THE IMPACT CAUSED BY TOOTHED DREDGES REQUANTIFIED ON A PAN-EUROPEAN SCALE
Ref. N. 98/018
Study project in support of the Common Fisheries Policy, call for proposals XIV-C1D(98)
J.M. Hall-Spencer1, J. Grall2, G. Franceschini3, O. Giovanardi3, P.G. Moore1, R.J.A. Atkinson1 and I.D. Tuck4
1University Marine Biological Station Millport, Isle of Cumbrae, Scotland KA28 0EG, UK
2Institut Universitaire Européen de la Mer, LEMAR UMR-CNRS 6539, Plouzané, France
3ICRAM- Chioggia
4Fisheries Research Services, Marine Laboratory Aberdeen (MLA), Aberdeen, Scotland AB11 9DB, UK
Non-Specialist Summary
Scallops (Pecten, Aequipecten spp.) are harvested in European seas using toothed dredges that have been developed into a variety of configurations by fishers seeking to exploit, as efficiently as possible, different types of sea beds; from homogeneous, hardpacked sand to porous gravel and maerl beds. Maerl grounds are a unique type of sea bed formed from twiglets or nodules of Living chalky algae. The 18-month “REEFS” programme, begun in April 1999, involved collaboration between laboratories in Scotland, France and Italy. The basis of this work has been to compare and contrast the impacts of toothed fishing gear used to collect scallops on different bottom types: sand in the Adriatic (using rapido trawls) vs maerl in Scotland (using Newhaven dredges) and France (using Breton dredges). Rapido trawls are, as the name suggests, designed to be towed at speed over sandy grounds to catch scallops and flatfish. As such they are lighter in weight than the other types investigated. The rapido teeth penetrate surface sediment to 2 cm and serve to dislodge scallops that live recessed into the surface of the sea bed. The other dredges are heavier, being designed to cope with harder stony or bouldery grounds, and have longer teeth (10 cm).
Surface-living, immobile organisms are well known to be at considerable risk of damage from such gears. However, a substantial proportion of the community living in sediments may live at such a depth beneath the sediment as to be unaffected by the passage of toothed gears. This idea was tested experimentally in the field, using Before/After/Control/Impact experimental designs, at sites which were not fished commercially for various reasons: by virtue of proximity to a wreck (Italy); or the presence of an underwater communication cable (Scotland); or agreement by fishers (France). Bottom samples were taken using suction-lift techniques (water lifts in Italy, air lifts in Scotland and France) to depths of about 60 cm beneath the sea bed. In addition to contractual requirements, extensive sampling was undertaken using grabs (which were censused in detail), adding considerably to the value of the programme. The suction-lift samples were passed through 4 mm sieves to retain the largest species, while the grab samples were processed through 1 mm sieves to retain smaller organisms. The reason for including the latter samples was to check mainly for the presence of juvenile stages of the deep-burrowing elements of the fauna.
Representative samples were taken on each ground prior to impacting on defined control and experimental plots. Small-scale impacts were then made in the experimental plot on each site with the relevant commercial gear. The operation of the gear was monitored using underwater video cameras, and divers reported the appearance of the fished track soon after impact. The contents of the catches was assessed in terms of species composition, proportion of commercial catch to bycatch (as wet weight), and damage sustained by organisms. The longer term consequences of this single impact event were followed-up seasonally (on 4 occasions) over the following year in Scotland and in France.
These investigations revealed that the top 10 cm layer of sediments contained the majority of organisms numerically and also in terms of different types of organisms. However, the live weight of organisms in the deeper strata (10-30 cm, 30-60 cm depth) was often much greater than that at the surface; sometimes representing as much as 70% of the total. The deep-burrowing organisms were large, long-lived species (principally bivalve molluscs, mud shrimps and sipunculid worms). As adults, such organisms are protected from the passage of towed gears, however damaging these might be at the surface. Many of the animals (e.g. polychaete worms, small molluscs) living in the surface layer of the sediment, though disturbed by dredging, are sufficiently small to survive or they go through or under the dredges. The organisms that suffer most damage from these gears are those large sessile epifaunal species that are widely dispersed and hence poorly sampled by small-scale grab or quadrat methodologies. It is these organisms, plus their solid substratum elements (stones, dead shells) that form the bulk of the unwanted by-catch. The presence of such ‘trash' material contributes to the damage sustained by the captured organisms in the dredge bags. The discarding of these damaged organisms, together with the damaged organisms not retrieved and left behind on the dredge track, contribute an energy input to the bottom that is exploited by active bottom-dwelling scavengers including fish, crabs and echinoderms.
Of all the types of fishing gear towed over sea beds in European seas, toothed gears have acquired the reputation of being the most environmentally damaging. This project has shown that within one area, their impacts affect different aspects of sea-bed ecology differently. The impacts also depend on the type of ground and at what species the fishing is targeted. It has also highlighted the deficiencies of conventional approaches to the biologicai surveying of fishing gear impacts that often provide a very incomplete picture of the full range of effects.
Sandy bottoms are probably the most resilient grounds to towed gears of any sort, and are generally low-diversity habitats. Being highly mobile sea beds, any evidence of impact on sand is quickly erased by current and wave action, and their surface-dwelling communities are generally not of great conservation significance. Maerl grounds, being composed of living calcareous algae, are much more susceptible to dredging. The physical complexity of this habitat and its long-term persistence (maerl-forming species grow very slowly, taking centuries or millennia for beds to accumulate) makes maerl grounds suitable places for a wide range of animals and algae to live. Unfortunately, maerl is easily buried by dredging and then dies due to lack of light. Importantly, the two main species that form maerl beds in European seas are the only algae for which management is required under the EC Habitats Directive (1992). The physical damage caused by towing heavy fishing gear over these grounds can permanently alter the structure of the ground and reduce the long-term viability of the habitat.
In order to protect the best maerl habitats, it seems likely that defined areas will need to be selected where human intervention, in its most destructive forms, is restricted by regulation. Protected nursery beds could then support spawning scallops, enhancing the fishery potential of adjacent, less sensitive, areas. Many European maerl beds have already been adversely affected by exploitation and habitat degradation; probably few significant maerl grounds remain pristine. In order to achieve the desirable degree of protection necessary for the conservation of maerl ground biodiversity in a European context it will be necessary to upgrade the status of maerl-forming algae to achieve a higher category of protection under the EC Habitats Directive (1992) than that which currently prevails. Although in the ultimate best interest of stakeholders, acceptance of such ideas will depend on convincing fishers of the strength and validity of such arguments. Moves towards regional management of EC fisheries will help to encourage responsible attitudes to fishing, delivering better sustainability than currently prevails.
- PR REEFS 
