Australia: State of the Environment Second Technical Paper Series (Coasts and Oceans), Series 2
David Barratt, John Garvey and Jean Chesson
Bureau of Resource Sciences, Australia
Department of the Environment and Heritage, 2001
ISBN 0 6425 4745 9
"The area of sea and seabed subject to (a) demersal trawling, (b) other trawling, (c) exploration, and (d) mining"
(see indicator 4.5, 5.1 and 5.2; Ward et al. 1998, p45, p47-48)
This indicator is an amalgam of 3 indicators from Ward et al. (1998). Indicator 4.5 (Trawl fishing area) is from the Renewable Products group (Class 4), which is discussed in the introduction to the previous chapter on line and net fishing operations. Indicator 5.1 (Ocean exploration) and 5.2 (Ocean mining) together make up the Non-renewable Products group (Class 5). These indicators document a number of human activities that disturb benthic habitats with potentially damaging effects on benthic ecosystems.
Trawling is one of the most widely used commercial fishing methods in Australia (Chapman 1993). It can be divided into two types: demersal trawling and pelagic trawling. Most trawling in Australian waters is by demersal fish and prawn otter trawls, though two other demersal trawl gears are used: beam trawls and demersal pair trawls. All these gears will be dealt with together under this indicator.
Demersal trawling can have a substantial impact on marine ecosystems (see Harris and Ward 1999, and BRS 1998). The magnitude of the impact is variable and depends on a range of factors including the frequency of trawling, gear used, type of substrate trawled over and the species assemblage. Impacts arise from the removal of target and non-target species, the removal and disturbance of epibenthos and extend to the many mobile animals, including fish, crustaceans and molluscs, which live in close association with the sedentary benthic fauna. Repeated trawling may prevent the recolonisation of benthic species, both sedentary and mobile.
To date, very little trawling other than demersal has been carried out in Australian waters, although there has been recent interest in using pelagic trawl gear in some Commonwealth trawl fisheries. Pelagic, or mid-water, trawling is different to demersal trawling in that none of the gear makes contact with the sea floor. Pelagic trawls are characterised by having much larger mouths than demersal trawls. In overseas fisheries they can be truly enormous with nets over 100 m long and a mouth opening of nearly half a square kilometre. Most pelagic trawl gears currently used in Australia are substantially smaller.
Pelagic trawling off the south east coast of Australia is currently restricted to a small number of operators and reporting their activity is severely limited by confidentiality constraints. Consequently, pelagic trawling has not been considered further in this report. However, it is worth noting that since the primary impact of pelagic trawl gear is to sieve a large volume of water, and that they are as deep as they are wide, the area swept by this gear is probably an inappropriate indicator of the impact of this activity. It is recommended that the volume of water swept by this gear by specified region and time period be used as a measure of this gears impact.
As detailed in the previous chapter on line and net fishing operations, most fisheries management agencies collect information, via logbooks or fishing returns, on the area and intensity of fishing operations in order to measure fishing effort for stock assessment and management purposes. The type of information gathered often differs from agency to agency. Because of this, it is necessary to review the types of data collected by State and Commonwealth fisheries for their trawl fisheries in order to make recommendations on the spatial and temporal granularity with which this indicator can be presented nationally.
Offshore petroleum exploration and extraction in Australian waters is of fairly recent origin. The first offshore seismic surveys and drillings were carried out on the Gippsland Shelf at the beginning of the 1960s. Major oil and gas deposits were discovered on the shelf soon after and these fields have been the location of extensive production operations for almost 30 years. Less frequently, offshore regions of Tasmania have been the focus of intermittent petroleum exploration operations, but have yielded only limited gas production. Petroleum exploration off the north-west coast of Western Australia began in 1967 on Ashmore Reef (now a marine protected area). Oil deposits were discovered almost immediately and extraction from various parts of the North West Shelf continues today.
Exploration for hydrocarbons takes two main forms. The first is the conduct of geophysical (seismic) or airborne surveys to locate potential oil- or gas-bearing geological structures. Subsequently, exploratory drilling is used to confirm the potential prospectivity of a petroleum accumulation for commercial production. This may in turn lead on to the 'development' of the field in the form of full-scale extraction activity. Most of Australia's commercial oil reserves have been discovered in offshore reservoirs in the Gippsland Basin off south-east Victoria.
Exploration for petroleum and mineral resources may have local impacts on marine fauna, although these are poorly understood. Petroleum and mineral extraction activities however, may place substantial pressure on marine ecosystems. Not all activities have the same type or extent of impact and some have different effects depending on where they are conducted relative to the spatial and temporal distribution of local flora and fauna. Monitoring the location, extent and type of activities carried out will assist in assessing the importance of these activities in terms of their effects on local environments and regional ecosystems. This monitoring will also help illustrate the minimisation of potentially adverse effects where these activities are properly managed.
Commonwealth managed trawl fisheries and those managed by State and Territory fisheries agencies were identified. As with net and line fishing operations, information for these fisheries was available from reports and statistical documents published by Commonwealth, State and Territory fisheries agencies, and confirmed by interview with the general manager of commercial fisheries (or equivalent) of each fisheries management agency.
The fishing effort data collected by the Commonwealth and State fisheries agencies were reviewed to determine whether the data needed to implement this indicator are currently being collected. The types of information collected by the relevant trawl catch and trawl effort monitoring programs were examined to determine the spatial resolution at which demersal trawling effort can be analysed for any particular fishery. Assumptions required about the size of the gear, or other factors, when calculating trawl effort statistics were also identified. The logbooks and forms used were identified and their ability to provide the appropriate spatial, temporal and effort data was determined using the criteria described in the General Methods section of this report.
Data sources for petroleum and minerals exploration and extraction were documented. Work on minerals exploration and extraction was restricted to reviewing and documenting mining activities in the States and Territories. Spatial and temporal data were assembled for petroleum exploration and extraction only.
Seismic survey records and petroleum exploration drilling records dating from 1963 to 1995 were analysed. Spatial analysis of seismic survey activities was restricted to the south-east coast of Australia, including important sites off the Gippsland coast and in Bass Strait. The following analyses were conducted to quantify and illustrate the spatial extent and intensity of seismic surveys and exploration drilling:
- Gridded 2-D seismic survey intensity (distance streamed per 1 n mile grid) for years 1963-1995
- Gridded petroleum exploration intensity (number of drill sites within 15 n miles)
- 2-D seismic survey intensity (distance streamed) by IMCRA region (standardised by area) for years 1963-1995
- Petroleum exploration intensity (number of drill sites) by IMCRA region (standardised by area)
- 3-D seismic survey intensity (distance streamed) for years 1980-1995
Locations (start and end latitude and longitude) of seismic surveys are recorded by skippers using GPS. To generate a gridded representation of the relative intensity of surveys, the data had to be first assigned a unique identifier and assembled into arcs. Vector coverages of survey tracks were compiled using the ARC 'generate line' command. The ARC 'linestats' command was then used to convert the coverages into 1x1 minute grids of total streamer survey distance using a neighbourhood radius of 0.011785 degrees. This radius was ascertained to be the minimum needed to span from the centre of a 1x1 minute grid cell to the corner of the cell. A neighbourhood analysis radius describing a 1x1 minute grid cell was chosen because this represented at least the precision of the data and was still manageable in terms of computing resources. (NB: The ARC linestats command does not provide an option for specifying a rectangular neighbourhood, as was done using Spatial Analyst tools on point data in the Southern, Pelagic Longline and Danish Seine Fisheries analyses - see previous Indicator). The final grid was converted from floating-points to an integer grid using the GRID 'int' command.
