Seabed Biodiversity on the Continental Shelf of the Great Barrier Reef World Heritage Area

Roland Pitcher, Peter Doherty, Peter Arnold, John Hooper, Neil Gribble

July 2007

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NON-TECHNICAL SUMMARY:
a multiple use Marine Park, activities such as fishing and tourism occur along with conservation goals. Managers need information on habitats and biodiversity distribution and risks to ensure these activities are conducted sustainably. However, while the coral reefs have been relatively well studied, less was known about the deeper seabed in the region. From 2003 to 2006, the GBR Seabed Biodiversity Project has mapped habitats and their associated biodiversity across the length and breadth of the 210,000 km² shelf in the Marine Park to provide information that will help managers with conservation planning and to assess whether fisheries are ecologically sustainable, as required by environmental protection legislation (e.g. EPBC Act 1999).

Holistic information on the biodiversity of the seabed was acquired by visiting almost 1,400 sites, representing a full range of known environments, during 10 month-long voyages on two vessels and deploying several types of devices such as: towed video and digital cameras, baited remote underwater video stations (BRUVS), a digital echo-sounder, an epibenthic sled and a research trawl to collect samples for more detailed data about plants, invertebrates and fishes on the seabed. Data were collected and processed from >600 km of towed video and almost 100,000 photos, 1150 BRUVS videos, ~140 GB of digital echograms, and from sorting and identification of ~14,000 benthic samples, ~4,000 seabed fish samples, and ~1,200 sediment samples.

The project has analysed this information and produced all of the outputs as originally proposed; these included:

• Images and videos of seabed habitat types and fishes, including more than 150 substratum and biological habitat component types; and >300 fishes, sharks, rays and sea snakes attracted to BRUVS.
• An inventory of more than 5,300 species of benthos, bycatch and fishes, with catalogued museum voucher specimens, many of which were new species, and a database of more than 140,000 records of species distribution and abundance on the seabed.
• Identification of the key environmental variables likely to be important in structuring seabed distributions, including: sediment grain size and carbonate composition, benthic irradiance, current stress, bathymetry, bottom water physical attributes, nutrients and turbidity — and predictive models of bio-physical relationships between seabed species, their assemblages and the physical environment.
• Maps of the distribution and abundance of ~850 seabed species throughout the GBR shelf region.
• Estimates of the likely extent of past effects of trawling on benthos and bycatch over the entire shelf of the GBR region, which indicated that trawl effort had a significant effect on the biomass of 6.5% of 850 species mapped; with negative biomass change of -1% to -36% for 4.5% of these species and positive biomass change of +1% to +96% for 2% of these species.
• Estimates of exposure to trawl effort showed that about 70% of the 850 species mapped had low or very low exposure, and at the other extreme, about 33 species had high to very high exposure to trawl effort — of these species, after taking relative catch rates into account, five had high estimates of proportion caught annually and 28 were intermediate. The remainder (>800 species) had low or very low estimates of proportion caught annually.
• Indicators based on qualitative recovery ranks showed that about 15 species stood out as being at higher relative risk with respect to trawling. An additional, quantitative absolute sustainability indicator showed that three species (Fistularia petimba, Brachirus muelleri, Trixiphichthys weberi) exceeded a limit reference point (analogous to maximum sustainable yield, MSY), one species (Pomadasys maculatus) exceeded a first conservative reference point (≡0.8×MSY) and two others (Psettodes erumei, Sillago burrus) exceeded a second conservative reference point (≡0.6×MSY) — another 10 species were also listed due to uncertainty in parameters, though they were below the sustainability reference points.
• Evaluations of the environmental performance of several recent management interventions showed that generalized depletion trends up until the late 1990s have all been arrested and reversed — the 2001 buyback of fishing licences and subsequent penalties made the biggest positive contributions with the 2004 rezoning of the marine park making a small positive contribution for some species.
• Methods and tools for regional marine planning, including: representative survey design and techniques, spatial-statistical classification and prediction methods, and biodiversity and bycatch species risk assessment methods.

A key output from the project is the identification, by means of the trawl exposure and sustainability indicators, of species at risk or potentially at risk from trawling. Different species were highlighted by different indicators, though there was some overlap. The most quantitative indicator was directly related to sustainability, with biologically based reference points — while three species appeared to be at risk and another three species exceeded conservative reference points (as listed above), there was uncertainty that requires a more precautionary response. Hence, the top ranked 50 species, across all indicators developed, were listed herein and recommended to be considered for stakeholder consultation regarding future action. Options may include clarification of the identified uncertainties, monitoring of species at risk, management interventions that reduce risk or combinations of these actions.

It is also recommended that long-term monitoring of trends in ecological condition of identified key seabed habitats and constituent species be implemented to assess responses to regional pressures, including climate change. Candidate habitats should include those that have been demonstrated to be particularly species rich such as vegetated areas and epibenthic gardens. The seabed may be vulnerable to climate change, as climate change modelling has indicated that the thermocline is likely to deepen and upwellings to become weaker and less frequent, with potential consequences for productive habitat dependent on nutrients from such sources. Such a possibility should be investigated.

Further work is needed to address the uncertainties in the risk assessments that arise from uncertainties in estimates of catchability and natural mortality rates. Currently, the uncertainty is such that several additional species could exceed the reference points and many species with unknown mortality might be of concern. It is also possible that clarification of these uncertainties may show that species currently thought to be at risk or potentially at risk may be demonstrated to be of no sustainability concern. Thus, it is recommended that further studies of catchabilities and natural mortality rates be conducted to address this key uncertainty for affected species. Such results are likely to have wide application in risk assessments being conducted in multiple jurisdictions.

Many fisheries in Australia are conducting qualitative approaches to Ecological Risk Assessments. The results of the quantitative sustainability indicators applied here raise concerns about the reliability of the qualitative approaches, which have not been benchmarked because of the lack of a suitable test bed. Such a test bed is now available with the Seabed Biodiversity dataset and an assessment of the performance of the qualitative methods is recommended. The Seabed dataset also provides an opportunity to develop condition and trend and vulnerability indicators for seabed communities and ecosystems that are needed to meet the increasing requirement for ecosystem-based management approaches.

The results of the Seabed Biodiversity Project have been adopted for marine park zoning assessment by a follow on project supported by the Reef and Rainforest Research Centre and involving collaboration with marine park managers. This project has contributed to ongoing marine park planning with respect to meeting WHA obligations.

Another project supported by the CERF National Marine Biodiversity Hub will use the unique Seabed Biodiversity dataset in comparisons with other datasets to test the inter-regional utility of physical variables and cross-taxonomic patterns as surrogates for application in marine planning at a national scale.

Other further opportunities include: sorting and identification of remaining samples that could not be completed within the scope of the project, taxonomic work to properly identify the more difficult specimens, and quantification of visible species from the available towed camera video and digital photos to fill significant gaps in areas too rough for sampling and currently lacking species information. These activities would provide full utilization of the samples and deliver additional value, with expected benefits for greater understanding of the seabed ecosystem, fishery sustainability, zoning assessment and ongoing marine park planning.