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Executive summary
The effects of reef
line fishing on the productivity of targeted species and its impacts on
other reef species on the Great Barrier Reef (GBR) have been poorly understood.
Understanding the distribution, intensity, and effects of reef line fishing
is essential for successful management of both fishing and other recreational
and commercial activities in the GBR region, as well as for conservation
of the GBR ecosystem.
The GBR Reef Line
Fishery (RLF) comprises socially and economically important commercial,
charter, and recreational fishing sectors. The fishery has been undergoing
some change over the last decade, particularly manifest as considerable
increases in effort and catch in the commercial fishery since 1995. These
changes probably arise from several events, including changing management
arrangements in other fisheries, the introduction of Dugong Protection
Areas in in-shore areas, the process of reviewing management arrangements
for the Reef Line Fishery and the development of lucrative export markets
for live reef fish for consumption. Collectively, these influences have
resulted in nearly 50% increase in commercial effort and 40% increase
in catch since 1996. There also is potential for increased recreational
fishing pressure along the GBR coast simply because of population growth
and increased tourism. Management arrangements for the Coral Reef Fin
Fish Fishery are now under review, with new management arrangements likely
to regulate commercial effort in the fishery explicitly.
Conservation management
of the GBR Marine Park also is undergoing significant change. The current
zoning system is being substantially upgraded with the development of
a comprehensive, adequate and representative system of no-take areas for
biodiversity conservation of the GBR ecosystem – the Representative
Areas Program. This revision is likely to increase the area of the GBR
closed to reef line fishing. Realising the minimum regime of 20% of all
GBR bioregions being ‘no-take’ will inevitably result in significant
increases in the amount of coral reef habitat closed to the Reef Line
Fishery in some areas. These
factors, combined with limited historical information about the fishery
or its main target species, present significant problems for planning
appropriate management strategies of the fishery and the GBR World Heritage
Area.
These factors, combined
with limited historical information about the fishery or its main target
species, present significant problems for the development of appropriate
management strategies for the fishery and the GBR World Heritage Area.
In this research, we have quantified some of the primary impacts of the
RLF on targeted stocks and assessed secondary impacts on other components
of the GBR ecosystem. We have assessed experimentally the degree to which
area closure strategies are likely to have ameliorated those impacts.
Finally, we evaluated the prospects for alternative mixes of strategies
for conservation and fishery management in the region to realise the objectives
of diverse stakeholders.
Surveys of areas that
had been open and closed to fishing for over a decade showed that the
two main target species of the RLF, the common coral trout and the red
throat emperor, were significantly more abundant, larger and older in
areas zoned Marine National Park ‘B’ (and so closed to fishing)
than in adjacent General Use areas that have always been open to fishing.
The magnitude of these differences varied regionally, from near-zero around
Lizard Island to several-fold for some population characteristics in the
southern regions of the GBR. The pattern in apparent ‘effectiveness’
of past closures matched closely patterns in the amount of fishing effort
and catch and underlying patterns in the abundances of several harvest
and non-harvest species. We present circumstantial arguments that this
regional variation in the apparent ‘effectiveness’ of Marine
Protected Areas is likely to reflect long-standing regional variations
in the amounts of fishing and its impacts outside closed areas, rather
than wholesale subversion of zoning strategies by high levels of poaching.
That is, the lack of contrast between open and closed areas in the Lizard
Region probably arises because the open areas are lightly fished, whereas
the strong contrasts in the other regions arises because of relatively
heavy fishing in the open areas in those regions.
Experimental manipulations of reef zoning status and fishing effort verified
that fishing on reefs that had been closed historically reduced the abundances
of target species on those reefs to levels similar to surrounding open
reefs. In the absence of prior data with which to compare open and closed
reefs before zoning was implemented, these manipulations provide the most
convincing evidence that the Marine Park zoning strategies have been effective
in protecting sub-populations of the fishery resource from the impacts
of harvest. The protection of such refuges, with sufficient compliance,
thus has the potential to sustain high biomass of reproductively mature
populations of harvested species in spite of an active fishery on the
GBR.
Indirect effects of
line fishing on non-harvest fish were less conspicuous. Whilst differences
existed between open and closed reefs in abundances of the prey of targeted
species, the nature of the patterns varied regionally, through time and
with species or species group. In some situations the patterns in abundance
suggested that removal of a key predator (coral trout) by fishing might
have allowed populations of some prey to grow on fished reefs, but the
evidence was neither uniform nor convincing.
We have evaluated
prospectively the relative merits for managers and stakeholders of alternative
strategies for effort management and area closure on the GBR. We based
these evaluations on a set of simulation models (‘ELFSim’)
for the population dynamics and harvest of common coral trout on the GBR.
The population dynamics model is spatially structured, depicting nearly
4000 reef-associated populations of coral trout inter-connected via larval
dispersal. The reef-associated, post-settlement populations are age, size
and sex structured and we allow for variation in most of the key demographic
parameters, such as natural mortality, growth, recruitment, etc. The harvest
model predicts the allocation of fishing effort over the GBR by three
fishing fleets, parameterised with historical catch and effort data to
represent the commercial, charter and recreational sectors of the RLF.