To generate a smoothed gridded representation of the relative intensity of petroleum exploration drilling operations in Australian waters, a nearest neighbourhood analysis was applied to drilling location point data using ArcView Spatial Analyst tools. This analysis generated a 1x1 minute grid coverage, where the value of each cell was the sum of operations within a 15 n mile radius around the cell. This gridded output provides an indication of the relative intensity of operations around Australia and can be likened to a contour map of drilling intensity.
Two datasets were used to compile information on 3-D seismic surveys in SE Australia:
- A spreadsheet maintained by AGSO on seismic (and other marine) surveys which includes information on the operator, number of tracklines in the survey grid and start and end date of the survey. Location information is, however, generalised to the north and south latitude and east and west longitude of a rectangular box within which the survey took place.
- Data purchased from New Wave Geophysical Pty Ltd in Sydney which gives the exact location of seismic surveys in the form of coordinates of the outer bounding polygon of the survey grid, but generalises the survey date to the year level. The cost of this data was $1000. Data for the whole of Australia would have cost in excess of $10,000. New Wave Geophysical has a contract to maintain data in the public domain and petroleum exploration leaseholders are obliged to hand over copies of their data after two years.
These two datasets in combination provided both the spatial and temporal location of each 3-D survey grid. The location information was used to generate an ArcInfo polygon coverage of individual survey boundaries. Attribute data included the date and length of each survey and this information was used to plot the frequency and track line length of 3-D seismic surveys by year since 1980. In addition, the total length of all seismic surveys (both 2-D and 3-D) carried out since 1983 was plotted annually.
For this report, the Northern Prawn Trawl fishery was selected as a case study to illustrate the extent and intensity of demersal trawling in northern Australian waters between 1989 and 1998. The Commonwealth managed South East Trawl fishery and the NSW Ocean Prawn Trawl fishery were selected as case studies to examine the extent and intensity of demersal trawling in south-eastern Australian waters between 1989 and 1998. However, owing to delays in negotiating access to data from the NSW State Fisheries, the NSW Ocean Prawn Trawl logbook data has only been superficially analysed here to illustrate the type and quality of available data in comparison to Commonwealth trawl fisheries logbook data.
A total of 18 logbooks and catch and effort forms were identified that collect effort information on trawling and dredging (Table 7.1). Of these, 72% (13) met all of the spatial, temporal and effort criteria required for effective mapping of trawling effort as an indicator of marine disturbance. Thirteen logbooks met the spatial criteria, 15 the temporal criteria and 18 logbooks met the effort criteria, which are described in the General Methods section of this report. Appendix III contains the spatial, temporal and effort details collected by these logbooks and forms. Some of the details discussed below are the same as those discussed in the previous chapter, but are included again for completeness.
There were eight Commonwealth (AFMA) logbooks identified for trawling and dredging operations. Most of these logbooks collect latitude/longitude information for each operation. The exceptions to this are the NP12 and NP12A, which collect information on a daily basis and latitude/longitude of greatest catch only; and the BS02, which collects information on a daily basis and uses a 7.5'x7.5' grid. The BS02 logbook explicitly asks if a dredge has been lost during a trip. For other fisheries there is a requirement in the management act to report lost gear, but this may not be routinely practised. Tasmania also uses the BS02 for its scallop fishery.
One Queensland logbook was identified. This logbook currently gives fishers the option of reporting location of greatest catch by latitude/longitude position or a grid (6'x6') reference and collects daily fishing details. The trawl catch and effort database kept by Queensland contains data from 1988 to the present. The current Queensland Fisheries Act came into effect in 1995. The current logbooks are therefore no more than four years old, although the previous logbooks collected similar information.
Two New South Wales catch and effort forms were identified. NSW has used a system of Zones for the collection of spatial information of marine fishing since 1977. Each zone is 1 degree of latitude wide with no longitudinal delineation. There is some refinement of these zones in each logbook. Use of such a coarse zoning system limits the extent to which questions about area fished may be answered. All NSW forms are monthly returns and detailed effort data, other than days fished, has only been collected since 1997. However, the particulars of effort information being collected are currently being reviewed.
|Jurisdiction||Logbook or catch and effort form|
|Commonwealth||Antarctic Waters Catch Details Log (ANT03-CA)Australian North-West Slope Confidential Fishing Log (NWS02)Coral Sea Line, Trawl and Collector Fishery Log (CO01)Great Australian Bight Trawl Fishery Log (GB03)Northern and Torres Strait Prawn Fisheries Confidential Daily Fishing Log (NP12 and 12A)South East Fishery - Trawl Sector Daily Catch Log (SEF1B)Western Deepwater Trawl (WDT01)|
|Queensland||Queensland East Coast Otter Trawl Fishery (OT06)|
|NSW||Fish Trawl (FT02)Ocean Prawn Trawl (PT02)|
|Vic||Ocean ScallopTrawl - Inshore|
|Tasmania||General Fishing Log|
|Commonwealth and Tasmania||Scallop Fishery Logbook (BS02)|
|SA||Prawn Fishing Daily Log|
|WA||Trawl and Dive|
|NT||Fin Fish Trawl Daily Log Sheet|
Victoria has two logbooks that collect catch and effort information from it's marine trawl and dredge fisheries. Victoria currently uses a 10nm grid system for the collection of all spatial data and all logbooks collect information for each day fished. The Victorian Catch and Effort database contains data from 1978 to the present, though logbooks have changed in that time.
Two Tasmanian logbooks were identified. Tasmania's General Fishing Logbook is used to collect information for most fishing methods, including trawling. Currently Tasmanian scallop dredgers use the Commonwealth's BS02 logbook (with fishers tagging their catch with either AFMA or TAS codes). A block system is used for the collection of spatial data by all relevant logbooks. The General Fishing Logbook uses 30'x30' blocks and the BS02 uses 7.5'x7.5' blocks. Details are recorded for each day's fishing. The current General Fishing Logbook has been in use since 1995. It replaced the General Fishery Return, which did not collect the same detail of effort information nor at the same spatial resolution. Data collected by the General Fishing Logbook and the Scallop Fishery Logbook are stored in a database called Seals (Pisces). Catch and effort data from 1969/70 onwards is stored electronically, however not all of it is in Seals (Pisces).
