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Scotland's Marine Atlas: Information for The National Marine Plan

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BIOTOXIN MONITORING IN SCOTLAND

What are biotoxins?

Marine biotoxins are a group of natural substances produced by microscopic marine algae (phytoplankton, including diatoms and dinoflagellates). Biotoxins can accumulate in fish or shellfish as they feed on these algae. The biotoxins are a risk to human and animal health if these shellfish are consumed.

Biotoxin monitoring requirements

The cultivation, harvesting and sale of bivalve molluscs are important to the Scottish economy, with particular significance to the sustainable development of rural areas of the west coast and islands. Marine phytoplankton is the food source for these filter-feeding organisms.

The Food Standards Agency in Scotland ( FSAS) is required, under Regulation ( EC) No. 854/2004 to monitor shellfish harvesting areas around Scotland for the presence of toxins in shellfish and the occurrence of biotoxin producing phytoplankton. Fisheries are closed when necessary in order to prevent consumption of contaminated shellfish.

Three main groups of toxins are routinely monitored in Scottish waters. The potential for Paralytic Shellfish Poisoning is indicated by the presence of saxitoxin in shellfish and Alexandrium species in the phytoplankton. Toxins that cause Diarrhetic Shellfish Poisoning are generated by Dinophysis species in phytoplankton. The potential for Amnesic Shellfish Poisoning is indicated by the presence of domoic acid in shellfish flesh and the presence of Pseudo-nitzschia species in the phytoplankton. If the toxins are found above the maximum permitted level specified by the regulations the harvesting area is closed.

EU regulatory limits from Regulation ( EC) No 853/2004 and the causative phytoplankton species

Type of Shellfish Poisoning

Maximum Permitted Level of biotoxins in shellfish flesh

Main causative phytoplankton species in Scottish waters

Paralytic Shellfish Poisoning

80 µg Saxitoxin di- HCl equivalent /100g

Alexandrium species

Diarrhetic Shellfish Poisoning

Positive bioassay result

Dinophysis species

Amnesic Shellfish Poisoning

20 µg Domoic Acid + Epi Domoic Acid /100g

Pseudo-nitzschia species

Biotoxin monitoring in Scotland

Biotoxins are monitored at sites classified for the production of shellfish. In 2009 there were 260 classified shellfish sites around Scotland, with shellfish routinely sampled from around 115 of these for the presence of the three groups of toxins. Samples are collected from Representative Monitoring Points within a group or 'pod' of classified shellfish sites. Shellfish sites within each 'pod' are considered to be similar hydrographically and environmentally. The Representative Monitoring Point is considered to be the point most likely to be representative or indicative of any toxicity in the area. Representative Monitoring Points are primarily selected on a geographical basis, and according to the history of toxic events in the area, although the shellfish species harvested at the site and ease of access are also taken into account. Sampling frequency for each of the three groups of toxins is based on a risk assessment, according to seasonal risk and harvesting activity.

The detection, at the Representative Monitoring Point, of any of the three biotoxin groups at levels which exceed EU regulatory limits, leads to the closure of all classified shellfish sites within the pod. Toxin-producing phytoplankton are also routinely monitored at 36 sites (mostly Representative Monitoring Points) around the coast, with the majority concentrated in the major shellfish producing regions of Argyll and Shetland. Enumeration of phytoplankton species can be used as an 'early warning' of the possible occurrence of shellfish biotoxin events. To facilitate this, threshold concentrations, or trigger levels, of the main genera of phytoplankton have been determined by comparing phytoplankton counts with the presence of biotoxins in shellfish. While Paralytic Shellfish Poisoning, Amnesic Shellfish Poisoning, and Diarrhetic Shellfish Poisoning are of greatest concern, other toxins produced by phytoplankton and the trigger levels for the key toxin producing genera are used to predict shellfish biotoxin event. In addition, the monitoring programme will report unusually large occurrences of any of the other harmful species detailed on the governmental Oceanographic Commission ( IOC) taxonomic reference list of toxic phytoplankton http://www.bi.ku.dk/ioc/.

