Preliminary observations and comments regarding the selectivity of scallop dredges in Welsh waters.

Dr Greta Hughes, Hendy, Llanbedrog, Pwllheli, Gwynedd. LL53 7UA (


The king scallop, Pecten maximus, (Linnaeus, 1758) belongs to the pectinid group of bivalve molluscs. It is found on sand, fine gravel or sandy gravel between Norway and the Iberian Peninsula in depths ranging from shallow water to over 100 metres. The minimum landing size (MLS) for P. maximus is a shell length of 110mm in the Irish Sea and Eastern English Channel and 100mm elsewhere in United Kingdom waters.

A considerable amount of damage to the outer edges of P. maximus shells has been routinely observed during fishery inspections of scallops taken from the Cardigan Bay fishery. Shells that were 110mm or over in length prior to coming into contact with a dredge have to be returned to the sea because their length has been reduced to below this minimum landing size.

Life History

P. maximus is hermaphrodite with one shell releasing sperm and eggs into the water column where fertilisation takes place between spring and autumn. The larvae are planktonic for 20 to 30 days prior to settling on suitable substrate on the seabed where they rock from side to side to create a hollow. They are filter feeders feeding on phytoplankton and live for 10 to 12 and perhaps to 20 years.

Shell size increases by the daily addition of bands to all edges of the shell. Growth is rapid from April to June and almost ceases completely in winter when a clear shell ring appears allowing accurate ageing. Irregular rings or disturbance bands appear if the animal is disturbed for a period, perhaps by bad weather or fishing activity. Growth rate varies between areas but full maturity can be expected at about 3 years.

Regulation of the Scallop Fishery

The minimum size for P. maximus laid down by Council Regulation (EC) No 850/98 is 100mm across the widest part in all areas except in ICES Areas VIIa north of Latitude 52º 30’ North and ICES Area VIId where the minimum size is 110mm. ICES Area VIIa is the Irish Sea between Latitude 52º 00’ North and Latitude 55º 00’ North and Area VIId is the Eastern Channel west of Longitude 2º 00’ West to Latitude 51º 00’ North.

The Scallop Fishing (Wales) Order 2005 states that no scallop of less than 110mm across the widest part may be carried in Welsh waters. Stringent dredge specifications are required by the Order. Dredges must be of the Newhaven type (Figure 1). This dredge consists of a triangular steel frame with a spring-loaded plate to which a replaceable steel toothbar is attached. The bellies on the underside are constructed of no more than 8 rows of steel rings. The rings are attached to each other with wide washers. The backs are constructed of netting.

Dredges of 80cm or more in width are to have toothbars with a maximum of 9 x 12mm teeth and the bellies are to be constructed of a maximum of 8 rows of steel rings. There is no specific ring internal diameter requirement.

The dredge specifications required by the Scallop Fishing (Wales) Order 2005 mirrors those required by the Scallop Fishing Order 2004 for English waters although the scallop MLS requirement for the latter is 100mm except in the Eastern English Channel where it is 110mm.

Dredges are attached to a beam with rubber wheels at each end (Figure 2). A scallop dredger tows a beam on each side. No more than 4 dredges per side may be towed within 3 nautical miles of the shore and no more than 8 dredges per side may be towed in Welsh waters beyond 3 nautical miles from the shore.

The Cardigan Bay Scallop Fishery

Vessels participating in the Cardigan Bay scallop fishery range in size from 8.5 metres to 12 metres within six miles off the shore although 2 larger vessels fish between 3 and 6 miles off the shore because they had established rights to fish these waters prior to the introduction of North Western and North Wales’s Byelaw 9 which restricts fishing from vessels, other than those used in taking mussels or used for angling, to a maximum length of 12 metres overall to the west of a line drawn 000º (True) from The Old Lighthouse, Great Orme’s Head, Llandudno, to Cemaes Head, Cardigan. Increasing numbers of large over 12 metre vessels are seen working between 6 and 12 miles off the shore. Some of the vessels are purpose built scallop dredgers while others are multipurpose fishing vessels.

Some of the smaller boats fish 3 or 4 dredges per side within 6 miles of the shore. The dredges seen in use in the Cardigan Bay fishery have been 800mm wide Newhaven dredges. Toothbars are 700mm wide normally supporting 9 x 10mm wide teeth. Only 3 vessels have been seen to be fishing toothbars supporting 8 x 10mm wide teeth.

Belly rings of various internal diameters (observed to range from 50-80mm) are used and the effective internal diameter of a belly ring is dependent on a number of variables such as wear to the belly ring itself and the thickness of the connecting washers which are themselves subject to wear.

Gear Selectivity.