One logbook was identified for South Australian marine trawl fisheries. This logbook uses a grid/block system for the collection of spatial data. These grids/blocks vary in size depending on the region in question. Catch and effort details for each operation are collected. Catch and effort data has been collected in South Australia since 1976/77. However, GARFIS, the main catch and effort database only contains data from 1983/84. The Prawn database contains data from 1990 to the present.
One catch and effort form was identified for Western Australia. This logbook uses a 1 degree block system to collect spatial data. Use of this block system limits the extent to which questions about area fished may be answered. These data are collected on a monthly basis. Effort data for the Western Australian commercial fisheries has been collected since 1965, however it has only been stored electronically since 1975. A separate trawl research logbook program also exists. This logbook collects information for each operation with the GPS position of each operation being recorded. The research program has been running since 1963 using a variety of logbooks and differing levels of coverage.
One logbook was identified for Northern Territory. This logbook collects information for each operation with the GPS position of the start and finish of each operation being recorded. FISHDAT is the database used to store the Northern Territory's catch and effort, and licensing information. It contains data from 1993 to the present. A separate database exists which contains data collected from earlier logbooks for the period from 1984 to 1992.
NSW Ocean Prawn Trawl data analysis
The intensity of prawn trawling by NSW fishery zones between 1989 and 1997 based PT02 logbook records is shown in Figure NSW 1. The change in prawn trawling intensity between 1989 and 1997 is shown in Figure NSW 2. These maps illustrate the coarse resolution of spatial data collected from these logbooks, particularly compared with the spatial resolution of data from the South East Trawl Fishery. It is unlikely these two datasets could ever be combined in any meaningful way in the course of generating a national spatial report on the extent and intensity of trawling operations.
Petroleum and minerals exploration and extraction
Exploration for petroleum is the dominant offshore exploration activity in Australia. The number and size of all titles held for offshore petroleum exploration, production and retention since the commencement of activities in the early 1960s are recorded by and available from the Petroleum and Marine Division (PMD) of the Australian Geological Survey Organisation (AGSO).
The PMD, on an annual basis, and occasionally with greater frequency, publishes maps and map key listings identifying all petroleum titles and production licences held in Australia for both offshore and onshore regions, including all titles held under State jurisdiction. These maps are based on information held by State and Territory Departments of Mines/Mineral Resources and the Commonwealth Department of Industry, Science and Resources (DISR). Elements of the 'titles map' series published by PMD are currently being converted to GIS format for development as a comprehensive titling system. They are also the subject of an ongoing effort to develop a flexible and efficient digital titling and data referencing system.
Details of all exploration and extraction activity, including the extent of all drilling and rig activity are available in the form of yearly and quarterly activity reports. These are compiled and published by AGSO PMD using data from various sources such as the State and Territory Departments of Mines or Mineral Resources and the petroleum industry. Electronic versions of this information, as well as the location of new oil and gas discoveries, are also readily available via the AGSO website at http://www.agso.gov.au. Drilling locations can be made available in ArcView format following transfer from AGSO PMD database listings.
AGSO holds historical data on all titles since the commencement of offshore petroleum exploration in the early 1960s. AGSO also holds information on all relevant details of previous exploration, i.e. exploration prior to the granting of a title. Data on the nature and extent of all offshore petroleum exploration activity is available in a variety of formats. Some of the most recent records are presently being transformed into digital information suitable for display with commonly available GIS software.
Minerals exploration and extraction data available for Commonwealth waters include licence approval and renewal information, archived licence information, annual reports, seismic data and interpretations, drilling data and samples, geological interpretations and environmental data. Most of this information is held by the licence holder and is unavailable to the public under confidentiality provisions of the Act (until the licence is relinquished). For State and Territory waters similar information exists but with equivalent restrictions.
There is relatively little offshore exploration for minerals and there is no extraction of minerals, sand or gemstones in Commonwealth waters. There is, however, some mining activity in State and Territory waters. Data are collected and held by the relevant State or Territory regulatory authority. A list of active mineral exploration licences in Commonwealth waters is given in Appendix IV. An equivalent list for State and Territory waters is also shown in Appendix IV.
Between 1963 and 1995, 2-D seismic surveys in south east Australian waters were conducted over an area of approximately 75,000 n miles2. Operations were mainly focused on St Vincents Gulf in the west; off the south-west coast of Victoria in the Otway IMCRA region; off Strahan on the west coast of Tasmania; in central Bass Strait and on the Twofold Shelf and Hawkesbury Shelf (Figure SS 1). Surveys occurred in 12 IMCRA regions. The Twofold Shelf region had the greatest intensity of surveys calculated by IMCRA region (Figure SS 2). Areas of moderate or greater surveying effort are almost exclusively found in this area but are relatively concentrated around particular locations and therefore patchily distributed (Figure SS 1). The Otway and Central Bass Strait regions had moderate survey intensities, relative to other regions, though one very small area south of Port Macdonnell in the Otway region had very high survey intensities at the 1' grid level (Figures SS 1 and 2). Low survey intensities were recorded for the Hawkesbury Shelf, Franklin and St Vincents Gulf regions. Survey intensities calculated by IMCRA region were very low for the Manning Shelf, Flinders, Boags, Central Victoria and Coroong regions, relative to other regions (Figure SS 2).
The total area in which 2-D seismic survey effort was moderate or greater was only 1323 n miles2, less than 1% of the total area in which surveys had taken place. Very high surveying effort was restricted to 2nm2 south of Port Macdonnell in the Otway IMCRA region. Sixty percent of 2-D seismic surveys were located over the continental slope and ocean floor, beyond areas encompassed by IMCRA regions. However, with the exception of some areas immediately to the east of the Twofold Shelf region on the upper continental slope, the intensity of surveying in areas beyond the continental shelf was very light.
Twenty-three 3-D seismic surveys, totalling 103,180 km of track-lines, have been carried out in waters off Victoria since 1980. With the exception of one survey located just west of Cape Otway in 1994, all of these have been conducted on the Twofold Shelf (Figure SS 3). The level of seismic survey activity (both 2-D and 3-D) in Australian waters began increasing markedly in the early 1990s and reached a peak of nearly 1 million kilometres of survey tracks in 1998 (Figure 7.1). Seismic survey activity in the Twofold Shelf region has fluctuated markedly since 1961, with a small mode in the late 1960s and early 1970s and peak activity occurring in the years 1981 - 1983 and 1988 (Figure 7.2). The number and length of 3-D seismic surveys conducted per year since 1980 in the Twofold Shelf and Otway IMCRA regions is shown in Figure 7.3. While the greatest number of surveys took place in 1988 (4), maximum coverage occurred in 1993 when almost 42,000km of seabed was surveyed. In several years, particularly in the mid-1980s, no surveys were carried out at all.
