Summaries of reviewed publications – references 61 – 65.

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: 61



Sea Lamprey

Allis Shad

Twaite Shad

General effects

A review of site based information on these species, life history, distribution, habitat, reproductive biology and sources of threat. Together with recommendations to better assess and implement actions to help with the conservation of each species.


Potts G.W. & Swaby S.E. (1993). Marine Fishes on the EC Habitats and Species Directive.



REF: 62


Shallow inlets and bays

Mudflats and sandflats


Mechanised cockle fisheries

Review. Environmental effects fall into several broad categories the most obvious being (a) direct impacts, mainly on the benthic biotopes and on the discarded undersize by-catch (b) indirect interactions with predators and scavengers, including shorebirds, (c) ancillary disturbance from the vessels and vehicles, including effects at the shore access points.

Habitat effects - Hydraulic dredge tracks can be seen at low tide days or weeks later, persistence depending on the stability of the sediment surface and the prevailing tide or wave conditions. On areas of cohesive sediment the tracks appeared to act as lines from which erosion of the surface layer spread out therefore appearing to accelerate the erosion phase of a natural cycle of cohesion of the surface sediment by worm tube mats. Where dredging has been carried out in a sheltered area with eel grass (Auchencairn Bay) breaking the sward allowed erosion that produced clearly visible grooves down the shore. Long-term effects on benthic diatoms on and in the surface of intertidal flats were considered unlikely.

Species and community effects - Shell breakage occurs with overall damage rates to cockles and Macoma baltica in screen rejects from hydraulic dredgers 12.6% and 5.3% respectively. In experimental plots where damage rates from tractor dredging were determined these were 9.3% in an area of muddy sand and 8.2% in a sandy area but only impinged directly on about 80-85% of the area of the plots. Dredged areas often had a lot more dead shell scattered on the surface, an effect which can persist for several months whereas in undisturbed beds most dead shell is normally under the surface which can create a shell layer limiting the depth to which small drainage channels can normally erode into a cockle flat.

Observation on other species include the tendency for some motile species, like the amphipod Bathyporeia sarsi to temporarily leave disturbed areas, lugworms producing normal casts in dredge tracks as soon as the tide falls, tubes of the sand mason worm L. conchilega still standing, apparently to nearly their full extent in the hydraulic dredge tracks. Results from a study of tractor dredging in the Burry Inlet recorded declines in other invertebrates (particularly H. ulvae, P. elegans and N. hombergi), the greatest fall being 14 days after dredging for the less mobile species in the muddy areas, and increases in some species Urothoe sp., M. balthica, A. tenuis. Localised additional bird activity has also been reported in some areas following dredging. In a study on the Solway Firth it was concluded that because natural changes are very large the fishery may not have a significant effect on bird numbers unless a high proportion of the cockles are harvested. On sandy areas the effect on most invertebrate populations was considered to be causing some thinning of stocks rather than persistent patchy defaunation. In muddier, more cohesive sediments tracts may persist for months. Persistent hydraulic dredging has in some cases been reported to have changed the sediment structure which may have medium term consequences for deposit feeding benthic species. The most undesirable effects are where the surface is bound by swards of eel-grasses.

Various UK sites

Rees E.S. (in press). Environmental effects of mechanised cockle fisheries: a review of research data. A report commissioned by the Ministry of Agriculture Fisheries and Food.

REF: 63


Intertidal Shellfisheries

Report develops a predictive model to explore the effect of different shellfishery management options on the mortality rates of the migratory shorebirds that feed on shellfish on intertidal wintering grounds in Europe. Effects incorporated include disturbance and reduction of abundance of the shellfish stocks. Application to the Exe estuary was successful in predicting levels of oystercatcher winter mortality in previous years. Main conclusions were:

Given a number of conditions it is possible to exploit shellfish stocks without increasing the winter mortality of shorebirds.

Effects of a given intensity of shellfishing depends crucially on local conditions of the climate and the general abundance of food.

