Many animals and plants live attached to the seabed where they form benthic habitats used by a diversity of other species.
These habitats can be affected by human uses in the marine environment and a range of management measures have been implemented to reduce these impacts. This project sought a better understanding of the types of taxa that form benthic habitats: their distribution, sensitivities and management measures that provide protection from pressure. It produced the first regional-scale distribution maps for benthos in the South-East Marine Region (SEMR), and assessed the impacts of human uses (including cumulative effects) and the efficacy of existing management strategies for epibenthic fauna.
Closures, assemblages and taxa types in the South-east Marine Region
ABOVE: (a) Post-1985 footprints of the main bottom fisheries (cross-hatching) over four main 2013 trawl closure types: [D] Deep water closure, [C] CMRs, [B] Bass Strait closure and [G] Gulper shark closures. (Light grey shading: open areas.) The inset plot shows actual trawl effort time-series and alternative effort scenarios.
(b) Bio-physical characterisation with 15 predicted species assemblages (environments in which biological composition is expected to be relatively similar, and between which composition is expected to vary). Their degree of similarity is indicated by the proximity and colour of the legend symbols. Assemblages may not be spatially continuous. The inset plot shows the inclusion of assemblages in closed areas, and exposure to trawling.
(c─l) Images and predicted distributions of 10 major taxa types of habitat forming benthos (relative density: blue=low through to red=high). Each inset plot shows predicted abundance time-series, relative to 1985, from simulation modelling of trawl effort on the taxa type distribution (from no management interventions to all interventions).
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Approach
A trawl-simulation model developed for tropical regions was reconfigured to quantify and assess cumulative threats, risks to benthic biodiversity, and the effects of discrete management actions in the SEMR. The model incorporated predictions of biodiversity assemblages and habitat-forming benthos, in addition to their exposure to fishing and levels of protection (derived from existing data sources).
Survey data were collated for benthic species on the continental shelf and upper slope, as well as information on impact and recovery rates in relation to human uses. Biophysical modelling was used to characterise, predict and map patterns of biodiversity assemblages (spatially unique mixtures of all species, including mobile invertebrates and fishes), and the distributions and abundance of the major habitat-forming taxa such as sponges, coral, gorgonians and bryozoans.
Fishing and other human activities that affect the SEMR seabed were mapped from collated data. For the fishing industry (particularly trawling) this included historical annual fishing effort by area, fishing operations and management actions (including effort reductions, closures to fishing and the Commonwealth marine reserves system). Information was also collected in relation to oil and gas infrastructure.
Inclusion of benthic biodiversity in CMRS and fishery closures, and exposure to human uses
| SEMR shelf and slope: area affected |
15 spatially unique assemblages: area affected | Overlap with habitat-forming benthos taxa types (by abundance)* | |
|---|---|---|---|
| CMRs | ~9% (excluded 1.1% of historical trawl effort) | 0–41% | 7–19% |
| Trawl closures | ~39% (excluded 5.5% of historical trawl effort) | 1–81% | 33–60% |
| CMRs and closures | ~44% (excluded 6.2% of historical trawl effort) | 1–83% | 40–63% |
| Exposure to trawl effort since 2007 | ~6% of seabed trawled (across ~23% of the region) | 0–43% (3 most exposed assemblages trawled 2–3 times yearly on average) | 1–9% (most-exposed taxa trawled ~2 times yearly on average; annual overall trawling impact ~1–8%) |
| Longlining (widespread, but lighter impact) | ~0.03% of seabed long-lined (spread over ~17% of the region) | 0–0.12% | 0.01–0.03% (annual impact 0<0.01%) |
| Scallop dredging (Commonwealth only) | ~0.01% dredged (spread over 0.38% of region) | 0–0.05% | 0–0.02% (annual impact 0–0.01%) |
| Oil and gas facilities and pipelines | ~0.05% (spread over 0.52% of region) | 0–0.20% | 0–0.05% |
Inclusion of benthic biodiversity in CMRS and fishery closures, and exposure to human uses.
* Survey data for habitat-forming benthos are sparse and patchy, thus prediction maps have high uncertainty.
Key findings
The effects of fishing were modelled for 15 spatially unique species assemblages and 10 habitat-forming benthos taxa types that had been predicted and mapped from survey data*. Simulation of the bottom trawl fishery from ~1985, (when consistent logbook records were available), showed that all 10 benthos taxa types declined in abundance in trawled areas until the mid-2000s. At this time fishing effort reduced due to economic pressures and licence buy-backs, and large areas were closed to trawling. A complex picture emerged, with patterns and responses varying spatially according to the distribution of benthos taxa types, trawling distribution, and type of management action.
The lowest total regional abundance (status) of habitat-forming benthos taxa types across the SEMR was ~80–93% of pre-trawl status, after effort peaked during 2000–2005. Subsequently, all taxa were predicted to recover by varying extents (~1–3%) in the following decade. Had none of the management actions been implemented, benthos status was predicted to stabilise or recover slowly, and with all management actions in place, the rate and magnitude of recovery was greater. Reductions in trawl effort universally improved the status of habitat-forming benthos, with the larger 2006 licence buy-back leading to greater improvements than the 1997 buy-back.
In some cases, spatial management that excluded trawling, (particularly deepwater fishery closures), led to improved status of some benthos taxa types. Most fishery closures and CMRs had little detectable influence on status, though in some cases they worsened the status of some taxa in some locations. This was because displaced trawl effort moved to areas in which some taxa were more abundant.
New knowledge and opportunities
The new approaches can be used to evaluate existing and potential future management measures, and also be applied to assess the status of benthos in other marine regions. There are opportunities to reduce the uncertainties and data gaps in the map of benthos taxa types, by conducting new surveys of the distribution and abundance of habitat-forming benthos taxa.
Outputs and outcomes
An unprecedented level of data integration led to improved knowledge and understanding of benthic biodiversity distribution, protection, vulnerability and status. It enabled the first regional scale quantitative analysis of pressures and cumulative impacts, and the first quantitative evaluation of management strategies for benthic biodiversity conservation. This advanced capacity to evaluate existing and potential alternative biodiversity management options on and off reserves, as well as the relative impact of alternative pressures including marine industries, supports evidence-based decision making in these difficult-to-observe offshore environments.
Contact
Roland Pitcher
roland.pitcher@csiro.au
(07) 3833 5954