Coles et al., 2008:

Summary 

"There is a complex system of spatial closures in the Great Barrier Reef World Heritage Area (GBRWHA) and since the Great Barrier Reef Marine Park Representative Areas Program of 2004 there have been considerable reductions in available fishing area.

The Queensland East Coast Otter Trawl Fishery (ECOTF) effort in the GBRWHA is highly clumped and trends in distribution of trawl effort shown by the Vessel Monitoring System (VMS) data suggest the location of the main fishing grounds where most fishing effort is expended have not changed markedly between 2002 and 2005.

Information now available (on seabed biodiversity, seagrass distribution, distribution of turtle and dugong populations and interactions with populations of other marine species of conservation interest), suggest the recent changes to the area available for trawling, reduction in effort, implementation of gear restrictions, and other fishing regulations have reduced the impact of trawling on marine habitats and marine ecosystems. While fishing effort remains in its present locations and at current levels, there does not seem to be a strong case for additional spatial closures in the GBRWHA. 

The effects of otter trawling on benthic communities can be influenced or reduced by modifications to gear type and deployment, reductions in overall effort in the fishery, restrictions on areas open to trawling, reductions in the amount of time any one location is fished, and/or a combination of these factors. The distribution of effort and spatial and temporal restrictions is highly correlated and if poorly implemented can increase fishing effort in some locations at the expense of others as fishers respond to changing circumstances. Nevertheless limiting the location of fishing effort is a powerful tool when there is sufficient information on the type and complexity (biodiversity) of benthic communities warranting enhanced protection by spatial arrangements.

Two programs of the CRC Reef Research Centre, the deep water seagrass mapping project and the Seabed Biodiversity Project have modelled, based on transect and point source data and GBRWHA wide biophysical data sets, probability gradients of benthic communities and more abundant individual taxa/species. This enables an estimation of the exposure of benthic communities and species (at least in the GBRWHA) to potential impacts from otter trawl nets. 

Effort trends in the fishery

Effort in the fishery has been declining more or less constantly since a peak in numbers of just over 1400 vessels in the early 1980’s. There are presently approximately 318 vessels recorded fishing in the GBRWHA and approximately 202 vessels operating in the area south of the GBRWHA (Figure 1). Catch has remained relatively constant with a slight downward trend since 1996 (Figure 2). Effort reduction results from three separate sources: a reduction in the area allowed for fishing particularly since 2004, a reduction in the number of vessel and fishing nights allowed, and a reduction in the actual number of boat nights fishing (possibly because of poor economic conditions in recent years). Boat nights in the GBRWHA have declined from 58 687 nights in 1990 to 26 865 in 2006 and from   22 080 nights in 1990 to 15 441 nights south of the GBRWHA in 2006 (Figure 3).

 

 

 

Location and spatial extent of trawl effort

An overall large scale spatial constraint on the ECOTF is the long thin shape of the coastline and fishing area, approximately 2500 km north to south but only 400 km from the coast to the outer trawl grounds at the widest point and much narrower in most locations. The effect of this is that the trawl fishing grounds also stretch in a long north south strip and effort is clumped in the east - west direction. On the outer reef slopes and east of Moreton Island this is accentuated by topography and the depth range favoured by the target prawn species. Trawling along a constant depth contour favours a north south trawling pattern. On the inner reef lagoon trawling follows the coast and inner edge of the coral reef string and with the exception of large bays also follows a north south pattern.

The clumped nature of the trawl fishing effort is evident from the GBRWHA VMS data that shows of the modelled VMS (1 km x 1 km) grid cells that are being trawled, only between 30 per cent and 40 per cent are fished for more than five hours in a year (Figure 4). Less than 0.5 per cent of VMS grid cells were fished for more that 100 hours in 2005 (the last year that corrected VMS data are available).

 

Changes to the ECOTF are noticeable in the GBRWHA where there was a downward trend in the area trawled from 2002 - 2005. There was a small decline in the 2004 year which resulted from an increase in the spatial trawl closures designated in the rezoning of the Great Barrier Reef Marine Park (Great Barrier Reef Marine Park Zoning Plan 2003). However, many of the areas removed from the total area allowed for trawling by the rezoning were not trawled or not trawled regularly. This has resulted in the reduction in the potential area available for trawling not having a comparative impact on the area of the GBRWHA being trawled (Figure 5). The actual percentage trawled has been in slow decline as the number of boats has reduced and economic circumstances have focussed trawling in areas historically producing higher catch rates.

