Summaries of reviewed publications – references 41 – 45.

Details are limited to information relevant to the UK marine habitats and species listed in the Habitats Directive and the Birds Directive.

[pr] indicates that the paper is from a peer reviewed journal or report


Natura 2000 Habitats & Species

Fishing Technique




REF: 41

Harbour porpoise

Bottlenose dolphin

Grey seal

Common seal

Fish farming, fisheries in general

Review paper

Seals are still killed around the Scottish coast where they interact with fishing or fish farming interests but it is difficult to assess the impact. Probably localised and limited in extent, but could have a significant effect on some local populations. Seals and cetaceans may be caught accidentally in fishing gear and anti-predator nets around fish farms. Grey and common seals, harbour porpoises and common dolphins are the most commonly caught species in UK waters. Currently the assessment of the significance of the potential threats is hampered by lack of data on the nature of the threats and the dynamics of the populations concerned.

Scottish waters

Thompson P.M. (1992). The conservation of marine mammals in Scottish waters. Proceedings of the Royal Society of Edinburgh.

100B: 123-140


REF: 42

Shallow inlets and bays


(Mixed sediment chiefly sand and shell gravel with varying quantities of silt, shells, gravel, stones and cobbles)

Scallop dredging

Pre-dredging surface followed by qualitative and quantitative assessments (although not at the same stations), photographs and sediment samples.

Habitat effects - Conspicuous tracks on the seabed about 4m wide. At each site a ridge of stones, shells and shell fragments approx. 15cm high and 30cm wide. Inside ridges shallow grooves formed by rubber bobbins at the ends of the towing beam. Examination of tubes of the anemone Cerianthus lloydii in the dredge paths suggested top 2-4cm had been removed. Passage of dredge created a thick sediment cloud the heaviest constituents of which settle out rapidly and close by. Fine sediments were carried away by the tide.

Species and Community effects - Dredge bags contained shells and stones most of which supported sponges, hydroids, small anemones, tube-worms, barnacles, ascidians and bryozoans. Remains of several P. folicacea and large numbers of small crustaceans (chiefly Pilumnus hirtellus), molluscs (especially Trivia spp.) and juvenile echinoderms within the folds of the colonies. Also several sponges (mostly Suberites spp.) and a large number of epibenthic echinoderm species in the catch. Predators and tidal currents removed much evidence of killed or injured animals in the 24 hours after dredging but dead or damaged tubeworms, crabs, squat lobsters echinoderms and P. foliacea were found. Large numbers of C. lloydii present in dredge path. Broken tops of l. conchilega tubes were common in dredge paths but large numbers of intact tubes suggested that the worms had survived and rebuilt their tubes. Large mobile epifauna generally absent from dredge path except for occasional scavenging A. rubens although within 48hrs smaller mobile species such as hermit crabs were present. Counts of infauna in and immediately alongside dredge paths showed these species were unaffected by the level of dredging. Sessile species found during presurvey but not seen in dredge paths include "shell fauna", C. celata, Suberities spp. A. digitatum and P. foliacea


Bullimore, B. (1985). An investigation into the effects of scallop dredging within the Skomer Marine Reserve. Skomer Marine Reserve Subtidal Monitoring Project. Report to the Nature Conservation Council.

REF: 43


Harbour porpoise

Bottlenose dolphin

Grey seal

Common seal

Gill nets (including trammel nets and tangle nets)

Report on the nature and scale of European gill net fisheries and review of accidental catches of non-target species. Incidental catches reported for common dolphins, bottlenose dolphin, striped dolphin, harbour porpoise, common seal, grey seal, sharks (especially blue sharks), loggerhead turtles, guillemot, razorbill, shag and loon.

Around the UK catches of grey seals in tangle net fisheries high in the Barra fishery and for Cornwall appeared to be higher than other areas. Catches of common dolphins often reported in southwest fisheries amounting to perhaps some hundreds per year. Bottlenose dolphins rarely recorded but porpoises fairly frequently found in gill net fisheries especially in the North Sea. Drift net fisheries catch most but most of these are released alive. Total drownings in gill nets throughout the country may be in high tens to low hundreds. Impact on porpoise population not known. Bird catches widely reported but little studied. Catches of non-target fish poorly known but crabs are taken in very large numbers.

Regarding impact on marine mammals the study clarified importance of North Sea cod fishery and Atlantic hake fishery both already suspected of taking significant number of harbour porpoises and common dolphins respectively. With no populations studies on this species in Europe the impacts of these fisheries and the recently implemented tuna drift net fishery, remain speculative. There are apparently significant catches of birds in the salmon driftnet fisheries in Ireland and Denmark and catches in coastal and lagoon fisheries in Portugal and Italy. It has been estimated that breeding populations of guillemots at two sites in northern Norway have declined by 95% from the early 1960's to 1989 and that this decline could be explained entirely by gill net mortalities based on observed catch rates.

Impacts on non-target fish poorly documented, but where examined a wide variety of species recorded. Probably most acutely seen in the swordfish driftnet fishery. May be an impact on benthic communities because of cumulative effect of exposure to netting (including lost netting) on certain seaweeds, seagrass or pedunculate invertebrate communities may be important but little investigated.