Methods of shellfishing which disturb birds can be significantly more damaging to the bird’s chances of survival.

Numbers of birds using alternative food sources is an early warning that a change in shellfishery practice is beginning to have an effect on the birds.

Key factor in determining the impact is the proportion of the shellfish stock that is affected

Cumulative effects of small increases in shorebird morality in winter can over a period of years greatly affect stable population size.

As fishing effort increases, shorebird mortality may be hardly affected initially but then may suddenly increase dramatically once a threshold level of fishing effort has been reached.

Model tested on Exe estuary

Stillman et al., (1996) Models of Shellfish Populations and Shorebirds: Final Report. Institute of Terrestrial Ecology Report to the Commission of the European Communities, Directorate-General for Fisheries.

REF: 64


Shallow inlets and Bays

Mudflats and Sandflats


Bivalve mariculture

Reviews current knowledge of environmental modification or conflicts with other species at seed collection, seed nursery and on-growing, and harvesting stages of the cultivation process.

Seed collection - subtidal dredging for seed mussels likely to be confined to relatively small areas of seabed because they occur in dense aggregations in discrete areas. UK licensed areas from unstable beds which are likely to be lost anyway. Non-target species probably adapted to large-scale natural disturbance so likely to recolonise rapidly but in extensive heavily exploited fisheries, such as the Wadden Sea, the entire mussel stock was removed in 1990/1 resulting in increased mortalities for eider duck and reduced breeding success for oyster catchers. May be some effects associated with intertidal collection (trampling, disturbance of foraging birds and removal of winter food source). Few impacts likely from spat collectors, continuous relaying of cultch leads to habitat modification which may increase diversity. There are also risks of introduction of alien species.

Ongrowing - effect depends on habitat, type and scale of cultivation. Introduced structures effect local hydrography and provide a settlement surface, high densities increases local oxygen demand and elevates input of organic matter however beds used to be extensive and they fulfil an important role in the retention of phosphorus and nitrogen. May be eutrophication beneath mussel lines if not enough tidal flow to disperse particulate matter. Decreases in abundance of macrofauna and increases in meiofauna beneath oyster trestles been measured. In the USA insecticide is sprayed on intertidal areas and ground may be harrowed prior to cultivation. Addition of gravel or shell, formation of mussel mud anduse of protective netting induces localised changes in benthic community composition. Small-scale culture seems to have only very limited effects on local benthic communities. Cultivation sites may conflict with bird feeding or roosting sites but probably only problematic if cultivation areas cover significant part of the feeding grounds.

Harvesting - restriction harvesting to early winter could ameliorate site restoration if main mechanism for recolonisation is by larval settlement. Suction dredging or mechanical rakinking affects the habitats. Recolonisation rates likely to differ between habitat types.

Management considerations in light of the reported effects are discussed and potential beneficial effects mentioned such as the proposal that integrated fish/bivalve mariculture systems can ameliorate undesiratal impacc ts of nutrient rich effluents from fish farmining, or for restoration of enclosed, polluted water masses.


Kaiser et al., (1998) Environmental impacts of bivalve mariculture. J.Shellfish Res. 17(1):59-66.


REF: 65

Shallow inlets and bays



Starfish and decapod Crustacea are among the most important megaepibenthic scavengers that aggregate in areas of fishing activity but recent work indicates that scavengers are far more selective than presumed previously. They avoid carrion that is phylogenetically similar and may avoid carrion that attracts potential predators.

The authors suggest that additional food resources arising from fishing activities are distributed unequally between sympatric populations of hermit crabs as a consequence of differences in their competitive abilities. This may provide a mechanism whereby fishing activities could lead to changes in the structure of crustacean scavenger populations. This type of effect has been well document for seabirds where fisheries-generated offal and discards have been linked to the increase in populations of larger scavenging seabird species.

Kaiser, M.J., Ramsay, K & Hughes, R.N. (1998) Can fisheries influence interspecific competition in sympatric populations of hermit crabs? J.Natural History 32:521-531.


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