Objectives for the
future status of coral trout populations and for the RLF were developed
by a diverse set of stakeholders in the fishery and the GBR World Heritage
Area, in association with the Reef Line Fishery Management Advisory Committee
(ReefMAC). Contributing stakeholders included state and federal managers,
commercial, charter and recreational fishers, conservation organisations,
and researchers. Stakeholder objectives included preserving near-virgin
biomass of coral trout on reefs closed to fishing, ensuring satisfactory
levels of populations available for harvest, maintaining economically
viable commercial catch rates and recreationally rewarding recreational
catches of coral trout, and minimising variation in harvests from year
to year. Quantitative articulations of these and other objectives were
derived and agreed with stakeholders, together with associated performance
indicators.
The same set of stakeholders
advised on the mix of potential strategies to be considered for achieving
their respective objectives. We were asked to compare the efficacy of
three levels of fishing effort, ranging from half of 1996 levels to 1½
times 1996 levels, and three levels of area closure, ranging from current
closures to nearly three times current closures. The outputs from these
Management Strategy Evaluations provide comparative assessments of the
likelihood that each of the stakeholder objectives will be met by each
combination of effort control and area closure strategy. The results are
not intended to prescribe which strategy mix should be adopted, but to
provide a basis for stakeholders to negotiate such an outcome based on
the degree to which different combinations of strategies meet their needs.
Harvest-related objectives
(e.g., maintaining CPUE, increased chance of catching a large fish, preserving
biomass available for harvest) were most likely to be achieved when effort
was lowest under any area closure strategy, but were less likely to be
achieved as increasing amounts of area were closed to fishing. The principle
stock-conservation objective, represented by preserving the spawning biomass
of the whole population, was most likely to be achieved by increasing
the amount of area closure and was only relatively slightly impacted by
increasing fishing effort within each area closure strategy.
Importantly, the observed
increase in fishing effort in recent years is most likely to impact most
negatively on the performance indicators for areas open to fishing, especially
those reflecting what fishers would consider satisfactory performance
of the fishery (e.g., catch rates and sizes of fish). The increase in
area closures under the Representative Areas Program is likely to exacerbate
the depreciation of fishery performance, but our results suggest that
growth in fishing effort will be considerably more influential than changes
in areas available to the fishery. Our results suggest that the currently
elevated levels of effort (~1.5 time 1996 levels) will reduce significantly
the prospects of fishers in all sectors realising their objectives in
future years, irrespective of the inevitable increases in protected areas
under the Representative Areas Program.
Reducing effort, conversely,
is the strategy of those considered in our evaluations most likely to
realise direct fisheries-related objectives. The conundrum in these results,
however, is that the improved prospects from effort reduction would apply
only to those fishers remaining in the fishery. We are unable to assess
the magnitude of financial costs likely to be incurred by those fishers
excluded through the effort reductions that would now be necessary to
achieve the two lower effort scenarios we considered. Changing effort
had relatively little impact on most performance indicators for closed
areas, especially conservation of spawning biomass of coral trout within
Marine Protected Areas, even allowing for low levels of infringement of
closed areas. The most effective mechanism by which to increase total
spawning stock biomass over the GBR domain, therefore, was increasing
the area closed to fishing, presuming that compliance with those closures
was relatively high.
It is important to
note that the status of coral trout populations in areas open to fishing
remained relatively robust under all strategies we considered. For example,
even under the most ‘adverse’ scenario of maximum effort constrained
to the smallest fishable area, spawning biomass (in the open areas) remained
above 50% of virgin spawning biomass and biomass available for harvest
(i.e., above the minimum legal size limit) remained above 30% of virgin
available biomass. These statistics generally would be considered acceptable
for a harvested stock. In large part, this is likely to be the consequence
of the biologically precautionary minimum legal size limit on harvest
of common coral trout, which ensures that most fish can spawn in at least
one year before reaching harvestable size.
Sensitivity analyses
for the simulations showed that the qualitative relationships among scenarios
were robust to changes in model parameters. Accordingly, the conclusions
about the relative merits of increasing or decreasing fishing effort or
area closures are robust to most changes in model assumptions. It should
be noted, however, that our evaluations relate only to the populations
and harvest of common coral trout (P. leopardus). Though several
other species harvested in the Reef Line Fishery are taxonomically close
to P. leopardus, they are generally considered to be less abundant
and longer lived than P. leopardus and their populations dynamics
are perhaps less resilient to harvest than that of P. leopardus.
Accordingly, conservative regulations for the harvest of these other species
would be prudent at this stage.
This research has
laid bare some of the inevitable trade-offs among different scenarios
for managing the harvest of common coral trout by the RLF in the GBR World
Heritage Area. Most importantly, the trade-offs have been assessed in
relation to objectives and performance indicators specified by diverse
stakeholders in the fishery and the World Heritage Area. We present the
tradeoffs in ways that allow direct comparisons among disparate objectives,
essentially providing a common currency for comparing performance across
fundamentally different types of objectives. In so doing, we hope that
the costs and benefits of different management options are more transparent
to all stakeholders than might otherwise have been the case. We hope that
such transparency aids in the negotiation of acceptable and effective
future management arrangements for the Great Barrier Reef World Heritage
Area and the Reef Line Fishery.
Download a complete copy of the report (Adobe Acrobat file 5.07MB).
For a hard copy of the report contact CRC Reef on info@crcreef.com.
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