Exploration drilling for petroleum in Australian waters has been largely focused on the Twofold Shelf and Pilbara IMCRA regions (Figure PW 1). Some level of drilling has occurred on much of the continental shelf to the northwest and southeast of the Australian mainland, and beyond the shelf in the northwest in particular. (Figure PW 1). Drilling operations have been conducted in 42 IMCRA regions. The Twofold Shelf region has had the greatest intensity of operations calculated by IMCRA region (Figure PW 2). Drilling intensity in the Pilbara (offshore) and Pilbara (inshore) regions has been moderate to low, relative to the Twofold Shelf. The intensity of exploration activities in all other regions has been very low by comparison (Figure PW 2).
Figure 7.1. Total length of offshore seismic survey tracks (2-D and 3-D combined)
Figure 7.2. Number of seismic surveys (2-D and 3-D combined) per year in the Twofold Shelf Region, South-east Victoria.
Figure 7.3. Number and length of 3-D seismic surveys per year in the Twofold Shelf and Otway IMCRA regions.
Over 550 exploration wells have been drilled in Australian waters in the past 10 years. A record level of offshore exploration drilling was reached during 1998 when the number of exploration wells drilled (74) easily surpassed the previous record of 64 wells drilled in 1990. This number subsequently declined to 51 in 1999 (Figure 7.4). Overall, there has been a steady upward trend in both the number and length of exploration wells drilled over the past 10 years (Figure 7.4).
A total of 34 petroleum production facilities have been established in Australian waters since the mid 1960s. The locations of these wells are shown on Figure PW 2. The majority of the production activity has occurred in the Pilbara (offshore) and Twofold Shelf IMCRA regions, with 14 wells sunk in each. Three wells have been sunk in the Oceanic shoals IMCRA region, two in the Pilbara (inshore) region and one in the North West Shelf region. The temporal trend for development wells is similar to that for exploration wells, though in this case peak activity occurred in 1997 rather than 1998 (Figure 7.4).
Sixteen man-made structures are present on the Twofold Shelf and deliver petroleum to onshore facilities via a network of pipelines and other seabed facilities. The number of active rigs has averaged 10 over the 5-year period from 1995 - 1999 with a range of 5 - 15 being operational at any given time (Figure 7.5). At the end of 1996, offshore petroleum licences in force or pending renewal in Australia covered about 350,000 km2. This was an increase from 317,100km2 in 1995. Daily crude oil production in 1996 averaged 536,000 barrels.
Figure 7.4. The number and length of wells in Australian waters (1989 - 1999)
Figure 7.5. Number of active offshore rigs (1995 - 1999)
Logbook records for prawn trawlers in the Northern Prawn Fishery (NPF) database from 1989 to 1998 were analysed. This database holds catch and effort details collected from prawn trawlers operating in the NPF. Catch and effort data from this fishery has been collected since 1969 by a series of logbooks. The current logbooks (NP12 and NP12A) were introduced in 1999. A description of the data collection in this fishery can be found in Sachse (1994). Further details of the fishery can also be found in Pownall (1994).
Each record in this dataset holds information for a single days fishing. Each vessel records the daily catch of each prawn species and the latitude and longitude of the greatest catch. Also recorded are the total hours searched and trawled.
The locations (latitude and longitude) of greatest catch are currently recorded by skippers using GPS or by reference to a 6x6 minute grid overlaid on the fishery area. Even larger grids were used very early in the fishery. The widespread recording of GPS positions is relatively recent, and many records from early years are reported by 6x6 minute grid, whilst some activities refer to the earlier grids. To standardise this positional information for analysis, the positional data were converted to the centre position of the 6x6 minute grid in which the reported daily position fell. A small number of records (<1%) that gave positional information using grids larger than 6x6 minute were rejected, as were records where geocode co-ordinates were missing, or the reported position was beyond the EEZ or on land.
Only vessels engaged in fishing for tiger prawns were analysed. The NPF can be divided into two sub-fisheries; one for banana prawns and the other for tiger prawns. While most vessels will fish in both fisheries, the characteristics of each are very different. Banana prawn fishing is concerned with fishing aggregations of banana prawns, with the majority of the day spent searching for these aggregations and very little time trawling. On the other hand, tiger prawn fishing is often restricted to the night time, and most available time is spent trawling (Robins and Somers, 1994). Over the last decade, many more days have been spent by the fleet trawling for tiger prawns than banana prawns (Robins and Somers, 1994). Using the method detailed in Robins and Somers (1994), approximately 23% of records from 1989 to 1998 were identified as banana prawn fishing and rejected from the analysis.
The following analyses were conducted to quantify and illustrate the spatial extent and intensity of prawn trawling within the fishery and changes in the spatial extent and intensity of operations between 1989 and 1998:
- Gridded fishing intensity (area trawled in 6 n miles2) for years 1989-1998
- Gridded trend in fishing intensity between 1989 and 1998 (change/SD)
- Fishing intensity (area trawled) by IMCRA region (standardised by area) for years 1989-1998
- Trend in fishing intensity between 1989 and 1998(change/SD) by IMCRA region
Area trawled per vessel per day (km2) was calculated from each record from the reported time spent trawling that day, the vessel's annually reported gear details (number of nets and the length of each net in fathoms) and an estimate of the average speed of vessels while trawling (3 knots), using the following equation;
Area = h*s*n*l
where: h = hours
s = speed of trawler = 3 knots = 5.556 km.hour-1
n = number of nets
l = headline length (fathoms*0.0018288 fathoms.km-1)
The techniques used to generate a gridded representation of the relative intensity of operations throughout the fishery in each year from 1989 to 1998 are described in the General Methods chapter. Unlike analyses conducted on the south-east fisheries, because data were converted to the centre position of the 6x6 minute grid in which the reported daily position fell, these surfaces represent the total area trawled in each 6x6 minute grid cell. The accuracy of these figures is, of course, dependent upon the accuracy and completion of the Northern Prawn Fishery database and the assumptions made in calculating the area trawled per operation (see above). To help remove random geocoding errors and to better define the main body of the fishery, cells having less than 2 boat-days within a 6x6 minute neighbourhood over 10 years were masked.
Between 1989 and 1998, prawn trawling operations in the Northern Prawn Fishery were conducted over an area of approximately 87,000 km2. The fishery extended from King Sound in the west, to Torres Strait in the east, and north to around 10°S (Figure NPF 1). Trawling occurred in 17 IMCRA regions, but at significant levels (> 10km2 over 10years) in only 16 regions (Table 7.2).