2009 Biotoxin Monitoring Sites

2009 Biotoxin Monitoring Sites

Shellfish sampling frequency for Representative Monitoring Points in Scottish waters during 2009

Type of Shellfish Poisoning

Weekly

Fortnightly

Monthly

Paralytic Shellfish Poisoning

April to October

November to March

Diarrhetic Shellfish Poisoning

April to December

January to March

Amnesic Shellfish Poisoning

June to October

May

November to April

Phytoplankton species monitored in Scottish waters and the trigger levels for key toxin producing genera

Phytoplankton

Trigger level (cells/l)

Associated toxin

Pseudo-nitzschia (genus)

50,000

Amnesic Shellfish Poisoning

Alexandrium (genus)

Presence

Paralytic Shellfish Poisoning & spirolides

Dinophysis (genus)

100

Diarrhetic Shellfish Poisoning including:
Okadaic acid ( OA), Dinophysis toxins ( DTXs) and Pectenotoxins ( PTXs)

Prorocentrum lima

100

Diarrhetic Shellfish Poisoning

Prorocentrum minimum

None specified

Venerupin (hepatoxin)*

Lingulodinium polyedrum

None specified

Yessotoxin

Protoceratium reticulatum

None specified

Yessotoxin

Protoperidinium crassipes/curtipes

None specified

Azaspiracids#

* The toxicity of P. minimum and the production of hepatoxin are currently under scientific debate.
# The causative organism of azaspiracid toxins has recently been identified as the small dinoflagellate Azadinium spinosum, with the heterotroph P. crassipes/curtipes being a vector for these toxins if it ingests A. spinosum, along with other heterotrophic phyto and micro zooplankton.

Detection of biotoxins in shellfish monitored in Scottish shellfish harvesting areas during 2009

Between March and October, Amnesic Shellfish Poisoning was detected in samples from 38 separate sites ranging from 1 µg/g to 13 µg/g. All of these were below the maximum permitted level of 20 µg/g. These samples originated from sites predominately on the West Coast, Western Isles and Shetlands. The results for these Representative Monitoring Points coincided with areas at which the largest bloom densities of Pseudo-nitzschia were detected.

During 2009, 100 samples from 30 Representative Monitoring Points recorded positive results for Diarrhetic Shellfish Poisoning toxins. January, March and October to December were the only months with no positive results. The samples originated predominately from the West Coast, Western Isles and Shetlands. Again, these results coincided with the areas at which the largest bloom densities of Dinophysis were detected.

During 2009, Paralytic Shellfish Poisoning toxin was detected in a total of 32 samples between March and June. Only 5 of these samples contained levels of Paralytic Shellfish Poisoning toxin above the maximum permitted level. These were sampled from 3 sites located in the west coast, between late April and early June 2009. Although significant Alexandrium blooms were detected in Shetland during 2009, these did not appear to lead to Paralytic Shellfish Poisoning toxins being detected in shellfish sampled from Representative Monitoring Points in this area. Paralytic Shellfish Poisoning toxicity in Scottish waters is thought to be generated by toxic (group I) ribotypes of Alexandrium tamarense. However, recent evidence indicates the co-occurrence of non toxic (group III) A. tamarense in Scottish waters. Blooms of these organisms would not generate shellfish toxicity. The biogeography of Alexandrium, and the factors driving this, are subjects of active current scientific investigation.

Detection of biotoxin producing phytoplankton in Scottish waters during 2009

During 2009, a total of 1,031 phytoplankton samples were screened for biotoxin producing phytoplankton. Of these, approximately 12%, 29% and 15% exceeded the regulatory threshold for the three major toxic genera Pseudo-nitzschia, Alexandrium and Dinophysis, respectively. The largest bloom density at each sampling location is depicted in the maps.

Dinophysis species

Dinophysis species
© SEPA

Pseudo-nitzschia species

Pseudo-nitzschia species
© SEPA

What do the results indicate?

During 2009, blooms of the phytoplankton species responsible for production of Amnesic Shellfish Poisoning, Diarrhetic Shellfish Poisoning and Paralytic Shellfish Poisoning were detected across Scotland, with blooms in Shetland and the West Coast resulting in the closure of commercial harvesting areas for Diarrhetic Shellfish Poisoning and Paralytic Shellfish Poisoning toxins in shellfish. These monitoring programmes confirm the importance, to both human health and the rural economy, of regular water sampling for the early detection of phytoplankton blooms and the prediction of biotoxin events in shellfish.

Levels of Pseudo-nitzschia species detected at phytoplankton monitoring points during 2009

Levels of Pseudo-nitzschia species detected at phytoplankton monitoring points during 2009

Levels of Dinophysis species detected at phytoplankton monitoring points during 2009

Levels of Dinophysis species detected at phytoplankton monitoring points during 2009

Levels of Alexandrium species detected at phytoplankton monitoring points during 2009

Levels of Alexandrium species detected at phytoplankton monitoring points during 2009