REF: 6

Estuary

Experimental dredging of sandflats with mechanical cockle dredge. Two distinct sites sampled.

Site A: Poorly sorted fine sand with small pools and Arenicola marina casts with some algal growth.

Site B: Well sorted fairly coarse sand, surface sediment well drained and rippled as a result of wave activity.

Habitat effects - Dredge track visible after 6 months at Site A (stable sediments). No alteration in sediment parameters by dredging at Site B (mobile sediments).

Community effects - Effects of dredging on biota apparent at Site A after 3 months may be attributed to destruction of seabed algal covering, destruction of permanent tube dwellings, mortality of eggs/broods, interference with predator prey relationships or changes in sediment characteristic. Seasonal perturbation eg produced by winter storms produce community changes of greater magnitude than those caused by dredging in unstable high energy environments such as Site B.

Site A (stable sediments): Decreased number of Pygospio elegans no recovery to pre-dredging numbers by six months. Disappearance of Scoloplos armiger from some dredged plots. Distribution of Nephtys hombergii disturbed by dredging recovery after six months. Large decline in numbers of Hydrobia ulvae, statistical difference between dredged sites and control sites up to six months post-dredging. Cerastoderma edule numbers reduced by dredging, significant reduction in numbers compared with the control still apparent up to six months post-dredging.

REF: 7

Red throated diver

Review paper. Coastal net fisheries have been implicated in declines of numerous seabird populations but there are substantial difficulties in establishing cause of a population decline. Synthetic nets have been implicated as a major contributor to the decline of several auk populations.

REF: 8

Estuaries

Shallow marine inlets

Mudflats and sandflats

Sandbanks

Reefs

Grey seal

Common seal

Harbour porpoise

Bottlenose dolphin

Longline

Gill nets

Scallop dredging

Mussel dredging

Purse seine

Hydraulic dredging

Review paper covering many fishing techniques.

Habitat effects - Subtidal rocky habitats characterised by encrusting communities that are resilient to predation and invasion are extremely vulnerable to mussel dredging as these organisms often have poor dispersal mechanisms and slow growth rates. Desertification of such habitats recorded in Italy following intensive and destructive mussel dredging. Reefs extremely vulnerable to fishing as they often represent islands in seas of soft sediments making recolonisation from surrounding areas unlikely. Intertidal and subtidal soft sediment communities are vulnerable to fishing and as they are often close to areas of population density, heavily fished.

Bottom fisheries have resulted in the destruction of Zostera beds and saltmarsh vegetation. Calcareous algal bed of maerl destroyed by 8 passes of a dredge in Scotland. Reef building polychaete Sabellaria spinulosa, seagrass Zostera marina and oyster beds Ostera edulis destroyed by trawling. Hydroid and brozoan habitats lost in English Channel.

Zostera marina indirectly impacted by increased turbidity, replaced by deposit feeding polychaetes, community composition shifts such as these may resist the recovery of suspension feeding species. Epifauna often play key roles in influencing the structure and stability of benthic communities, modifying benthic boundary flow which further influences sediment characteristics and so the settlement of larvae. Epifauna may also provide a refuge for juvenile species from predators. Organisms which stabilise the seabed can also mitigate the effects of natural disturbances such as storms. Modification of microbial activity induced by bottom fishing, resuspension of pollutants, increased benthic/pelagic nutrient flux. With repeated trawling the intense disturbance may select for species with the appropriate facultative responses, communities will become dominated by juvenile stages, mobile species and rapid colonists.

Large amounts of discards falling to the seabed cause anoxia in bottom sediments the discards decay using up oxygen, kills scavenging organism attracted by the discards. Decaying discards may also harbour disease and have caused the elimination of a scallop fishery in Australia.

Dayton P.K., Thrust D.F., Agardi M.T. & Hofman R.J. (1995). Environmental effects of marine fishing. Aquatic conservation: marine and freshwater ecosystems. 5:205-232.

REF: 9

Estuaries

Shallow inlets and bays

Reefs

Mudflats and sandflats

Sandbanks

Grey seal

Common seal

Harbour porpoise

Bottlenose dolphin

Beam and Bottom Trawling

Review paper.

Habitat effects - Towed fishing gears such as bottom and beam trawls physically disturb the seabed causing alterations in microbial communities, resuspension of particles, nutrients and pollutants and the relocation of stones and boulders. Inshore fisheries have led to destruction of reefs built by species such as the polychaete worm Sabellaria or by calcareous algae. Fishing has led to structural changes in habitat that have resulted in changes in species assemblages.

Species and community effects - Fixed nets such as gill nets are more likely to entangle non-target species. Diving seabirds are especially vulnerable to entanglement in fixed nets such as gill nets. No evidence that mortality due to entanglement has precluded the observed increase in population size of many species of seabirds which has taken place during this century in the North Sea. Harbour porpoises especially vulnerable to entanglement in gill nets. Recent estimate of the by-catch of the Danish gill net fishery in the eastern North Sea gave an annual by-catch of 4629 porpoises. Incidental by-catch could be a significant contributing factor to the overall decline harbour porpoise abundance in European waters. Seal populations have been able to sustain or increase their populations whilst subject to fishery induced mortality. No species exists in isolation, fishery-induced changes in the density of one species will have repercussions on its predators, prey and competitors

Heavy towed gears in contact with the sea bed can kill or injure animals living in the top most layers of sediment. The percentage of benthic organisms caught in a beam trawl which die varies from zero for hermit crab, whelks and starfish to 100% for shells such as Artica islandica. Beam trawl is the most important fishing gear which penetrates the seabed. General fisheries generated mortality results in reduced abundance of long-lived benthic species and increased abundance of short-lived species. By-catch and offal produced by gutting the fish at sea thrown overboard provides food for seabirds and other scavenging animals. Changes in the amount of discards may affect the relative and absolute abundance of various species of seabirds. Increased abundance of scavenging seabirds since the start of the century. Large or unattractive discard items will fall to the seabed where they can become available to sub-surface scavengers.

Gislason H. (1994). Ecosystem effects of fishing activities in the North Sea. Marine Pollution Bulletin 29: 520-527.

REF: 10

Grey seal

Harbour porpoise

Red throated diver

Gill nets

Notes on recorded entanglement casualties in Cardigan Bay.

Species effects - Potential threat to red-throated divers from gill and tangle nets high. May have knock on effects at the birds breeding grounds. During 14 inspections of beach set nets between September 1991 and December 1992 no seabird by-catch was noted despite red-throated divers observed diving within 20m of nets.

Ten harbour porpoises Phoecoena phocoena reported as casualties of gill nets in 1991. Author considers that Harbour porpoise is the only cetacean under severe threat of extinction from static fishing gear in Cardigan Bay. 24% of UK deaths of harbour porpoises caused by entanglement in fishing gear.