Core fishing areas (defined as areas in the 30-100 percentile range of trawl intensity values and shown on maps as moderate or greater fishing intensity) covered approximately 6,300 km2 and were located off the east coast of Arnhem land, the west coast of Cape York Peninsula, and in the south of the Gulf of Carpentaria, in the Groote, Carpentaria, Pellew, Wellesly, Karumba-Nassau and West Cape York IMCRA regions (Figure NPF 1).
The Groote region had the highest trawl intensity when calculated by IMCRA region (Figure NPF 2). At the 6 minute grid level, trawl intensities were very high north-east of Blue Mud Bay in the central-north of this region (Figure NPF 1). Relative trawl intensity was calculated to be high, moderate and low for the Wellesley, Pellew and West Cape York regions respectively (Figure NPF 2). However, areas of very high trawl intensity were observed in 6 minute grids north of Mornington Island in the Wellesly region, and south-east of Albatros Bay in the West Cape York region (Figure NPF 1). Trawl intensity in the Karumba-Nassau and Carpentaria regions was low relative to other IMCRA regions in the fishery, though areas of moderate trawl intensity were recorded in 6 minute grids in the west of the Karumba-Nassau region and in the south-east of the Carpentaria region (Figures NPF 1 and 2).
|Van Diemens Gulf||2||13||9||0||0||0||24||0||0||0|
|North West Shelf||13||12||0||30||0||0||0||0||0||0|
|West Cape York||7066||4079||743||1445||1051||3017||3923||8714||6832||999|
The area over which operations were conducted each year decreased from approximately 72,000 km2 in 1989 to around 45,000 km2 in 1998 but was lowest in 1995 (Figure 7.6). Trawling effort also decreased from 70,651 km2 in 1989 to 59,370 km2 in 1998. Trawl effort and trawl area showed similar trends over time, though the area over which operations were conducted was consistently lower than the total area trawled from 1991 onwards (Figure 7.6).
The Anson Beagle IMCRA region showed a very large increase in trawling intensity between 1989 and 1998 compared with changes in intensity in other regions (Figure NPF 3). This was due to very low trawling effort in 1989 and 1991 and a peak in intensity in 1998. However, trawling intensity in the region is highly variable from year to year (Table 7.2). A moderate increase in trawling intensity in the Pellew IMCRA region (Figure NPF 3) was due to a large jump in trawling effort in 1998 (Table 7.2 and Figure 7.7). Prior to 1998, trawling intensity in the region was generally declining to a low in 1997 (Table 7.2). There was a small increase in trawling intensity in the Cobourg region (Figure NPF 3). However, annual variation in trawling effort was very high and this result was basically a consequence of relatively high trawling intensity in 1998 (Table 7.2 and Figure 7.7).
Figure 7.6. Change in trawling effort (km2) in the Northern Prawn Trawl Fishery and the area of the fishery (km2) between 1989 and 1998.
A large decrease in trawling intensity was recorded for the West Cape York IMCRA region (Figure NPF 3). As with most of the fishery, trawling effort in this region was highly variable between years and the declining trend was primarily due to relatively low trawling effort in 1998. Moderate decreases in trawl intensity were calculated for the North West Shelf, Tiwi, Carpentaria and Wellesly regions (Figure NPF 3). Trawling did not occur on the North West Shelf after 1992 (Table 7.2). Trawling effort in the Tiwi region peaked in 1991 and was subsequently relatively low (Table 7.2 and Figure 7.7). Trawling intensity in the Carpentaria region fell significantly after 1994, but was increasing in 1998 (Table 7.2). The Wellesly region had generally lower levels of trawling after 1991 than before 1991, but had a peak in trawling intensity in 1995.
Small declines in trawling effort between 1989 and 1998 were calculated for the Kimberley, Bonaparte Gulf and Karumba-Nassau IMCRA regions (Figure NPF 3). However, trawling effort was highly variable between years in these regions. No trawling was recorded in the Kimberley region after 1996. Trawling intensity in the Bonaparte Gulf region was lowest in 1993 before reaching a peak in 1997 followed by a sharp decline again in 1998 (Table 7.2). Trawling effort in the Karumba-Nassau region has been declining steadily following a peak in 1995.
Figure 7.7. Trawling intensity (km2 trawled per 6' grid) by IMCRA region in the Northern Prawn Trawl Fishery between 1989 and 1998.
The direction of change in trawling intensity (increase or decrease) was spatially variable within IMCRA regions at the 6' grid level (Figure NPF 4). Both increases and decreases in intensity were recorded in different parts of the same IMCRA region in all regions.
The Northern Prawn Fishery showed a substantial decline in both fishery area and trawling effort between 1989 and 1993. This was mainly due to a buy-back scheme that operated between 1987 and 1993, which reduced the number of vessels operating from 230 to 125 (Die et al 1998). Since 1993 the number of boats operating has remained stable. Although the amount of trawling undertaken each year and the total area of the fishery have increased since 1993, trawling effort and the area of the fishery are still less than that observed in 1989. The small increases in trawling area and intensity since 1993 appear mainly due to a small increase in the number of boats operating in the fishery and the lifting of gear restrictions in 1994.
The amount of trawling in the fishery decreased less than did the total area of the fishery between 1989 and 1993 and increased more than did the total area of the fishery between 1993 and 1998. Consequently, while the size of the fishery and the amount of trawling are both less in 1998 than they were in 1989, trawling intensity in the reduced fishery area is greater. This is because much of the reduction in the area of the fishery occurred at its edges, particularly in the Carpentaria and Kimberly IMCRA regions, where trawling intensity was relatively low.
The amount of trawling in the fishery fluctuated greatly from year to year between 1989 and 1994, particularly in the Groote IMCRA region. In contrast, trawling effort for the fishery has been relatively constant from 1994 to 1998. Interestingly, trawling effort has fluctuated spatially over this period. As trawling in the Groote region has decreased, trawling in the West Cape York and Wellesly regions has increased, and vice versa, resulting in a relatively stable overall trawling effort for the fishery since 1994.
Logbook records for otter trawl vessels in the South East Trawl database from 1989 to 1998 were analysed. This database holds catch and effort details collected from otter trawl and Danish seine fishers operating in the South East Trawl Fishery. Catch and effort data from this fishery has been collected since 1985 by a series of logbooks. The current logbook (SEF1B) was introduced in 1999. A description of the logbook data collection practices for the South East Trawl fishery can be found in Garvey (1998b).
Each record in this dataset holds information on an individual fishing operation. Records with geocode errors (i.e., where geocode co-ordinates were missing, or the reported position was beyond the EEZ or on land) were removed from the dataset prior to analysis. Records where fishing operations were not conducted were also removed, as were records where the start and end values for either latitude and longitude were separated by more than 30' of latitude or longitude.
