Extent of Beds

Patch Dynamics

Ageing Mussels

Condition of Mussels

Embryo Assay

Associated Fauna

Aerial photography with image processing and combined with ground estimates of biomass has been found suitable for extensive surveys of the Wash (Walker et al., 1990). The method compares favourably with transect based survey methods. Aerial surveys in Denmark calibrated by ground transects were reported by Munch-Petersen & Kristensen (1989). Ideally surveys should be conducted annually within cSACs with good examples of Mytilus reefs.

Where the mussels form a shore belt in land-locked fjords Olafsson & Hoisaeter (1988) used a two stage stratified sampling design to estimate populations per length of shore. This involved initial mapping of 86 km of steep rocky shoreline into short lengths of shore, each described according to four subjectively chosen categories of mussel density. Representative samples of these smaller shorelines were then subsampled by removing and weighing strips of mussels (using divers due to the inaccessibility of the shores) and the results extrapolated to produce biomass figures. Full details of the method, which is claimed to give reliable results from minimal sampling, are given in Olafsson & Hoisaeter (1988). Similar methods might be useful where, for any reason, aerial surveys are impracticable.

Although monitoring may indicate beds as a whole may be stable, patches within it are much more dynamic (Svane & Ompi, 1993).

Mussels are more reliably aged by the annual growth bands in shell sections using acetate peels than by external marks (Richardson & Seed, 1990; Seed & Richardson, 1990). Furthermore, samples of M. edulis taken from offshore platforms exposed for known periods suggested that year classes found from sections do not coincide with apparent size-class frequency nodes (Richardson et al., 1990).

Because they are widely available sessile filter feeders, mussels of the genus Mytilus are routinely used in biomonitoring and as model organisms for toxicological investigations (see reviews in Gosling ed., 1992). Duration of survival out of water, provides a simple measure that integrates stress effects caused by accumulations of contaminants (Smaal, et al., 1990). Various measures of condition index were compared by Davenport & Chen (1987).

In bivalves, embryonic development is very sensitive to reduced water quality. The D larva stage in Mytilus edulis is reached in 48 hours and comparisons between numbers reaching this stage between artificial fertilisations done in control and test waters provide useful indications of environmental quality (Quiniou & Toularastel, 1992).

Laser diffraction techniques could prove useful in measuring the short-term effects of various environmental conditions, such as episodes of high sediment loads from resulting dredging projects, which can affect growth (Sushko & Freeman, 1991).

The use of meiobenthos in pollution monitoring studies was reviewed by Vincx & Heip (1991). Since there are similarities between the biodeposition of faecal matter from mussels with organic loads higher than the surrounding sediment there are obvious parallels to be drawn between enrichment due to sewage and that due to the deposits from high concentrations of filter feeding invertebrates. Techniques for monitoring the infaunal macrobenthos should also broadly follow the protocols laid down for pollution monitoring. It is however essential that there is much more precision in the placing of the samples than when ordinary soft sediment locations are monitored. Sampling needs to be carefully stratified so that replicates truly come from biogenic reef communities.

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