Trawling directly affects colonial and sessile invertebrates such as sponges, tunicates, bryozoans, hydroids and corals through smothering by sediment, dislodging them, breakage or prevention of further growth. It also affects mobile epifauna that are either captured in the trawl or damaged, or their habitat or food source is removed.

 

There is a substantial amount of literature documenting the effects of trawling on the benthos. Watling and Norse (1998b) have likened heavy trawling to forest clear cutting.  They identify the impacts, both immediate (physical damage and destruction of organisms and communities; reduction of habitat complexity and 3-D structure; sediment resuspension and burial by settling plumes; water quality; bycatch) and long-term (changes in community structure and composition; reduction in numbers of sensitive long-lived, slow-growing, fragile and “tall” (epifaunal) species; increases in numbers of opportunistic species). Watling and Norse (1998a) argued that the “use of mobile fishing gear is on a par with agriculture as humankind’s most important physical disturbance of the biosphere” (p. 1178). A comprehensive review of the environmental effects of trawling is given by Jones (1992). It includes a history of trawling, effects of scraping off the epibenthos and ploughing soft sediments; sediment resuspension; destruction of non-target benthos; dumping of bycatch and how this may modify feeding behaviours seabirds and fish predators, as well as theoretical considerations including the rate of recolonisation, recovery, and the types of communities that are most sensitive. Harris and Ward (1999) have recently reviewed the information on bycatch collected during Australia’s commercial fisheries activities, and provides breakdown of the bycatch in all the major fisheries.

 

Several studies have been undertaken to attempt to quantify the impact of trawling. However, as pointed out by Dayton et al. (1995), there are difficulties distinguishing effects from natural variability, because the long fishing history makes it difficult to find undisturbed control areas. A study on the effects on seafloor habitat and associated invertebrate taxa in the Gulf of Alaska, USA, found that boulders were displaced, and large epifaunal invertebrates removed or damaged just by a single trawl pass. These structural components of habitat were the dominant features on the seafloor. There was a significant decrease in density, and an increase in damage, to sponges and anthozoans in trawled versus reference transects. Changes in density, or damage to most motile invertebrates were not detected (Freese et al. 1999).

 

CSIRO undertook a major review of the fisheries on the North West Shelf during the 1960-1970’s (Sainsbury 1987). Comparisons of sponge catch rate in 1967-73 (Shu et al. 1973a, b) with that recorded by a CSIRO survey in 1979 showed that during the 16 year interval there was a significant reduction in sponge frequency. Along with the loss of these sponges and associated benthos there was a reduction in certain fish species (Sainsbury 1988). The bycatch in terms of diversity and biomass was significantly greater in “lightly” fished areas than in “heavily” fished areas (Russell et al., in Sainsbury and Poiner 1988). This suggests that habitats with three-dimensional structure tend to be more sensitive to fishing disturbance than communities with mobile sandy sediments and little three-dimensional structure (Collie et al. 1997; Hansson et al. 2000; Jennings et al. 2001). Loss of three-dimensional structure changes the habitat, leading to reductions in populations of animals dependent on it for a range of biotic reasons including shelter, food or spawning. Another study in Torres Strait also recorded a similar phenomenon (Poiner unpublished data quoted in Hutchings 1990) and anecdotal evidence suggests that large sponges were also abundant on the shelf areas around much of NSW, Victoria and South Australia prior to the advent of frequent trawling.

 

Demersal fishing activities provide food for scavengers in the form of dead or damaged animals left in the tracks of the trawl or dredge or discarded as bycatch. Responses by motile benthic invertebrate scavengers to trawling can vary, as studies on benthic scavengers to experimental trawling in the UK have shown. For example, the numbers of hermit and swimming crabs, as well as starfish decreased in some sites but the density of hermit crabs increased in others (Ramsay et al. 1998).

 

Dredging and bottom trawling affect water quality and turbidity. Pilskaln et al. (1998) suggest that the resuspension of sediment from trawling may have important implications for regional nutrient budgets (input of nitrogen and silica into the water column). While trawling had no detectable effect on sediment grain size, tracks made by trawl doors were readily visible on the sea floor 10 weeks later; in some cases they were still faintly visible after one year. Trawling had also increased roughness of surficial sediments, reduced surficial biogenic sediment structure and the abundance of flocculated organic matter (Schwinghamer et al. 1998). Studies of trawling and mussel dredging in Danish waters have shown that these activities increased particulate matter and ammonia levels and decreased levels of oxygen, which may affect phytoplankton primary production (Riemann and Hoffmann 1991).

 

In New South Wales there are plans by State Fisheries to investigate the effects of trawling (most of the NSW shelf and upper slope are heavily impacted by trawling), but the proposal, which proposes to investigate the infauna as well as the epibenthos, has yet to be funded.



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Department of Environment and Heritage