Because of their different catching principles, otter trawls, beam trawls and scallop dredges are likely to have different physical impacts on the sea bed. Unfortunately, most impact studies give an inadequate description of the gear used to impose disturbances.
The catching principle of otter trawls is different from that of beam trawls and scallop dredges (Figure 4). Demersal otter trawls are designed to catch fish and shrimps that stay above the sea bed, from close to the bottom to several metres from the bottom. Beam trawls and scallop dredges, on the other hand, are used to target species that stay on the bottom or that are partly buried in the sediment. Accordingly, the tickler chains of a beam trawl and the teeth of a dredge are specifically designed to disturb the sea bed surface and penetrate the upper few centimetres of the sediment. Chains and teeth, respectively, are mounted along the whole width of the two gears (beam trawl: 4 to 12 m, scallop dredge: 0.75 to 3 m).
Otter trawls are rigged with different types of ground gears (e.g. bobbins, rockhoppers) depending on the bottom type fished and the species targeted (Figure 5). The functions of the ground gear are to ensure close contact with the bottom and to enable fishing on rough bottoms without damage to the trawl net. When targeting flatfish, a chain may be used as ground gear to chase the fish off the bottom. The trawl doors keep the trawl mouth spread open laterally and create the sand clouds that herd fish into the opening of the net (Figure 6). The creation of the sand cloud and, for some types of doors, the spreading force are the only reasons why part (i.e. the doors) of the otter trawl needs to penetrate into the sediment. Other parts, such as the warps, sweeps and net, do not normally remain in continuous contact with the sea bed. Owing to their different catching principles, otter trawls are likely to have different physical impacts on the sea bed from those of beam trawls and scallop dredges.
Furthermore, there are considerable variations in size and weight among trawls, beam trawls and scallop dredges, and their levels of impact are likely to vary according to these. It is obvious that a groundfish otter trawl with 2 300 kg doors, 140 m door spread and 53 cm diameter rockhopper gear (Kutti et al., in press) will cause different disturbances from those of a shrimp trawl with 125 kg doors, 30 m door spread and 4 cm diameter ground rope with 250 g lead rings (Hansson et al., 2000). The gear, in particular the parts that are in contact with the sea bed (doors, ground gear, tickler chain, sole plates, teeth), should therefore be properly described in order to interpret the effects on the sea bed and the response of the benthic assemblages studied. Unfortunately, most impact studies give poor and inadequate descriptions of the trawls used to cause the benthic disturbances.
Trawl doors may cause furrows of up to 20 cm deep depending on the door weight and the hardness of the sediment. Such marks are likely to last longer in sheltered areas with fine sediments.
Several studies on the effects of otter trawling on benthic communities describe physical disturbances of the sea bed. On a sandy bottom of the Grand Banks of Newfoundland (Canada), intensive trawling (300 to 600 percent coverage, i.e. a given site on the sea bed was trawled on average three to six times) had an immediate effect on the topography of the sediment surface owing to berms and furrows created by the trawl doors, which were readily seen by side-scan sonar and video observations (Schwinghamer et al., 1998). RoxAnn data also showed changes in sediment surface characteristics in that repeated trawling over the same bottom increased surface relief or roughness, but did not affect sediment texture (hardness). Changes in the acoustic properties of the upper 4.5 cm of sediment suggested decreased habitat complexity through the destruction of biogenic structures such as tubes and burrows (Schwinghamer, Guigné and Siu, 1996). These observations indicated that the physical habitat recovered from trawling disturbance within one year. However, it is important to note that significant interannual changes were also observed in the acoustic properties of the sediment that could not be attributed to trawling disturbances.
Side-scan and video recordings of a sandy/gravel bottom in the Barents Sea also showed physical disturbance from trawling, with highly visible furrows (10 cm deep and 20 cm wide) and berms (10 cm high) caused by the doors and smaller depressions created by the rockhopper gear (Humborstad et al., 2004; Figure 7). Five months later these marks had disappeared. RoxAnn data showed that intensive trawling (700 percent coverage) caused a decrease in sediment hardness and a slight increase in surface roughness, whereas moderate trawling (230 percent coverage) did not cause changes in these properties of the sediment. As was the study on the Grand Banks, this investigation was conducted on a high-seas fishing ground exposed to strong currents.
Tuck et al. (1998) conducted a trawl disturbance study in a sheltered Scottish loch (United Kingdom). Side-scan recordings showed clear evidence of physical disturbance from tracks left by the trawl doors. These tracks were still seen 18 months after the trawling treatment had ended, although they were very faint by this time. RoxAnn measurements also showed changes in sea bed topography, in that surface roughness increased in the trawled area. This effect of trawling had recovered after six months. The RoxAnn data showed no effect on sediment hardness.
