he primary aim of the Research and Monitoring Section's Effects of Fishing Program is to ensure that fishing activity in the Great Barrier Reef Marine Park is consistent with nature conservation and World Heritage values. The Great Barrier Reef supports a widespread commercial and recreational reef-line fishery that is of great economic and social importance to the people of Queensland. It is a multi-species fishery, with most fishers targeting around 10-20 species of the coral reef fish found on the Great Barrier Reef. The major species targeted by the fishery is the common coral trout (Plectropomus leopardus).

Reef closures in the Great Barrier Reef Marine Park are seen as a management strategy that can effectively lead to the rehabilitation of reef fish stocks. One of the Effects of Fishing Program's projects is titled Bramble Reef Replenishment Area. The research is being carried out by Sea Research who is studying fish population density changes resulting from closing and re-opening a reef to reef-line fishing. From this research the Program is looking at the management implications and effectiveness of reef closures.

Bramble Reef, in the northern Central Section of the Great Barrier Reef Marine Park, was designated a Replenishment Area and closed to fishing in January 1992 (see Fishing: The effects on the Great Barrier Reef?, Reef Research, December 1996). The objective of the closure was to replenish commercially and recreationally targeted fish stocks, particularly coral trout (Plectropomus leopardus). A monitoring program was set up to collect data on coral trout density concurrently with estimates of chaetodon, lethrinid and lutjanid populations and measures of coral cover. The research was designed to detect temporal changes in the density of these reef fish populations and the coral cover on Bramble Reef in contrast to the control reefs (John Brewer, Lodestone and Davies Reefs). A baseline study prior to the closure showed that the mean coral trout density was lowest on Bramble Reef relative to the control reefs, and that the mean density of coral trout on all reefs in the study area was at a low level. It was perceived that a reduction of this species on these reefs was due to fishing pressure. After one year of closure, a further three control reefs (Britomart, Little Trunk and Trunk Reefs) were included in the study as these reefs adjacent to Bramble Reef were subject to effort displacement from the closure.

Figure 2. Legal size (> 38 cm total length) coral trout mean density per hectare for all reefs in the study. An increase in legal size coral trout density occurred on Bramble, John Brewer, Lodestone and Davies Reefs since 1991 until the re-opening of Bramble Reef in July 1995. The legal size coral trout density on Bramble Reef was protected by the closure, but was quickly reduced to pre-closure levels soon after the re-opening.
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Annual underwater visual surveys carried out during the 3.5 years of closure (January 1992 to July 1995) found more than 300% increase in the density of legal size coral trout (greater than 38 cm total length) on Bramble Reef. Three of the control reefs (John Brewer, Lodestone and Davies), however, also had similar increases in legal size coral trout density (figures 1 and 2).

The surveys indicated that there was a highly successful pulse in coral trout recruitment on Bramble, John Brewer and Lodestone Reefs the year Bramble Reef was closed, i.e. 1992 (figure 3). The resulting cohort was instrumental in increasing the density of coral trout on these reefs. This 1992 cohort was protected on Bramble Reef by the closure, as indicated by the significantly higher coral trout density on Bramble Reef in 1995. In contrast, the control reefs, which were open to fishing, had lower densities of coral trout (figure 2). This was most likely attributable to fishing effort.

Figure 3. Coral trout recruits mean density per hectare for all reefs in the study. There was a highly successful recruitment pulse on Bramble, John Brewer and Lodestone Reefs in 1992, the same year Bramble Reef was closed to fishing. After the closure of Bramble Reef the recruitment levels on Bramble, John Brewer, Lodestone and Davies Reefs were not as high.
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Bramble Reef was re-opened to fishing in July 1995 and the first post-opening survey was carried out 8 weeks later to assess the impact of fishing on fish stocks following the re-opening. This survey showed that following the re-opening coral trout density on Bramble Reef decreased by approximately 30% and the legal size coral trout density had been reduced by approximately 60% (figures 1 and 2). There were also similar density decreases on all of the control reefs after the re-opening of Bramble Reef.

A second post-opening survey was carried out twelve months after the re-opening and showed that the coral trout density on Bramble Reef had decreased by approximately 50% and that the legal size coral trout density had decreased by approximately 80%. There were also further decreases in coral trout densities on all of the control reefs (figures 1 and 2).

These post-opening surveys suggest that there have been continuing high levels of fishing effort on Bramble Reef and all the control reefs since the re-opening of Bramble Reef. The density of legal size coral trout on Bramble Reef is now at the same level as before the closure and at similar levels of surrounding control reefs. There are low numbers of juvenile fish on most of the reefs in the study area due to decreased recruitment levels since the high recruitment pulse that occurred in 1992.

The results of this study illustrate that if the fish population densities of a replenishment reef are assessed in isolation to recruitment surveys and control reefs, an incorrect picture of the success of the reef closure would be given. This study showed that success of annual juvenile recruitment will dramatically increase stock size on closed and open reefs. Also, once a closed or replenished area is re-opened to fishing, fish stocks can be quickly reduced to levels found on surrounding open reefs. Naive populations of coral trout, i.e. those that are not often fished, are far more susceptible to capture than exploited populations. Therefore a reef should not be re-opened to full exploitation after a closure. Rather a staged temporal and/or spatial opening strategy should be used. Long-term recruitment data should be used in determining the duration of the closure and the timing of the re-opening to take advantage of successful recruitment pulses and subsequent strong cohorts. The study points out that unless the intended objective for the reef closure is well defined and the appropriate management strategies for re-opening the reef are decided, the benefits of the closure will diminish in a relatively short time.

The Bramble Reef Replenishment Area study is an ongoing research project with annual coral trout population density surveys. The information gained from these surveys is crucial for determining the appropriate management strategies for Bramble Reef and other reefs in the Great Barrier Reef Marine Park.