Ecological effects of trawling

Historically, the effects of trawling on benthos caused concern as early as 1376 when a petition was made to the English parliament by fishermen concerned over the damage done to the seabed and fisheries by bottom trawling (De Groot 1984). This was despite the gear used by sailing vessels in those days being relatively light and towed at slow speeds and in shallow water only. When steam trawlers were developed in the early 1900s, everything changed. The weight and size of trawls increased and use of tickler chains (mounted on the bottom rope to disturb bottom-living fish upwards and into the trawl net) were of great concern, although studies done in the 1970s to allay the fears of fishermen did not find long-term effects on macrobenthos (Jones 1992). At the end of World War II the otter trawl was developed and its use became widespread. This and the beam trawl (see Fig. 8.4) were (and still are) the types of gear most widely used to fish the seabed. A further gear that is widely used and can be severely damaging is the scallop dredge, a rectangular metal box to which a metal-mesh bag is fixed. As an example of the change in technology, Jones (1992) reports that whereas the average weight of a beam trawl in the 1960s was 3.5 t, by 1980 the average weight was over 10 t. Yet it was the tickler chains that were thought to be most damaging aspects of the gear. The French Institut Scientifique et Techniques des Peches Maritimes made studies around Corsica and demonstrated the damaging effects on the benthos, which resulted in a ban in the use of such gear in the Mediterranean. A similar ban was sought on the French Atlantic coast, mainly fished by Dutch beam trawlers, but fisheries laboratories bordering these areas were not convinced that large negative effects occurred and so no ban was imposed (De Groot 1984). Even the influential Intergovernmental Council for the Exploration of the Sea (ICES) was not convinced of the negative effects of trawling in the late 1980s, and it was only in 1990 that a working group was established to evaluate the effects of fishing on the marine ecosystem, including effects on marine mammals, birds, and benthos (Jones 1992).

Figure 8.1 A conceptual model ('horrendogram') of the ecosystem effects of trawl fishing (from McLusky and Elliott 2004, based on case studies in Blaber et al. 2000)

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