Water Quality — Seafish


Various species

Water Quality

The consideration of water quality in oyster farming is important. Oysters feed by filtering phytoplankton and in doing so they can accumulate and concentrate bacteria or viruses, some of which can be a risk to human health. These are often derived from land-based activities such as water treatment, storm drainage and diffuse agricultural run-off. Bio-toxins may also be contained in seasonally occurring marine algae (Harmful Algal Blooms or HABs)1, 2 which the oysters can ingest.

Rigorous controls need to be in place in regards to harvesting shellfish such as mussels in order to protect consumers; ensuring that those sold into the supply chain meet strict food safety (health and hygiene) standards3. The UN Food and Agriculture Organisation (FAO) has developed the Codex Alimentarius or “Food Code” which is a set of voluntary standards, codes of practice and guidelines covering food and its production, including those for bivalve molluscs4, 5. The aim of the Codex is to protect public health and to support balanced trade relationships in food, and all World Trade Organisation (WTO) signatories are obliged to observe them. In practice, most countries have laws that are very broadly equivalent to Codex guidelines but differ from them in the detail.

European Hygiene Regulations6 state that shellfish business operators are responsible in ensuring that bivalve molluscs meet strict standards. The UKs Food Standards Agency, in compliance with European regulations, has classified shellfish harvesting areas and beds on the basis of the level of the bacterium E.coli in mollusc flesh. Depending on shellfish production area classification (A, B or C) certain procedures must be followed to enable harvested mussels to enter the market. Molluscs harvested from Class A areas can go straight to market, but they are often purified (depurated) to provide additional assurance of quality; those harvested from Class B areas must be purified before being sold to consumers. All molluscs sold on the UK market must contain less than 230 cfu (colony forming units) of E.coli per 100 g of flesh7, 8.

Depuration is a technique whereby live shellfish and bivalves, that may contain undesirable substances (e.g. sand, silt), pollutants, parasites or organisms of possible harm to human beings (e.g. pathogenic bacteria), are placed in systems with continually circulated and sterilised seawater for a specific period of time in order to clean themselves9. Depuration is effective in removing many undesirables, but it is not in regards to removing viral contamination (e.g. norovirus)10. As oysters are generally eaten raw, norovirus viability can remain an issue, however appropriate cooking can eliminate the norovirus risk. End product testing of live shellfish is also undertaken in countries such as the UK, to ensure shellfish products that enter the market are safe to eat11, 12.

For many reasons there is an on-going societal need to reduce coastal water pollution, and this is of particular importance to protect and enable the growth of bivalve aquaculture. Legislation exists to control water pollution, for instance that in the UK13, but pollution incidents do occur, and efforts continue to improve water quality14, 15.

Active Management is continuous, real time decision making, which takes into account of all the available information enabling shellfish producers to decide what action to take in regards to when and where to harvest. It is an increasingly important tool, especially in inshore waters, to reduce the risks from poor water quality and HAB episodes16.


  1. CEFAS
  2. Seafish
  3. Seafish
  4. FAO Codex Alimentarius
  5. Codex Standard for Live and Raw Bivalve Molluscs
  6. CEFAS
  7. FSA
  8. FSA
  9. Seafish
  10. Seafish
  11. Seafish
  12. Seafish
  13. OFWAT
  14. Defra
  15. Seafish, 2018. Intermittent Microbial Water Quality Barriers to Bivalve Shellfish Production: Improvement and Management Options for Change in Relation to Prioritised Aquaculture Areas in England. In prep
  16. Seafish