The impact of Rapido trawling for scallops, Pecten jacobaeus (L.), on the benthos of the Gulf of Venice

J. M. Hall-Spencer C. Froglia R. J. A. Atkinson P. G. Moore 0 Present address: Institute of Biomedical & Life Sciences, Glasgow University , Glasgow Rapido trawls are used to catch sole around the coast of Italy and to catch scallops in the northern Adriatic Sea but little is known about the environmental impact of this gear. Benthic surveys of a commercial scallop ground using a towed underwater television (UWTV) sledge revealed an expansive area of level, sandy sediment at 25 m characterized by high population densities of scallops (282 m"2 Aequipecten opercularis but fewer Pecten jacobaeus) together with ophiuroids, sponges, and the bivalve Atrina fragilis. Rapido trawls were filmed in action for the first time, providing information on the selectivity and eYciency of the gear together with its impact on the substratum and on the benthos. The trawls worked eYciently on smooth sand with ca. 44% catch rate for Pecten jacobaeus, of which 90% were >7 cm in shell height. Most organisms in the path of the trawl passed under or through the net; on average by-catch species only formed 19% of total catch by weight. Of the 78 taxa caught, lethal mechanical damage varied from <10% in resilient taxa such as hermit crabs to >50% in soft-bodied organisms such as tunicates. A marked plot surveyed using towed UWTV before, then 1 and 15 h after fishing by Rapido trawl showed clear tracks of disturbed sediment along the trawl path where infaunal burrow openings had been erased. Abundant, motile organisms such as Aequipecten showed no change in abundance along these tracks although scavengers such as Inachus aggregated to feed on damaged organisms. There were significant decreases in the abundance of slowmoving/sessile benthos such as Pecten, Holothuria, and Atrina. Juvenile pectinids were abundant on the shells of Atrina. The introduction of a scheme of areas closed to trawling would protect highly susceptible organisms such as Atrina and enhance the chances of scallop recruitment to adjacent areas of commercial exploitation. - Fisheries research has traditionally focused upon gear eYciency and stock management in terms of securing sustainable yields of commercial target species, with little consideration being given to the eVects of fishing on non-target species and ecosystem structure. The past decade, however, has witnessed an increasing awareness world-wide of the wider detrimental ecological eVects of commercial fishing (Gislason, 1994; Jennings and Kaiser, 1998; Auster and Langton, unpubl.). One towed gear about which little is known in terms of its environmental impact is the Rapido trawl. Bini (1960) described early designs of Rapido trawl that were towed at 34 km h"1 to catch sole (Solea vulgaris) and use of this gear has spread rapidly throughout the Adriatic (Piccinetti, 1967). In the late 1960s a Rapido fishery for scallops (Pecten jacobaeus and Aequipecten opercularis) opened on sandy grounds in the Gulf of Venice. This scallop fishery is now usually based on one or two consecutive year-classes of P. jacobaeus, around the minimum commercial size (10 cm). Scallop Gulf of Venice Adriatic Sea populations and landings there have shown large fluctuations over the past 30 years, determined both by the intensity of fishing eVort and by mass mortalities due to hypoxic conditions that have occurred episodically over wide areas of the northern Adriatic (Froglia, 1983; Ott, 1992; Orel et al., 1993; Mattei and Pellizzato, 1996). The numbers of vessels now using Rapido gear to catch scallops in the northern Adriatic varies from <50 medium to large trawlers (250800 hp) operating yearround, up to ca. 100 vessels when this activity becomes more profitable than otter trawl or pelagic fishing, such as following large recruitments to the exploitable scallop population. Average annual landings of P. jacobaeus in recent years are estimated to be around 1000 metric t (including shells) with a first-sale value of over US$ 2.5 million. Here we report on video observations of modern Rapido gear in action and investigate its impact on the large pinnacean mollusc, Atrina fragilis, and other members of the benthos. Materials and methods Fieldwork took place on 48 May 1995 aboard RV Salvatore Lo Bianco (30.2 m) in the Gulf of Venice. As a result of Holocene changes in sea level, relict residual sands characterize sediments of a wide area of the northern Adriatic (Colantoni et al., 1979). With the exception of some outcrops of beachrocks and biogenic hard substrata, locally called tegnue and trezze (Newton and Stefanon, 1975), most of the area is trawlable, particularly along courses following the ancient coastal profiles. Extensive exploitable beds of scallops (P. jacobaeus, A. opercularis, and A. glaber) are present on these bottoms (Froglia, 1983; S{imunovic, 1997). A rather undisturbed area with high densities of P. jacobaeus was located using local knowledge of the region and underwater television (UWTV) sledge data (see below). The area covered ca. 10 km2, centred at 45)13.5*N 12)47.1*E at a depth of 25 m (ca. 40 km SW of Venice, Fig. 1). The sea bed was level sand with scattered coralline rhodoliths. Rapido trawl Modern Rapido gear resembles a toothed beam trawl (Fig. 2a). It is lightly built and towed at 1013 km h"1, far faster than the 35 km h"1 that is typical for the heavier dredge designs that are used to catch scallops world-wide (Prado, 1987). Italian scallopers shoot one trawl per warp with 26 independent trawls towed at once (Fig. 2b). Fishing is continuous; while the catch from one trawl is retrieved and sorted, the other trawls remain on the sea bed. In our investigations, a single trawl was towed, so the eVects reported here are less than that expected from commercial operations. The gear used was of typical commercial design; 3 m wide with four skids, each 12 cm wide. A wooden plank (300#30#2 cm) fitted to the front of the iron frame, at an angle of ca. 27) to the ground, acted as a spoiler to keep the trawl in contact with the seabed. There were 32 teeth along the trawl mouth, each was 4 mm wide and extended 2 cm below the skids. The teeth were fixed (cf. spring-loaded in UK scallop dredges) and were spaced 7.8 cm apart. A square 8 cm mesh nylon net bag was tied to the trawl frame to retain the catch. This extended 6.8 m behind the tooth bar. The belly of the net was protected by reinforced rubber diamond-mesh matting (stretched mesh size 24#15 cm). In air, the trawl weighed 170 kg in total and 120 kg without the net, chafer, and wooden spoiler. Video surveys of the benthos before and after trawling The scallop ground (Fig. 1) was sought using a low-light sensitive colour video camera (SIMRAD Osprey, Aberdeen, Model OE1362) mounted in conjunction with a compact underwater lamp (beam angle 70); SIMRAD Osprey, Model OE1132) on a benthic sledge with 10 cm wide runners spaced 1 m apart, manufactured to specifications given by Shand and Preistley (unpubl.). A surface cont (...truncated)