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Pressure

Note: Some of the material contained in this chapter is out of date and currently under review (1/7/2004)

Pressure

The two most readily identifiable sources of direct human pressure on the biodiversity, ecosystem function and ecological integrity of inter-reefal and lagoonal benthos are trawling and increased run-off from the land (including sediments, nutrients and other pollutants). Relatively speaking, far more information exists about trawling and its effects on seabed communities than about the effects of increased run-off or fishing other than trawling such as line fishing and netting on these areas.

Pressure: Trawling

In 2000, the ECTF produced some 8,300 tonnes of product. The mean annual Gross Value of Production of the fishery in the Great Barrier Reef for 1996-2001 was $87.3 million. Most of the Queensland catch is taken from the Great Barrier Reef World Heritage Area with prawns, scallops, bugs and squid being the main fishing targets. In addition, sand crabs, barking crayfish and a few other species are caught. Only seven of the 22 species of penaeid prawns found in the World Heritage Area are of commercial significance. Prawns and other by-product species are also taken by beam trawlers, however beam trawlers account for only five per cent of the trawl harvest and only operate in estuarine and inshore areas.

The Trawl Plan capped the number of days trawlers can operate at 1996 levels

In order to take part in the ECTF, a boat must have a licence from the Queensland Fisheries Service. The allowable size of fishing vessels is currently limited to 20m. In October 2002, there were 517 licensed trawlers of which 485 were both licensed and operationally capable of fishing in the Great Barrier Reef Marine Park with otter trawl gear. This is a significant decrease from the 870 licensed vessels operating in the GBRMP in 1998. In 2001, the average number of days fished per boat was 110 days with a total of 75,351 days fished by the fleet. With the introduction of a new fisheries management plan for the ECTF, the Trawl Plan, in December 2000, the number of fishing days was capped at 1996 levels and reduced by 15 per cent through a structural adjustment scheme. The capping of fishing days has removed latent effort and reduced the effort (measured by the number of allowable fishing days) from peak levels recorded between 1995 and1998. Nevertheless, while one of the main objectives of the Trawl Plan is to reduce effort, the current number of allowable days is based on the number of days fished in 1996 which was the third highest recorded, and is not derived from biological stock assessment or sustainability data. The current number of allowable fishing days is some 22,000 days more than that in 1988 and technological advances have increased fishing efficiency, further increasing the potential pressure on stocks, bycatch and the ecosystems on which they depend.

Trawling produces large amounts of bycatch

Trawling effort is spread along the Queensland east coast. However, most of the catch comes from the GBRMP with more than 50 per cent of the catch taken from less than 30 per cent of the area. About eight per cent of the GBRMP is trawled in excess of 100 boat days per year. Trawling pressure is concentrated in the lagoonal area because it tends be the preferred habitat for commercial prawn species, other than the red spot king prawn which is associated with reefal and inter-reefal habitats. Within the GBRMP the greatest catch comes from Princess Charlotte Bay and the waters offshore from Townsville.

Trawling is a very unselective method of fishing with no ability to selectively catch target species. In addition to the take of target species, trawlers bring up very large amounts of plants and animals that are of no use to the fishers. These plants and animals are known as bycatch. Compared to other kinds of fishing, bottom trawling, especially trawling for tropical prawn species, generally generates the highest amounts of bycatch. Some trawl fisheries catch eight to ten times more weight of bycatch than of target species. Bycatch is typically thrown back into the sea, where most of it dies.

In 1998, the Commonwealth Scientific and Industrial Research Organisation and the Queensland Department of Primary Industries & Fisheries completed the most detailed study of the effects of trawling on tropical seabed communities yet undertaken. The research was carried out in the Far Northern Section of the Great Barrier Reef Marine Park. The results show that the effects of trawling are complex and highly variable according to the frequency of trawling on a particular area of seabed and the physical and biological characteristics of the area. The results of the study indicate that trawling affects three different types of biological community:

  • the community of animals and plants that live more or less permanently attached to the seabed (sessile organisms);
  • the community of animals (mostly fishes) that live near to the seabed; and
  • various groups of animals that scavenge on trawler discards (including aquatic animals on the seabed and in the open water, and birds).
Repeated trawls can remove between 70 percent and 90 percent of seabed biomass.