Physical disturbances caused by flounder trawling on a macrotidal habitat in the Bay of Fundy (Nova Scotia, Canada) were assessed by eye when the trawl tracks were exposed at low tide (Brylinsky, Gibson and Gordon Jr., 1994). The trawl doors scoured furrows of 30 to 85 cm wide and 1 to 5 cm deep, which persisted for two to seven months. Less pronounced marks (10 cm wide) were made by each of the rollers of the ground rope, whereas the bridles left no visible marks. About 12 percent of the area between the outer edges of the doors was visibly disturbed, i.e. by the two doors and the ground gear.
Other experiments have also demonstrated clear marks created by the trawl doors, but do not give more detailed information on the physical disturbances than those already described. An experiment conducted on the Spanish Mediterranean coast showed evidence of marks left by the trawl doors, but no additional mark that could be attributed to the ground gear or the net (Sanchez et al., 2000). Video observations from another Mediterranean study demonstrated scrape marks on the sea bed made by the trawl doors and wires, as well as a general flattening of the micro-topography caused by the nets and ground ropes (Smith, Papadopoulou and Diliberto, 2000). Marks persisted throughout the closed fishing season of four months.
The physical interaction of trawl doors with the sea bed has been simulated in a test tank in order to examine closely the physical disturbance, the biological damage and the forces generated by scouring trawl doors (Gilkinson et al., 1998). A full-scale door model towed at a pre-set sediment depth (2 cm) created a shallow furrow bordered by a single 55-mm high berm on the inner edge of the scour. Bivalves that were originally buried in the scour path were displaced to the berm and, in two replicates, 58 and 70 percent of displaced specimens were completely or partially exposed on the surface. Of a total of 42 specimens in the scouring zone, only two showed major damage, although all specimens had been displaced. This low incidence of damage to bivalves was explained by a buffer effect in which small bivalves were mixed with sediment, excavated and then displaced into the berm bordering the scouring furrow. The authors stated that bivalves may suffer higher levels of damage on coarser and less smooth sea beds.
The works cited were all conducted on soft muddy or sandy bottoms. Video observations from a research submersible were used to assess trawling disturbance on a hard bottom (pebble, cobble, boulder) habitat of the continental shelf in the Gulf of Alaska (Freese et al., 1999). The dominant substrate type was pebble (< 6.5 cm), averaging 93 percent of the total substrate. Observations were made in the path of the 60-cm-diameter tyre gear of an otter trawl. The path was visible as a dark band on compact substrate, and as a series of furrows on less compact substrate. The depth of the furrows caused by the tyre gear ranged from 1 to 8 cm. The trawl gear displaced 19 percent of the large boulders (> 75 cm) in its path.
In conclusion, marks on the sea bed from one or more parts of the trawl have been demonstrated in all studies using acoustic (side-scan sonar) or visual (cameras) tools for sea bed assessment. Most studies were conducted on soft or sandy bottoms. These observations show that the most noticeable marks are those caused by the doors, and only faint marks are created by other parts of the trawl. Trawl door marks have been shown to be from 1 to 5 cm deep (Brylinski, Gibson and Gordon Jr., 1994), but may reach about 20 cm in certain parts of the tracks (Krost et al., 1990). The penetration depth depends on the weight and performance of the doors (type, angle of attack, speed) and on sediment grain size and hardness, being deeper in mud than in sand (Churchill, 1989; Krost et al., 1990; Tuck et al., 1998).
Data on the persistence of trawl marks in different environments are relatively scarce because only immediate physical effects are observed in most studies owing to their relatively short time frames. Trawl door marks were shown to disappear within less than five months in an area of strong currents in the Barents Sea (Humborstad et al., 2004). In a sheltered Scottish loch, however, faint marks could still be seen 18 months after the trawling treatment (Tuck et al., 1998), and the same trawl track could be identified for almost five years in a sandy mud area in Kiel Bay that is not exposed to tidal currents (Bernhard, 1989, cited in Krost et al., 1990). The persistence of marks produced by trawl doors depends on their original depth, the sediment type, the current, wave action and biological activity (Tuck et al., 1998; Fonteyne, 2000; Smith, Papadopoulou and Diliberto, 2000; Humborstad et al., 2004). Scouring marks are likely to last longer in deep water and in sheltered areas with fine sediments (Tuck et al., 1998).
The most noticeable physical effect of beam trawling and scallop dredging is a flattening of irregular bottom topography and the elimination of bioturbation mounds and faunal tubes.
The North Sea is intensively trawled by beam trawls, and certain fishing grounds are fished many times a year (Rijnsdorp et al., 1998). The physical impacts of beam trawling were studied in an area with hard, sandy sediments in the southern North Sea (Bergman and Hup, 1992). The penetration depth of the tickler chains of a heavy 12-m beam trawl was estimated by comparing the catches of the heart urchin with its length-dependent depth preference. The beam trawl was shown to catch large individuals of heart urchin, which indicated that the tickler chains penetrated at least 6 cm into the sediment. Tracks of the beam trawl shoes were still detectable by side-scan sonar after 16 hours, but their actual penetration depth could not be established.
