Indicator: CO-20 Non-target effects: Area of seabeds trawled

Data

No recent Australian studies appear to have been undertaken regarding the impact of trawl fishing on the seabed since the five year study of prawn trawling in the far northern Great Barrier Reef completed in 1996 and referenced in the 2001 National State of the Environment Report. This Report showed that 13 trawls of the seabed removed 70% to 90% of the seabed.

Several earlier studies from Australia and some more recent studies from elsewhere in the world (see “Further information”, below), provide insights into the potential impacts of benthic disturbance by both trawling and oil and gas exploration.

Coleman and Williams provide several examples of widespread marine ecosystem engineers that increase the structural complexity of the habitat, the local biomass, and the local biodiversity, with additional ecological influences distinctive to each category. Most of this engineering takes place on the sea bed. While some of the engineers are at risk of direct harvesting by fisheries, the habitat they have created is also at risk of destruction by trawling.

  • Corals, oysters, vermetid gastropods, sabellid worms and crustose coralline algae construct large solid mineralized reefs. These provide settlement substratum for other organisms and provide refuge from predation.
  • Marine plants (e.g. seagrasses and kelps) form canopies of vegetation in nearshore waters. They modify water flow, entrain larvae and provide refuge from predation.
  • Bivalve molluscs (e.g. mussels and clams) build thick shellfish beds and mats on rocky shores and in soft sediments.
  • The structure provided by shells and by byssal threads of molluscs serve to ameliorate environmental extremes, deposit organic matter, fertilize sediments and promote growth of marine plants.
  • Tilefish, groupers, clams, amphipods, specific types of shrimps (callianassid, alpheid), sea cucumbers, fiddler crabs and worms form excavations and burrows, sometimes meters deep.
  • While foraging, herbivorous sea turtles and dugongs create large gaps in seagrass beds. Dugongs ‘bulldoze’ through vegetation and sediments.

Source: Coleman, F.C. and Williams, S.L. 2002, 'Overexploiting marine ecosystem engineers: potential consequences for biodiversity', Trends in Ecology and Evolution, vol. 17, pp. 40-44, viewed 24 May 2006, http://72.14.203.104/search?q=cache:TX5SjDcc
_acJ:www.bio.fsu.edu/mote/colemanTREE_01.02.pdf++%22overexploiting+marine+ecosystem+engineers
:+potential+consequences+for+biodiversity%22&hl=en&gl=au&ct=clnk&cd=1.

What the data mean

No data were found on estimated seabed removed by trawling in Australian waters since 2001. However, the various studies show the extreme risk of seabed trawling to marine ecosystems.

Data Limitations

No data available.

Issues for which this is an indicator and why

Coasts and Oceans — Direct pressure of human activities on coasts and oceans - Pressure of fishing 

Area of the seabed trawled would provide a surrogate for the extent of benthic ecosystems potentially disturbed by commercial fishing activities. It is a crude indicator because it does not estimate either the sensitivity or the ecological importance of different areas of seabed disturbed, nor the extent or particular effects of the actual disturbance. However, in the absence of any more sophisticated assessment tools, it would give us a rough idea.

Other indicators for this issue:

Biodiversity — Pressures on biodiversity - Pressures on marine biodiversity: pressures of fishing 

Removal of seabed by trawling can place pressure on marine biodiversity. Area of the seabed trawled would provide a surrogate for the extent of benthic ecosystems potentially disturbed by commercial fishing activities. It is a crude indicator because it does not estimate either the sensitivity or the ecological importance of different areas of seabed disturbed, nor the extent or particular effects of the actual disturbance. However, in the absence of any more sophisticated assessment tools, it would give us a rough idea.

Other indicators for this issue:

Biodiversity — Utilisation and value of biodiversity - Harvesting and trade in wildlife 

Harvesting of wild fish is the principal commercial harvesting of wild animals in Australia.

Other indicators for this issue:

Further Information

Source: Black, K.P, and G.D. Parry 1999, 'Entrainment, dispersal, and settlement of scallop dredge sediment plumes: field measurements and numerical modelling', Canadian Journal of Fisheries and Aquatic Sciences, vol. 56(12), pp. 2271-2281.

Source: Moran, M.J, and P.C. Stephenson 2000, 'Effects of otter trawling on macrobenthos and management of demersal scalefish fisheries on the continental shelf of north-western Australia', ICES Journal of Marine Science, vol. 57(3), pp. 510-516.

Source: Pitcher, C.R, C.Y. Burridge, T. Wassenberg, G.P. Smith, R. O'Connor, P. Jones, N. Ellis, and G. Fry 1997, 'Recovery of seabed habitat from the impact of prawn trawling in the far northern section of the Great Barrier Reef', CSIRO Marine Research, vol, p. 200, p. 200

Source: Poiner, I, J. Glaister, R. Pitcher, C. Burridge, T. Wassenberg, N. Gribble, B. Hill, S. Blaber, D. Milton, D. Brewer, and N. Ellis 1998, The environmental effects of prawn trawling in the far northern section of the Great Barrier Reef: 1991-1996, Division of Marine Research, CSIRO, Cleveland, Queensland, p. 554

Source: Goni, R 1998, 'Ecosystem effects of marine fisheries: an overview', Ocean and Coastal Management, vol. 40, pp. 37-64.

Source: Clark, M 1999, 'Fisheries for orange roughy (Hoplostethus atlanticus) on seamounts in New Zealand', Oceanologia Acta, vol. 22, pp. 593-602.

Source: Roberts, C.M 2002, 'Deep impact: the rising toll of fishing in the deep sea', Trends in Ecology and Evolution, vol. 17, pp. 242-245.