Environmental factors influencing the microbiological contamination of commercially harvested shellfish.

Filter-feeding bivalve molluscs (such as oysters, clams, mussels and cockles) can concentrate contaminants from the water column. The extent of faecal contamination in shellfish is usually estimated by determining the concentration of faecal coliforms and/or Escherichia coli. Three sample points in each of three geographically separate commercial shellfisheries were selected for analysis for the effect of season, spring/neap and high/low tidal cycles, rainfall and wind direction on the results of routine E. coli monitoring. General linear modelling was used for the analyses. The principle factors affecting the contamination of shellfisheries were season, high/low tidal cycle and rainfall. The effects varied between harvesting areas and between individual sampling points within harvesting areas. Undertaking such analyses for all harvesting areas would contribute to the management of monitoring programmes and assist in the evaluation of potentially contaminating sources, such as sewage discharges. The type of analyses undertaken on E. coli monitoring data would also be pertinent for the analysis of putative viral indicators, such as F+ coliphage, and could be extended to data on bacterial and viral pathogens.