reef shark species was twice as high on the Townsville reefs surveyed in the present study as on the Cairns reefs surveyed by Robbins et al. (2006) (figures 13 & 14). Comparisons of the shark counting performance of Robbins and Ayling were made during a previous study of the Cocos-Keeling Islands: the two observers obtained very similar results (Robbins 2006).

Shark counts using the same method have also been made by A.M. Ayling on the offshore north-west Australian reefs of Ashmore  (12 counts), Scott (6 counts), Clerke (9 counts) and Impereuse (11 counts), as well as on the Coral Sea reefs of Herald Cay (17 counts), and the Indian Ocean outpost of Cocos-Keeling Island (Choat., 2004). Whitetip reef shark density on Townsville pink reefs was higher than that recorded in any of these other locations but the density recorded on the relatively undisturbed Cocos-Keeling Island was almost 75 per cent of this level. Densities of whitetips on Townsville green and blue reefs were similar to those recorded in the other locations with the exception of Scott Reef where no sharks were seen (figure 15). Grey reef shark densities on offshore Townsville pink reefs were more than twice those recorded in the other locations with the exception of the pink reef densities recorded by Robbins et al. (2006) (figure 16)."

 

Figure 13. Comparison of whitetip reef shark density on Townsville reefs with that on northern Cairns Section reefs from Robbins et al. (2006). Mean number of sharks per ha is shown from all survey reefs in each zone. Error bars are standard errors.

Figure 14. Comparison of grey reef shark density on Townsville reefs with that on northern Cairns Section reefs from Robbins et al. (2006). Mean number of sharks per ha is shown from all survey reefs in each zone. Error bars are standard errors. 

 

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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

Abundance patterns of predatory fishes