Shellfish
The London Port Health Authority covers approximately 775 square kilometres of estuarial waters including the rivers Roach and Crouch. Within this area there are many shellfish layings which are commercially harvested and the London Port Health Authority is responsible for sampling the shellfish. Shellfish sampling from the Thames.
Shellfish samples are collected monthly from the Thames for classification purposes and bio-toxin monitoring. The London Port Health Authority Launch, MV Lady Aileen dredges for cockles in the Thames Estuary and samples of cockles, mussels, native oysters, pacific oysters and clams are collected by hand from other locations.
Bivalve molluscs are filter feeders. Whilst feeding they can ingest toxins which accumulate in their flesh. This accumulation of toxins does not harm the mollusc but can cause serious problems to humans. You can find information on the harm that this can do to humans further down the page.
The samples are sent either to the Public Health Laboratory at Colindale or to the Centre for Environment, Fisheries & Aquaculture Science (CEFAS) in Weymouth. Samples of sea water are sent to the CEFAS Phytoplankton Laboratory in Lowestoft for biotoxin testing.
Classification samples
Regular samples of shellfish are obtained from the layings in order to establish levels of contamination and to enable the classification of the layings under UK and EU legislation.
These samples are used to find out the level of faecal contamination present in the shellfish which then determines the classification of the beds and the level of processing required before being able to be sold.
Shellfish areas are classified as follows:
- CLASS A – for direct sale for human consumption bivalves must not exceed 230 colony forming units of Escherichia coli in 100g of flesh and intra-valvular liquid.
- CLASS B – all bivalves must be purified before they can be sold live. A maximum limit of 4600 colony forming units of Escherichia coli in 100g of flesh and intra-valvular liquid.
- CLASS C – the bivalves have to be relayed in designated ‘relaying sites’ for at least two months, then purified if required or subjected to heat treatment before they will be deemed fit for human consumption. A maximum limit of 46000 colony forming units of Escherichia coli in 100g of flesh and intra-valvular liquid.
Maps of classified areas under the control of LPHA are on the CEFAS website.
More information on shellfish classification is on the Food Standards Agency website.
Biotoxin monitoring
Flesh and sea water samples are collected as part of the Food Standards Agencies Biotoxin Monitoring programme and more information is on the CEFAS website.
Issues related to marine biotoxins include
Algal blooms
Most individual phytoplankton are too small to be seen with the naked eye. When present in high numbers their presence may appear as a dramatic discoloration of the water. This population growth can be rapid and typically occur when temperature and nutrient levels rise, usually in late spring and autumn. It is commonly known as an “algal bloom”. The colour of a bloom can vary from a green to a dark red colour depending on the phytoplankton present.
While blooms can provide more food to organisms higher up the food chain, too much phytoplankton can do harm. Dissolved oxygen becomes rapidly depleted as the phytoplankton die, sink to the bottom and decompose. This can result in the death of other organisms including shellfish, crabs and fish.
More information is on the Irish Marine Institute website.
Shellfish poisoning
There are five main types of shellfish poisoning associated with algal toxins.
- Diarrhetic Shellfish Poisoning (DSP) – symptoms diarrhoea, nausea and vomiting
- Paralytic Shellfish Poisoning (PSP) – symptoms headache, dizziness, diarrhoea, nausea, vomiting leading on to muscular paralysis, in severe cases respiratory failure can occur.
- Azaspiracid Poisoning (AZP) – symptoms same as DSP (more acute) with headaches and chills.
- Amnesic Shellfish Poisoning (ASP) – symptoms diarrhoea, nausea, vomiting, abdominal pain, short term memory loss
- Norovirus in Oysters
There is a link between the consumption of live oysters and outbreaks of the Winter Vomiting Bug ‘Norovirus’. More information is on the FSA website or the CEFAS website
Local action groups
The London Port Health Authority co-ordinates the Local Action Groups (LAGs) which were set up in 2006 under measures introduced by the Food Standards Agency for the Long Term Classification of Designated Shellfish Harvesting Areas.
There are three LAGs in the London Port Health Authority:
- LAG1 - River Roach and River Crouch (South Side)
- LAG2 - River Thames North Side
- LAG3 - River Thames South Side (including the River Swale)
Each group has a Local Action Plan which is brought into operation when trigger levels for contamination are exceeded. There are three tiers of response depending on the levels of the micro-organism E-coli found in water monitoring samples.
A Tier 1 response triggers a minor investigation whereas a Tier 3 investigation is much more extensive and could result in closure of the shellfish harvesting area under investigation. The Action Groups share information by email.
More information on shellfish classification is on the Food Standards Agency website.
Live bivalve mollusc and live shellfish registration documents
Shellfish registration documents are issued to commercial fishermen who meet the relevant standards to enable them to transport their catch to processing plants.
All batches of shellfish must be accompanied by a registration document from the point of harvest until its final destination.
Registration documents can be obtained from the Denton Office
01474 363033 or email the Port Health Denton Office.
Permanent transport authorisations
In some cases where a gatherer operates the purification centre, relaying area or processing plant, the London Port Health Authority can issue a Permanent Transport Authorisation instead of individual registration documents. This only applies when all the gatherer's establishments are within the London Port Heath Authority.
Personal consumption
It is not safe to eat shellfish especially bivalves (mussels, oysters, clams, razor etc.) that you have collected from the seashore.
The bivalves bought in the shops are supplied from areas that are strictly controlled by the Food Standards Agency. This ensures that the shellfish are free from contamination and that the bivalves have been depurated before they are sold.
Depuration (the process of purifying) enables the shellfish to purge themselves of toxins in a highly controlled environment.
More information
Please contact the Denton Office by telephone on 01474 363033 or email the Port Health Denton office.