Only tooth spacing and belly ring size affect selectivity. Although tooth spacing is less reliable than belly ring size as a selective device (Lart et al., 1997), most of the damage done to scallops occurs at the first point of contact with the dredge, i.e., at the toothbar. Lart et al., (2003) found 8 x 10mm teeth on a 700mm bar and 9 x 10mm teeth on a 700mm bar to be the optimal number of teeth for selecting 110mm and 100mm MLS scallops respectively. Selecting scallops at the seabed also results in a 75% reduction in stress compared with deck sorting (Lart et al., 2003).

Differences in selectivity are found with different internal diameters rings and also different ring wire diameters (Lart et al., 1997). The gaps between rings as well as ring internal diameters must also contribute to selection. Bellies also become more selective as they wear making it difficult to establish and maintain the appropriate dimensions. Rings with an internal diameter of 85mm were optimal for selecting 100mm scallops but larger 88mm and 92mm ring diameters were investigated for 110mm scallops but full efficiency was difficult to achieve (Lart et al., 2003). The Scallop Fishing (Wales) Order 2005 has no specific ring size requirements.

Undamaged scallops can survive repeated dredging so it is necessary to reduce damage as much as possible. Damage rather than stress leads to increased vulnerability to predation (Lart et al., 2003).

Adverse Consequences of Legislation

Most of the toothbars seen to be in regular use in the Cardigan Bay scallop fishery contain the maximum number of teeth, and therefore the minimum tooth spacing, allowed by the Scallop Fishing (Wales) Order 2005, i.e. 9 teeth on the 700mm bar used on a 800mm wide dredge. To date, only three boats have been seen to be using 8 teeth on a 700mm bar. Preliminary observations indicate that there is less shell edge damage on scallops landed by these three boats.

Bars with 9 teeth appear to select for 100mm scallops and not the 110mm scallops required by the Scallop Fishing (Wales) Order 2005. As a result, many scallops in the 100mm – 110mm size range are seen with what may be avoidable shell edge damage. This has several consequences:

1) Scallops returned to the sea with damaged shells may die.

2) Stress effect on damaged scallops increases vulnerability to predation.

3) Many scallops seen in the 106mm - 109mm size range on deck are over 110mm prior to sustaining damage while passing through the dredges. Fishermen are required to return these to the sea.

The internal diameter of belly rings observed in the Cardigan Bay fishery to date range from 50mm to 80mm. Lart et al., 2003 state an optimum internal diameter ring of 85mm for selecting 100mm scallops although larger (88mm and 92mm) internal diameter rings did not prove to be efficient selectors of correspondingly larger 110mm scallops. The Scallop Fishing (Wales) Order 2005 has no ring size requirement, therefore, the introduction of a ring size requirement of at least 80mm would be beneficial.

Environmental Impact

Depending on the substrate, scallops make up only a small percentage of the volume of a catch. Lart et al., 1997 recorded that only an average of 11% of the catch consisted of scallops while the remaining 89% consisted mainly of stones. A considerable proportion of a vessel’s power must, therefore, be used to move stones over the seabed.


The enforced use of larger spacing on toothbars has several significant benefits for the environment, the fishery and the fishermen.

Less initial disturbance to the seabed resulting in fewer stones in the dredges.

2) Reduced shell damage resulting in reduced discard stress and mortality.

3) Improved selection of 110mm scallops at the point of initial contact.

4) More scallops may be retained due to reduced damage to the edges of >110mm scallops which effectively reduces them to <110mm.

5) Reduced shell damage to all scallops in the catch caused by interaction with stones in the dredges during towing, hauling and emptying.

6) Increased vessel energy efficiency because less of the vessel’s power is used to move stones over the seabed.

Suggestions for further action

Amend The Scallop Fishing (Wales) Order 2005 as soon as possible to:

a) reduce the maximum number of teeth allowed on 800mm wide dredges from 9 to 8.

b) require a minimum of 80mm for the internal diameter of belly rings.

Make the above specifications a requirement of North Western and North Wales Sea Fisheries Committee’s Byelaw 12 permit for the coming 2009/2010 scallop season if The Scallop Fishing (Wales) Order 2005 cannot be amended in time.

Investigate the possibility of optimising the internal diameter of belly rings to select for 110mm MLS scallops.


Council Regulation (EC) No 850/98 of 30 March 1998 for the conservation of fishery resources through technical measures for the protection of juveniles of marine organisms.

Lart, W., Horton, R. & Campbell (1997) "Scallop Dredge Selectivity. Contribution of tooth spacing, mesh and ring size; Part I. West of Scotland sea trials." Sea Fish Industry Authority, Seafish Report No. 509.

Lart, W. et al., (2003) "Evaluation and improvement of shellfish dredge design and fishing effort in relation to technical conservation measures and environmental impact; Ecodredge Final Report" May 2003 to the Commission of European Communities.

The Scallop Fishing Order 2004. Statutory Instrument 2004 No. 12.

The Scallop Fishing (Wales) Order 2005. Welsh Statutory Instrument 2005 No. 1717 (W.132).



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