The following analyses were conducted to quantify and illustrate the spatial extent and intensity of trawling within the fishery and changes in the spatial extent and intensity of operations between 1989 and 1998:
- Gridded fishing intensity (distance trawled in a 15 n mile radius) for years 1989-1998
- Gridded trend in fishing intensity between 1989 and 1998 (change/SD)
- Fishing intensity (distance trawled) by IMCRA region (standardised by area) for years 1989-1998
- Trend in fishing intensity between 1989 and 1998(change/SD) by IMCRA region
The techniques used to generate a gridded representation of the relative intensity of operations throughout the fishery in each year from 1989 to 1998 are described in the General Methods chapter. To help remove random geocoding errors and to better define the main body of the fishery, cells having less than 3 km of trawl lines within a 15x15 minute neighbourhood over 10 years were masked.
Between 1989 and 1998, fish trawling operations in the South East Trawl Fishery were conducted over an area of approximately 58,000 n mile2. The fishery extended from south of St Vincents Gulf in the west, along the 200m bathometry contour around Tasmania in the south and then, also predominantly following the 200 m bathometry contour, north to Port Stephens (Figure SET 1). Fishing occurred in 13 IMCRA regions but was significant (> 5km trawled over 10 years) in only 9 regions (Table 7.3). Operations conducted on the continental slope adjacent to these regions accounted for 60% of all trawling between 1989 and 1998.
Core fishing areas (defined as areas in the 30-100 percentile range of trawl intensity values and shown on maps as moderate or greater fishing intensity) covered approximately 7,500 n mile2. These areas were located south and east of Portland in the Otway IMCRA region and on the adjoining continental slope; on the slope immediately west of the northern part of the Franklin region; throughout most of the Batemans Shelf region and northern parts of the Twofold shelf region off Eden, and along the edge of the continental shelf south-east of Ninety Mile Beach (Figure SET 1).
The Batemans Shelf region had the highest trawl intensity when calculated by IMCRA region as shown in Figure SET 2. However, trawl intensities in the region at the 15' grid level were moderate compared with parts of the Twofold Shelf region (Figure SET 1). Overall trawling effort was moderate in the Twofold shelf region, but areas of very high trawling intensity were evident south-east of Eden in the northern part of the region at the 15' grid level (Figures SET 1 and 2). The Hawkesbury Shelf, Freycinet and Bruny regions all had low trawling intensities, relative to other regions. Overall trawling effort was calculated to be very low for the Flinders, Davey and Franklin regions (Figure SET 2). The Otway region also had a very low overall intensity of trawling, though areas of moderate trawling intensity were apparent at the 15' grid level to the south-east of Portland on the edge of the continental shelf (Figures SET 1 and 2)
The area over which operations were conducted each year in the South East Trawl fishery decreased from approximately 46,000 n miles2 in 1989 to around 41,000 n miles2 in 1993 before increasing to approximately 48,000 n miles2 in 1998 (Figure 7.8). The amount of trawling also decreased from 2,389 km in 1989 to 2,004 km in 1992, before increasing to 5,108 km in 1997. Fishery area figures for each year were only indicative approximations, owing to the poor spatial resolution of much of the data in earlier years, and the spatial analytical methods applied. Consequently, a smoothed curve (3rd order polynomial) was fitted to these data to help illustrate the trend over time (Figure 7.8). The decline in the area of the fishery from 1990 to 1993 is probably largely a function of increased use of GPS by skippers to record operation locations and a decline in the use of the fishery grid system.
There was a very large increase in trawling intensity in the Twofold Shelf region, relative to other regions, between 1989 and 1998 (Figure SET 3). Trawling increased significantly after 1992 and peaked in 1997 (Table 7.3 and Figure 7.9). More moderate increases in trawling intensity were recorded for the Hawkesbury Shelf, Batemans Shelf, Bruny and Franklin IMCRA regions (Figure SET 3). Trawling on the Hawksbury Shelf peaked in 1997 after almost disappearing in 1992 (Table 7.3 and Figure 7.9). Similarly, trawling intensity on the Batemans Shelf increased steadily from a low in 1991 to a peak in 1996 before falling away slightly in 1997 and 1998 (Table 7.3 and Figure 7.9). Trawling in the Bruny region was relatively light in all years, but peaked between 1993 and 1996 after a low in 1989 (Table 7.3 and Figure 7.9). Trawling intensity in the Franklin region was very low in all years, though trawling on the continental slope immediately adjacent to the northern half of this region was moderate at the 15' grid level and increasing (Figures SET 1 and SET 4).
Figure 7.8.Change in trawling effort (km) in the South East Trawl Fishery and the area of the fishery (n miles2) between 1989 and 1998.
The large decrease in trawling calculated for the Otway region between 1989 and 1998 (Figure SET 3) was predominantly due to a substantial drop in trawling intensity in 1998 (Table 7.3). Trawling intensity in the Otway region was low and quite variable between 1989 and 1998, with a peak in 1990 and trough in 1992 (Table 7.3). Small decreases in trawling intensity were calculated for the Freycinet and Davey IMCRA regions, relative to other regions (Figure SET 3). The trend in the Freycinet region was very weak and solely due to a peak in trawling intensity in 1989, as shown in Table 7.3. Trawling effort in the Davey region was extremely light in all years (Table 7.3).
The direction of change in trawling intensity (increase or decrease) was spatially variable within IMCRA regions at the 15' grid level (Figure SET 4). Both increases and decreases in intensity were recorded in different parts of the same IMCRA region in all regions except the Bruny region where trawling intensity increased in all areas.
Figure 7.9.Trawling intensity (km/n miles2) by IMCRA region in the South East Trawl Fishery between 1989 and 1998.
The amount of trawling in the South East Trawl fishery has almost doubled since 1989 and more than doubled between 1992 and 1997. However, the area of the fishery decreased significantly between 1989 and 1993 before increasing to a size in 1998 only slightly greater than in 1989. Overall trawl intensity in the fishery has therefore increased nearly twofold. This is due to a shift from target fishing to 'mixed-species' or 'market fishing' on both the continental shelf and upper-slope (Tilzey and Chesson 1998). This shift is because of a number of factors including a reduction in orange roughy quota, continued failure of the gemfish fishery and restrictions due to individual operator's quota holdings (Tilzey 1999).
Most of the increase in trawling effort has occurred in core areas of the fishery, particularly on the Twofold Shelf. Trawling intensity around the edges of the fishery is generally low and in many areas is decreasing. The greatest changes in trawling intensity though were increases on the upper continental slope due south of Portland and on the upper slope south-east of the Twofold Shelf; and decreases in the Freycinet IMCRA region and on the upper slope east of Batemans Bay. This is probably due to an increased interest in blue grenadier and ling, and an active avoidance of the winter gemfish run (Tilzey 1999). The majority (60%) of fish trawling in the South East Fishery takes place on the upper continental slope, including many areas of relatively high intensity trawling. Consequently, IMCRA regions are inappropriate analytical units to use for spatial studies of the fishery or for monitoring temporal trends.