In another North Sea study conducted on the Flemish Banks (Belgium coast) and off the Netherlands coast, the impacts of a 4-m beam trawl were investigated at sites consisting mainly of fine and medium sand (Fonteyne, 2000). Side-scan sonar observations showed clear marks immediately after trawling. The traces were too weak to determine the depth of penetration, indicating that this was not great. Side-scan observations were made several times during the 52 hours after trawling. Over this period the visibility of the trawl marks decreased gradually, and eventually they could be seen only as vague marks along parts of the original tracks. RoxAnn surveys showed that the sea bed roughness decreased and the hardness increased immediately after trawling owing to resuspension of the lighter sediment fraction. These sea bed characteristics returned to their original levels in less than 15 hours. The force exerted by a beam trawl on the bottom was also determined in this study, and found to be similar for heavy and light trawls. This similarity was explained by the fact that larger beam trawls have larger sole plates and are towed at higher speeds.
Boxcore sampling has been used in various sectors of the North Sea to determine the penetration depth of the tickler chains of beam trawls (Paschen, Richter and Köpnick, 2000). These observations showed a "wavy" penetration varying between 1 and 8 cm in depth. This pattern was explained by the heave motion of the vessel. The deepest penetration depth was measured in an area with softer sediments of finer muddy sand compared with the other areas studied, which were of fine to coarse sand.
The Irish Sea is also intensively trawled by beam trawls (Kaiser et al., 1996) and, as in the North Sea, there are few undisturbed areas suitable as control sites for impact studies. The physical impacts of beam trawling were investigated in an area of Liverpool Bay characterized by mobile mega-ripples in one part and by stable sediments with uniform topography in another (Kaiser and Spencer, 1996). RoxAnn measurements indicated that the sediment was less hard in corridors that were experimentally trawled than in adjacent unfished areas. In the part of the area with surface ripples and sand waves, the surface roughness was lower in fished corridors, probably owing to flattening of the ripples by the beam trawl.
Divers observed the effects of scallop dredging in an exposed bay of a Scottish loch (Eleftheriou and Robertson, 1992). Examination after each of several dredge treatments showed significant physical disturbance, indicated by furrows, elimination of natural bottom features (ripples and irregular topography) and dislodgement of shell fragments and small stones. The furrows were eliminated by wave and tidal actions shortly after the dredging operations. Similar observations were made by divers on a sandy sea bed in New Zealand (Thrush et al., 1995). The dredge broke down surface features (e.g. emergent faunal tubes, ripples), and its teeth (10 cm long) created grooves 2 to 3 cm deep.
The recovery of the physical impacts of scallop dredging was determined by Currie and Parry (1996) in Port Phillip Bay, Australia. Prior to dredging, the sea bed was dominated by low-relief mounds formed by callianassids (shrimp). Observations made by divers eight days after dredging showed that the mounds were flattened. The dredge typically disturbed the top 2 cm of sediment, but could penetrate up to 6 cm. Most callianassids appeared to have survived, and their density taken in grabs did not change significantly during the three months following dredging. Dredge tracks were still visible and the sea bed remained flat one month after dredging. Six months later the tracks had disappeared and callianassid mounds were abundant. After 11 months, the topography of the dredged site appeared similar to that of the adjacent control site.
Video observations were used to determine the effects of a Rapido trawl (resembling a toothed beam trawl) on sandy sediments in the Adriatic Sea of the Mediterranean (Hall-Spencer et al., 1999). The teeth of the trawl projected their full 2 cm length into the sediments and redistributed the surface layer. Observations taken one hour after trawling revealed extensive sediment redistribution with suspended particles reducing visibility 1 m above the sea bed from > 20 m (pre-trawling) to near zero. Disturbed sediment had settled 15 hours after fishing. Tracks of the Rapido trawl were seen as strips of flattened sediment that lacked evidence of bioturbation mounds or polychaete tubes, and were littered with smashed shells of crustaceans, bivalves and animal fragments.
A Rapido trawl rigged with 5 to 7 cm-long teeth caused disturbance of organic debris in the upper 6 cm of core samples (Pranovi et al., 2000). Side-scan sonar observations conducted one week after trawling showed trawl tracks. Diving observations showed that the trawl did not produce a clear furrow, but rather a flattened track with a small heap of sediment along its sides. Dead and damaged organisms and active scavengers (hermit crabs, brittle stars and gastropods) were also observed.
The studies reviewed here show that the most noticeable physical effect of beam trawling and scallop dredging is a flattening of irregular bottom topography by eliminating natural features such as ripples, bioturbation mounds and faunal tubes. The penetration depth of the tickler chain of beam trawls and the teeth of scallop dredges range from a few centimetres to at least 8 cm. Because few observations have been carried out on these marks beyond a few hours after trawling disturbance, confident conclusions on their persistence cannot be drawn. However, the scarce data reported indicate that the persistence of beam trawl and dredge marks varies from a few days in tidally exposed areas of the North Sea to a few months in an Australian bay (Currie and Parry, 1996; Fonteyne, 2000).