REF: 61

Sturgeon

Lampern

Sea Lamprey

Allis Shad

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

REF: 62

Estuaries

Shallow inlets and bays

Mudflats and sandflats

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.

REF: 63

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.

REF: 64

Estuaries

Shallow inlets and Bays

Mudflats and Sandflats

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.

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.

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.