Trawling effort in the South East Trawl Fishery does not fluctuate greatly between years. The amount of trawling was relatively stable between 1989 and 1992 before increasing consistently from 1992 to 1997. A small decrease was noted in 1998. These patterns reflect the trend in trawling effort in the Twofold Shelf IMCRA region and, to a lesser degree, in the Batemans shelf region. The Twofold Shelf region represents more than 60% of trawling in IMCRA regions and one quarter of all trawling effort in the fishery. Together, the two regions comprise over 80% of trawling in IMCRA regions and one third of all trawling effort in the fishery.
Area swept by trawling can be calculated in two ways, either by using start and end points (latitude/longitudes), or by using soak time and boat speed for each shot. However, there is uncertainty as to whether swept area should be calculated from between the ends of the wings or the ends of the boards. Except where specifically mentioned below, assumptions also need to made about the mouth size of nets.
The data collected by AFMA for Commonwealth fisheries are adequate for identification of fishing grounds and analyses of fishing intensity by grids or IMCRA regions. However, there is significant trawling effort beyond the continental shelf in areas not represented by the IMCRA regionalisation. All the relevant logbooks, except NP12, NP12A and BS02, collect latitude/longitudes for start and end positions and trawl time. NP12 and NP12A collect trawl time on log sheets and an indication of usual trawl speed is provided on the gear sheet. BS02 collects information on the number of hours dredged and the towing speed. The information collected in gear sheets is detailed enough to enable reasonable estimation of the average mouth area of nets and dredges.
Data collected by Victorian, South Australian, Western Australian, Queensland and Northern Territory trawl fisheries, and by the Tasmanian Scallop Fishery, are adequate for identifying fishing grounds and analysing fishing intensity by IMCRA regions. The zones used for spatial data collection in New South Wales and blocks used in Tasmanian fisheries other than the Scallop Fishery, are too coarse for reporting by IMCRA regions.
It is not possible to calculate the area swept by demersal or other trawling from fisheries data in Victoria, South Australia, Western Australia, New South Wales and Tasmania (except for the Tasmanian Scallop Fishery), as only soak time is collected.
For Queensland trawl fisheries, hours trawled and trawl speed are collected from the logbook and gear sheet respectively and an estimate of area trawled can therefore be calculated. Detailed descriptions of gear are provided on the gear sheet, so reasonable estimates for average mouth area of the net can be made. Soak time and trawl speed are collected in the Northern Territory, thereby also allowing area swept by trawling to be calculated. Information collected on gear is limited, so assumptions would still need to be made about the average mouth area of the nets. For the Tasmanian Scallop Fishery, the number of hours dredged and the towing speed are both collected, as is information on the dimensions of the dredge used. It is therefore possible to estimate the area dredged.
There is evidence that fishing operations that damage or reduce marine habitats, such as many types of demersal trawling operations, are likely to have a direct adverse impact on marine species that utilise those habitats. History has shown that populations are more vulnerable to a manipulation of their habitat than they are to a direct manipulation of their numbers (Caughley 1980). This phenomenon is not surprising when considered in relation to the principles of rate of increase. A population attacked frontally by passive fishing operations does not have to contend with a deteriorating habitat. The reduction in density occasioned by the fish harvesting leaves the quality of resources intact while increasing the quantity available to each surviving animal. The fishing automatically boosts the potential rate of increase. Populations will only consistently decline if the number of individuals harvested is greater than the sustainable yield.
It is an oversimplification, however, to suggest that trawling always has adverse impacts relative to passive fishing on marine populations. Recent modelling and simulation work by Harris and Huang (1998) suggests that while the Northern Prawn Trawl Fishery is likely to have a negative impact on most fish species, it may have a positive impact on the ecosystem as a whole in terms of increased total biomass, and increased total net primary production.
Petroleum exploration and extraction
The number of approval instruments or size of areas held under petroleum title are not necessarily synonymous with the pressure on ecosystems because the type and level of activity can vary considerably within those areas. Hence, they are probably poor indicators of marine disturbance by petroleum exploration and extraction activities. In some cases, areas held under petroleum title may not be subjected to any form of specific activity (e.g. Retention Leases). In other areas, only selected parts of the region may be subject to short term or periodic activity. This may take the form of geophysical or airborne surveys, or in the case of a specific location, the drilling of petroleum wells.
By contrast, the exploration activities themselves may give a much more accurate indication of disturbance. Seismic surveying involves the transitory passage of survey vessels trailing long cables towing an array of air guns through the water column within 10 metres of the sea surface. The air guns are discharged every 5 - 10 seconds and emit sonic signals in the frequency band of 20 - 150 Hz (Malme et al. 1986) which is within the auditory range of many marine species (Hawkins 1993). There are two strategies for surveying. The first is the 2-D survey which is typically a single line ("track") across, for example, a title. The second is the intensive 3-D survey which is a systematic grid coverage of a discrete area. 3-D survey tracks are typically spaced 25 m apart. The time frame for a geophysical survey can vary widely from a few weeks to several months but is generally undertaken over a 20 to 30 day period. As most parts of the continental shelf have already been subjected to geophysical survey operations, present day activities are generally in the form of a grid pattern over geological features of specific interest, in areas where industry activity is most concentrated. Such survey work is aimed either at in-fill coverage or at detecting subtle geological features.
Although 60% of 2-D seismic survey streamer lines are located over the continental slope, these tracks are generally very long and widely dispersed. IMCRA regions may therefore still be appropriate spatial analytical units for studies of relatively heavily surveyed areas and for monitoring temporal trends in these areas. It is currently feasible to spatially present annual trends in the area and intensity of seismic surveying and exploration drilling operations in Australian waters using fine scale gridded analyses given current data collection procedures.
A reduction in catch rates of some important commercial fish species (cod, haddock, rockfish) as a result of simulated seismic survey work has been demonstrated in the Barents Sea (Lokkeborg and Soldal 1993; Engas et al. 1996a, 1996b) and off the coast of California (Skalski et al. 1992). Similar effects on fish stocks in Australia may be possible.Work is currently underway to examine the effects of seismic surveys on fish stocks in the SE Trawl and Southern Shark fisheries but it is hampered by the coarse spatial resolution of most of the fisheries data. Whereas seismic survey locations may be known with great accuracy, fish catches are reported by large statistical blocks, making comparisons between the two sets of data difficult. By the same token, the difficulties associated with seismic survey data include integration over long time periods (weeks or months). Precise individual track line information is available from AGSO tape archives but is time-consuming to obtain. It can therefore be difficult to determine the exact location of the survey vessel on a particular day and thus to correlate survey activity with simultaneous catch rates of fish. There is also the issue of commercial-in-confidence provisions, with a normal embargo period of 2 years applying to petroleum exploration data. This leads to a time lag in monitoring the effects of these activities. Hence, it remains to be seen whether knowledge of the levels of seismic survey activity can be used to infer disturbance effects.