The effects of trawling on sessile organisms is highly dependent on the frequency of trawling. Trawling over a previously untrawled area removes about one tonne of attached seabed animals and plants per square kilometre trawled, typically about ten per cent of the biomass attached to the seabed. Repeated passes of a trawl over the same area can remove a large proportion of the seabed life. However, the total area of seabed that is trawled 13 or more times each year may be less than one per cent of the total inter-reefal and lagoonal habitat. After 13 passes of a trawl, 70-90 per cent of the biomass on the seabed was removed. Removal rates vary between about five per cent and 20 per cent per trawl for different species. Additionally, benthos that is not removed by the passage of a trawl net may suffer significant damage. Because various species are impacted to different degrees by trawling, intensive trawling can cause changes in the community composition of intensively trawled seabed. In practice, the typical pattern of commercial trawling is to trawl many times over high-yielding areas. Thus, areas subjected to such intensive trawling are likely to be significantly impacted.

In the Far Northern Section trawl study, 245 species of fishes were caught in prawn trawl nets out of the 340 fish species found living near the seabed. Prawn trawl nets caught 28 kg of fish per hour of operation, with fish comprising between one-half and three-quarters of the bycatch. Few of these species were of commercial or recreational fishing importance, with no trout or cod species being caught.

Trawling also has indirect effects on populations of scavengers such as seabirds, sharks, dolphins and small fishes and invertebrates living on the seabed. Scavengers that are highly mobile (mostly birds, sharks and dolphins) congregate around trawlers to feed on the bycatch as it is thrown overboard. Between 60 and 90 per cent of discards sink and thus are not available to surface scavengers such as seabirds, dolphins and some sharks. The effect of these discards on the populations of such scavengers is mostly unknown, however the population of at least one species of seabird (crested terns) in the Far Northern Great Barrier Reef may be unnaturally high due to the increased amount of food made available through trawler discards.

Pressure: Terrestrial run-off

Another pressure on lagoonal seabed communities is increased run-off of sediments, pesticides and nutrients from the land. This increased run-off is a result of development, particularly agriculture, on the mainland adjacent to the Marine Park, and the loss of coastal habitats such as rainforests and wetlands that would otherwise “filter” run-off. For more information on the processes involved, see Environmental Status - Water Quality. It is likely that increased run-off of nutrient laden sediments and other pollutants, and changes in the type of sediment run-off adversely affect nearshore seabed communities. Mechanisms by which this could happen include:

  • the smothering of benthos by increasingly fine sediment and marine snow (sticky mucous like tendrils of suspended marine mud that form in estuarine waters that have high nutrient and sediment levels);
  • increased turbidity which reduces the amount of light available to photosynthetic organisms such as seagrass;
  • direct toxic effects of pollutants such as heavy metals, and pesticides such as diuron that can cause abnormal growth and reproduction, and mortality;
  • algal blooms that cause changes in community composition (increase in the number of filter feeding organisms, decrease in the number of organisms that are susceptible to overgrowth by algae); and
  • metabolic changes in some organisms that lead to reduced reproductive output and lowered resistance to environmental stresses.
Run-off from the land can adversely affect inert-reefal and lagoonal benthos through a variety of mechanisms.

There is little research specifically directed to answering the question of whether or not nearshore seabed communities have been affected by these pressures. Furthermore, there are no scientific data available about the condition of nearshore areas prior to grazing and intensive cropping to use as a comparison. Nevertheless, unequivocal evidence that inter-reefal and lagoonal benthos were being adversely affected by runoff would likely only appear after these areas had been severely degraded. Meanwhile, it is widely accepted that land use patterns in the Great Barrier Reef catchment have significantly changed, that coastal wetlands and rainforest habitats have been severely degraded and that the amount of sediment, pesticide and nutrient runoff has significantly increased. There is also evidence that some coral reefs in inshore areas show signs of ecological dysfunction characteristic of nutrient and sediment stress. Considering these factors, the pressure from land based run-off on inter-reefal and lagoonal benthos is a significant concern.

Additional pressures

Other factors apart from trawling and terrestrial run-off exert pressure on the inter-reefal and lagoonal benthos. Factors such as climate change may have widespread effects on the condition of inter-reefal and lagoonal benthos, however the mechanisms by which these changes occur and the effects such changes may have are difficult to predict and are largely unknown.

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