European Community waters

Northridge S. di Natale A., Kinze C., Lankester K., Ortiz de ZarateV. & Sequeira M. (1991). Gill net fisheries in the European Community and their impacts on the marine environment. MRAG Ltd. A report to the European Commission’s Directorate General Environment.

REF: 44

Shallow inlets and bays


(Gravel sediment)

Scallop dredging

Otter trawling

Habitat effects - small differences in sediment type between dredged and undredged sites with dredged sites having a slightly higher frequency of small pebbles, and the undredged sites having slightly more larger pebbles and cobbles.

Species and community effects - Samples of benthic megafauna from disturbed and undisturbed sites showed that disturbed sites had lower density of organisms, biomass, and species diversity than undisturbed sites. Many of the species that were absent or less common in dredge sites were small, fragile polychaetes, shrimps and brittlestars. Most apparent difference was the lack of colonial, epifaunal taxa at the disturbed site. This study aimed to give a quantitative assessment of the impact using still photographs.

Comparison of deep sites showed that Filograna implexa had a high percentage cover at the undredged site and no epifauna and few animals visible at the dredged site. Significant effect between depth and dredging for both F. implexa and plant-like animals with effect on percentage cover greater at the deep sites. For plant-like animals the effect was higher at the shallow sites. Protula tubularia was significantly more abundant at undredged than dredged sites. There were no differences in the proportion of photographic sampling cells with bryozoans in them, but dredged sites had a significantly higher proportion of cells with abundant bryozoans than undredged sites. Spirorbis was more abundant at the deep sites and was in higher frequencies at the dredged sites than undredged sites. Most likely explanation is that the emergent epifauna at undredged sites concealed encrusting bryozoans and Spirorbis from view.

Depth had the greatest effect on the frequencies of non-colonial animals. Dredging had a lesser, but still significant effect on the frequencies of non-colonial species. Undredged sites had higher frequencies of almost all taxa except burrowing anemones, the earshell Sinum perspectivum and hermit crabs. Most of the non-colonial taxa seemed to be negatively affected by dredging but some seemed to profit from dredging. Burrowing anemones were more prevalent at dredged sites for example, perhaps because tentacles easily retracted to safety.

Results consistent with the hypothesis that gravel habitats are very sensitive to physical disturbance by bottom fishing and the primary impact is the removal of emergent epifaunal taxa.

Georges Bank, Canada

Collie J.S., Escanero G.A. & Hunke L. (1996). Scallop dredging on Georges Bank: Photographic evaluation of effects on benthic epifauna. ICES CM. 1996/Mini: 9

REF: 45

Shallow inlets and bays


Mudflats and sandflats


Grey seal

Common seal

Harbour porpoise

Bottlenose dolphin



Review report describing direct effects of fishing.

Habitat effects - all towed gears which exploit bottom-living species disturb the sediment and may therefore have an impact on the structure and processes at the seabed. Grain size distribution, sediment porosity and chemical exchange process are properties which may be affected. Another direct consequence is displacement of boulders which would otherwise be a surface for epifauna. A direct consequence of disturbance is an increase in suspended sediment load and the possibility of net transport of finer sediments. Resuspension may also influence uptake or release of contaminants, a shift in sediment-water exchange eg of nutrients. Reworking of sediments may result in burial of organic matter. Gears which disrupt the sediment most are beam trawls and shellfish dredges but method of rigging can have a profound effect on the level of disturbance.

Species and Community effects - Box cores revealed extensive changes to infauna before and after trawling. Significant reduction in burrowing sea urchin and the density of tube-building polychaetes. Survival rates for infauna and epifauna caught in net of beam trawl were high for starfish, many molluscs and crabs but poor for Arctica islandica. Trawl-caught whelks and hermit crabs largely unaffected. These results suggested that a relatively high proportion of some benthic species can be killed in the path of a beam trawling. In relation to scallop dredging epibenthic mortalities can be marked. Effects on seabed and benthos depend on substrate type, hydrographic features and community structure as well as the design and operation characteristics of the gears. Seabirds have been killed in gill and other static nets, no comprehensive studies of entanglement in the North Sea but available evidence indicates that it is likely to occur for diving birds in areas with fixed net fisheries. Gill net fisheries in some places have had a high by-catch of diving birds. Seals may be caught in gill nets, fyke nets and fixed nets for salmon. Gill nets killed the most cetaceans, catch rates varying seasonally. Around the British Isles several species of small cetacean have been reported as incidental catches but in the North Sea reported by-catches of species other than harbour porpoise are rare. As well as catch, fishing operations cause incidental mortality of fish which escape from the gear.

Gill nets, tangle nets and traps may continue to fish for some time after being lost of discarded. Length of time depends on factors such as current speed and fouling. On the bottom multifilament nets remain tangled, monofilament nets may, once clear of fish remains and crabs, disentangle, return to an upright position and resume fishing. Over time they build up an encrusting layer of marine organisms and become more visible to fish. Fragments of nets of all types may also entrap seabirds and marine mammals.

North East Atlantic, North Sea, Irish Sea

ICES (1992) Report of the study group on ecosystem effects of fishing activities. ICES C.M. 1992/G:11.


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