This indicator will be of regional relevance rather than national as the effects will be localised and will vary for different components of the biota. It is also important to consider the sub-components of geophysical exploration activities (i.e. 2D and 3D seismic surveys) separately because of the different levels of intensity of these two types of survey. Other forms of geophysical exploration such as magnetic and gravity surveys rely on passive rather than active sensors and therefore are unlikely to create any more disturbance than normal shipping activities.
The impact of production or extraction activities on marine species and habitats depends on the nature of the facility. There may be limited impact at the location of the production facility if processing is undertaken offsite. Locations where petroleum processing is undertaken may discharge Produced Formation water (PFW) and/or vent gas or exhaust fumes from the combustion of natural gas. PFWs can extend for many kilometres from a production facility and can contribute very large inventories of benzene, toluene and other compounds into marine systems over time. This is particularly so in a basin such as the Gippsland Basin, where fields are producing ever increasing (and very large) volumes of PFWs through their life. Even though the concentrations of such substances may be low, the net input can be large because of the high volumes of PFWs put out by the facilities. Relevant papers on this topic by King and McAllister, Heggie et al., Furnas and Mitchell, and Burns and Codi appear in the 1998 APPEA volume (Volume 38).
The impact of offshore exploration and extraction drilling operations is largely related to the presence of cuttings or drillings waste and drilling muds on the sea floor (Lavering 1994). As a result of monitoring studies, there is a reasonably good understanding of the ecological changes which disposal of these products may bring about. Four zones of effect have been identified during a period of active drilling and waste ("cuttings") disposal. The cuttings pile will comprise a footprint of less than 50m in diameter within which fauna will be effectively smothered. Some studies have found that this zone may be colonised by opportunistic species that are tolerant of both the material and environment of the cuttings pile.
Between 50 and 500m of the point of discharge, an area of impoverished and highly modified faunal assemblages is evident. This is a consequence either of the fine-grained material clogging the filter-feeding mechanisms of suspension feeders or alteration of the natural sediment type such that deposit or sediment-feeding species are unable to utilise it. As a result, the population size of some species will decline dramatically while others will become superabundant. These latter may be used as a key indicator of localised environmental disturbance.
Beyond the zone of major faunal change, and up to 2km from the point of discharge, there is a transition zone in which community composition gradually returns to normal. Some indications of deleterious impacts will still be evident in the form of changes in species composition but to a much lesser degree. Further out still, minute quantities of material derived from drilling waste may still be detectable but species composition, diversity and abundance return to normal.
The extent of these changes to benthic communities within each of the four zones has been shown to decrease after the discharge of drilling waste ceases. Generally, the environment will return to normal after a period of 18 - 24 months, although some traces of material derived from drilling will remain as additives to the sea floor sediment.
Where many production wells have been drilled from a major facility, the impact of disposed drilling waste is compounded and expands the impact zone on the sea floor (Terrens et al. 1998). The effect of the cumulative input of such material has however, proved to be limited and transitory. For further reading on this topic, the reader is referred to paper sited in the references of Terrens et al. 1998.
- The data from the South East Trawl fishery and from petroleum exploration and extraction operations indicate the Twofold Shelf region may be the most highly disturbed of the IMCRA regions in south-eastern Australia with respect to indicator 4. However, IMCRA regions are inappropriate analytical units for spatial and temporal analyses of the South East Trawl fishery because significant amounts of trawling occur beyond the continental shelf.
- The intensity of trawling in the South East Trawl fishery is increasing. Trawling intensity and area in the Northern Prawn Trawl fishery have decreased substantially in the last decade, but currently appear to be slowly increasing. Core fishing areas in trawl fisheries are generally small relative to the total area of a fishery. Consequently, areas of relatively high trawling intensity tend to be very small relative to the total area of a fishery.
- It is currently possible to calculate the area of seabed swept by trawl gear to spatially represent trawl effort for indicator 4(a), contingent on certain explicit assumptions. However, the coarse spatial resolution of some logbook data is a limiting factor, with only 13 of the 18 logbooks and forms identified as data sources for the indicator meeting the spatial data quality criteria that were developed to map the indicator.
- The spatial analysis of temporal trends in the area and intensity of trawling operations is currently confounded by changes in the spatial precision of logbook data over time.
- Levels of hydrocarbon exploration and production activity have fluctuated markedly from year to year, but overall there has been a small increase in both types of activity since the early 1990s, particularly in terms of the length, if not number, of survey tracks and wells.
- Petroleum industry activities and facilities occupy only a very small surface area of the sea bed and, in the case of exploration activities, are of short-term duration. They are concentrated almost entirely in the region of the Gippsland Shelf (Twofold Shelf IMCRA region), although recently new fields have been discovered and opened up for exploration on the Carnarvon Shelf off northwest Australia.
- The number of approval instruments or size of areas held under petroleum title are not useful indicators of the pressure on ecosystems because the type and level of activity can vary considerably within these areas.
- Whilst there is some potential to use the extent and intensity of seismic survey activities as an indicator of marine disturbance, a clearer understanding of their impacts awaits the outcome of current research activities.
- The number and location of active and recent drilling wells can be used with some confidence to infer the extent of impacts on benthic flora and fauna.
Based on our investigation of this indicator, we recommend that:
- The indicator should be split into the four indicators shown in Table 7.4.
- Management bodies should be encouraged to improve the accuracy and precision of information being received from fishing vessels. Logbooks should collect, as a minimum, the data listed in Table 7.5.
- Further research is necessary to improve and quantify our understanding of the relationship between seismic surveying and impacts on marine organisms if exploration effort is to be used as a surrogate of marine disturbance.
- Work on further sub-regionalisation of the current IMCRA regions and incorporation of the waters above the continental slope is required.The impact of trawling and mining on marine species and communities in the Twofold Shelf IMCRA region should be examined as a priority.
|1. Demersal trawling and dredging (seabed disturbance)||Area (trawled or dredged) per unit area|
|2. Mining (seabed disturbance)||Area disturbed per unit area|
|3. Pelagic trawling||Area trawled per unit area|
|4. Sonic surveying||Area sounded per unit area|
|Spatial||Fishing location given by latitude or longitude or by reference to a grid of 6 minutes square or less|
|Temporal||Fishing operations reported individually or details of fishing reported daily|