REF: 16

Shallow inlets and bays

Control and treatment type experimental investigation with pre and post dredge comparisons. Two spatially separated sites exposed to a single dredge with subsequent benthic sampling.

Site A, Lavan Sands NW Wales 3m above chart datum substrate very fine sand, extensively rippled, compact and firm, well oxygenated sediment. Site B, Blackshaw Flats, Solway Firth 5m above chart datum well sorted very fine sand, extensively rippled, compact and firm, well oxygenated sediment. Two experimental regimes.

Experiment 1: Effects of a single dredging activity.

Habitat effects - Dredging had no significant impact on the measured sediment characteristics due to the small percentage of fine material and the high degree of sorting.

Species and community effects - Rapid recovery of benthic infaunal communities as sediment exposed to regular disturbance from water movement - community already adapted to disturbance. Hydrobia ulvae, surface grazing gastropod, significantly affected by dredging.

Experiment 2 at Lavan Sands

80 sampling stations over an area of 400x300m used to assess the effects of a 3 month licensed commercial dredging operation using pre and post dredging data.

Habitat effects - No severe erosion of sediments occurred.

REF: 17

Great northern diver

Red throated diver

Comprehensive resume of recorded by-catches of marine mammals including dolphins, seals, porpoises and otters.

Incidental catches of marine mammals by no means rare and are reported in most fisheries in Britain. Data is still too sparse to enable a robust estimate of marine mammal by-catch. Gill net fisheries likely to account for the majority of marine mammal by-catches. 130 grey seals from the Farne Islands and the Orkneys may drown in fishing gear every year. Young animals more vulnerable to fixed nets. Cetaceans and seals only very rarely affected by long-line fisheries, creel, potting or salmon nets. Otters may be significantly affected by creel and eel fyke nets and the latter may have been a significant factor in the decline of otters in East Anglia. Salmon farming may have a significant effect on seal populations locally, estimates in the region of 100 seals caught in anti-predator nets annually with a further 1,000 seals shot by fish-farm operators. The number of seals caught in anti-predator nets, fishing nets in general or shot by fish farm operators does not seem to have had a deleterious effect on seal stocks. Harbour porpoise most vulnerable to incidental catches.

Possible solutions to conflicts with fishing discussed. Reflective knots at the intersection in netting may help prevent entanglement. Acoustic warning devices on nets may reduce the occurrence of entanglement. Certain nets and locations may precipitate large mammal catches these areas or methods may be avoided. Harbour porpoises more likely to be entangled during storms or at night, modification of fishing methods may reduce incidental by-catch.

REF: 18

Estuary

Mudflats and sandflats

Survey of intertidal benthic community and physical characteristics at a site of commercial clam cultivation on a shallow shelving mudflat during clam growth and post harvesting. Underlying sediment composed of London clay interspersed with shell debris and lignin deposits. Surface sediment of fine silt and sand with patches of clay.

Habitat effects - During clam growth no significant difference in particle size, organic content or photosynthetic pigment between control and clam lay sites. Harvesting by suction dredging removed upper sediment layers exposing clay which is unsuitable for larval settlement. Seven months post harvesting sedimentation had nearly restored the sediment structure.

Species and community effects - During clam growth no significant increase in faunal diversity under clam lay but density of benthic species individuals much greater. Community under clam lay significantly different from the control areas. Control area dominated by polychaete Nephtys hombergii, area under clam lay dominated by deposit feeding worms Lanice concilega and the bivalve Mysella bidentata. Nets may change hydrography reducing water flow and increasing sedimentation. This increases food supply and so may promote larval settlement. Adjacent areas may be influenced by commercial clam operation.

Suction dredge harvesting had a profound effect on the community structure. Large amounts of sediment and associated animal community (particularly crustaceans and bivalves) removed. Seven months post harvesting density of individuals decreased significantly to the point where there was no difference between control and harvested sites, with Neptys hombergii responsible for the similarity between treatment and control. The effect of clam harvesting barely detectable after 7 months. Clam cultivation increases productivity as the netting reduces wave action and other disturbances.

Lavan Sands, NW Wales

REF: 19

Otter

Grey seal

Common sea

Harbour porpoise

Gill nets

Longlines

Creels

Fyke nets

REF: 20

Estuaries

Shellfish aquaculture

Kaiser M.J. Edwards D.B. and Spencer B.E. (1994). Infaunal community changes as a result of commercial clam cultivation and harvesting. Aquatic Living Resources, 9: 57-63.