If it is assumed that VMS grid cells trawled for 5 hours or more in a year are VMS grid cells that have been fished more than once in a year, (a single trawl shot rarely exceeds 2.5 hours, N Gribble, DPI&F pers. comm.) between 2002 and 2005 less that 10 per cent of available grid cells (1 km² VMS grid cells) were visited by a trawl fishing boat more than once (Figure 6).  

 

Seabed Biodiversity report

The Seabed Biodiversity Project Report was completed in July 2007 (Pitcher et al. 2007) (FRDC project 2003/021). Using data from comprehensive benthic fauna and floral mapping, it focused specifically on the development of sustainability indicators and risk assessment of the effects of trawling on the sustainability of trawl bycatch, benthic species, assemblages, and seabed habitats within the GBRWHA.

The report developed Ecological Risk Indicators on the basis of exposure to trawling in areas that are open to trawling, areas where trawl effort is present, and areas where the intensity of trawling is taken into account and those three approaches are replicated for biomass. The study uses a 0.01 degree grid cells (approximately 1 km²) and assumes trawling is conducted uniformly within a grid. Risk rating incorporates both vulnerability and the ability of species / habitats / communities to recover. It is not possible to include a full assessment of the implications of the report in relation to adequacy of trawl closures in this initial assessment but the key implications can be summarised:

• The report provides survey, transect and point data and interpolated distributions (referred to as maps in the report) for 850 seabed taxa/species
• Estimates of the likely extent of the effects of past trawling activity shows trawling has had a negative effect on the biomass of only 4.5 per cent of these species and a positive effect on 2.0 per cent
• More than 800 taxa/species had a very low proportion of their estimated overall biomass in the sampling area caught annually by trawling –– 15 species had a higher relative risk when recovery capacity in addition to trawl exposure was included. There are 48 species listed in the project report which, because of the high levels of uncertainty in estimates and the limited knowledge of their life history biology, should be investigated further
• Recent management interventions –– the 2001 commercial trawl licence buyback and the 2004 RAP rezoning have arrested or reversed the bottom habitat deletion trends that had possibly been occurring
• Of the species listed in the present report only nemipterids (Nemipterus hexodon and N. peronii) have a biomass that is greater than 50 per cent in areas that could be trawled with 21 per cent and 37 per cent respectively in grid cells (approximately 1 km²) that have trawl effort. When effort levels are included N. furcosus is found in 25 per cent of trawled grids with an effort percentage of 32 per cent
• Two nemipterid species, N. peronii and N. furcosus were at the higher end of catchability uncertainty limits for catch/mortality estimates
• The species modelling process selected Trawl Effort Index as significant for only 6.5 per cent of species   – little more than would be expected by chance. This supports previous CRC Reef Research Centre results by Coles et al. (In Prep.) that found trawl effort was not a significant factor determining modelled predictions of seagrass distribution
• Habitat components from video data were examined individually, particularly marine plants. Pinnate and ovoid seagrass had an exposure ranging from 24 per cent to 15 per cent respectively, crustose coralline algae was most exposed at 44% primarily off Gladstone. Filamentous blue-green algae were the most extensive and had an exposure of 25 per cent. All other marine plants had an exposure 17 per cent or less. Available evidence is that catchability of marine plants in trawls is low in the GBRWHA and that the exposure to trawling risk for marine plants is similarly low.

As the models used assume that a trawler trawls novel ground on each trawl pass on any night these results represent the greatest likely impact. As trawlers do at times go up and back on the same ground the amount of area and percentage of biomass of a species or assemblage that is exposed would be expected to be less than estimated here although impacts on the area actually trawled could be higher."

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Citation and/or URL

Coles, R., Grech, A., Dew, K., Zeller, B. and McKenzie, L. 2008, A preliminary report on the adequacy of protection provided to species and benthic habitats in the east coast otter trawl fishery by the current system of closures. Department of Primary Industries and Fisheries, Brisbane.

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Spatial Coverage

Whole GBRWHA 

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Temporal Coverage

2002-2005

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Update Frequency

Not applicable 

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Other Information

None