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Correlation between algal presence in water and toxin presence in shellfish

Project Code: B04005

30/09/2003

FRS Marine Laboratory
Bresnan, E ; Fryer, R;
SAMS
Hart, M;
Centre for Environment, Fisheries & Agriculture Science (CEFAS) Weymouth
Percy, L

1.1 Background

Algal toxins are routinely detected in shellfish flesh from UK waters. High levels of paralytic shellfish toxins (PSTs), diarrhetic shellfish toxins (DSTs) and amnesic shellfish toxins (ASTs) have all resulted in closures of shellfish harvesting areas around the UK coast. Surveillance programmes for the detection of toxins in shellfish flesh and the causative organisms in coastal waters (fulfilling the requirements of EU Directive 91/492/EEC) have been in operation in the UK for a number of years. A lot of data has been collected under these programmes. However, using these data to examine the relationship between the occurrence of toxin producing algae in the watercolumn and toxicity levels in shellfish flesh has been difficult. Sampling frequency is often insufficient during toxic incidences to allow a robust statistical examination to be performed and in some cases shellfish flesh and phytoplankton samples were not taken at the same time or the same place.

This study established a monitoring programme at selected sites around the UK coast with the objective of enabling examination of any relationship between the occurrence of toxin producing algae in the watercolumn and toxicity levels in shellfish flesh.

Three sites were selected to participate in this monitoring programme:

Scapa Bay, Orkney: targeted toxins: PSTs, DSTs and ASTs
Scotland: in the blue mussel (Mytilus edulis)
Isle of Ewe, Scotland: targeted toxins: DSTs and ASTs in M. edulis ASTs in the King Scallop (Pecten maximus)
The Fal Estuary: targeted toxins: PSTs and ASTs in M. edulis
England:

A weekly sampling programme, for both phytoplankton and shellfish flesh, was implemented at these sites during 2001. This was increased to twice weekly sampling during 2002. Live samples of Alexandrium spp. and Dinophysis spp. were also taken for molecular examination during the course of this project.

1.2 Relationship Between Algal Numbers in the Watercolumn and Toxin Levels in Shellfish Flesh

The data collected during the sampling programme were analysed using standard time series analysis techniques. This involved estimating the correlation between algal numbers in the watercolumn and toxin levels in shellfish flesh for a series of time lags. Significant correlations indicate that the toxic events observed in the shellfish were associated with phytoplankton blooms.

For some time series, it was also possible to investigate whether the correlations persisted when any seasonal trends in the data were removed. Significant correlations now indicate that changes in shellfish toxicity during a toxic event were associated with changes in phytoplankton numbers.

The strengths of these associations were measured by the correlation coefficient. To aid interpretation, correlations less than 0.5 were classed as weak; correlations between 0.5 and 0.8 were classed as moderate; and correlations greater than 0.8 were classed as strong.

1.2.1 Pseudo-nitzschia spp. cell numbers and ASTs in shellfish flesh

There were moderate correlations between Pseudo-nitzschia cells in the water column and AST levels in M. edulis at the Isle of Ewe during 2002. Only a weak correlation was observed at Scapa Bay, with high AST levels recorded in M. edulis during periods where relatively low Pseudo-nitzschia spp. cell numbers were observed. A significant correlation was observed between ASTs and Pseudo-nitzschia spp. at the Fal estuary during 2001. Insufficient toxic events occurred during 2002 to examine the data in much detail.

AST levels in King Scallop (Pecten maximus) tissue were very variable. At best, there were only weak correlations between numbers of Pseudo-nitzschia spp. and AST levels in P. maximus gonad (lag of 3-4 weeks) and whole tissue (lag of 5-8 weeks).

1.2.2 Dinophysis spp., Prorocentrum lima and DSTs

The DSTs okadaic acid (OA) and Dinophysis toxin DTX-2 were targeted in the analysis. The Dinophysis population usually comprised several species, but the whole population was considered during the analysis. There were moderate / strong correlations between total Dinophysis/P. lima numbers and both OA and DTX-2 levels in M. edulis flesh.

1.2.3 Alexandrium spp. and PSTs in M. edulis

Problems in the chemical analysis of PSTs in M. edulis flesh made the data unreliable. Data from the Fal Estuary reveal moderate correlations between numbers of Alexandrium spp. and levels of the PST, saxitoxin.

1.2.4 Summary

The analysis highlighted the difficulties inherent in examining the relationship between toxic phytoplankton cells in the water column and toxin levels in shellfish flesh, in particular that phytoplankton cell numbers may not always be a good proxy for phytoplankton toxicity and hence the impact on shellfish flesh. However the data show correlations between the numbers of toxic phytoplankton in the water column and toxicity levels in shellfish flesh. Phytoplankton monitoring has the potential to act as a marker for shellfish toxicity, but only if sampling is sufficiently frequent.

1.3 Molecular Examination of Alexandrium spp. and Dinophysis Species

Molecular examination has identified Alexandrium spp. cells present in Scottish samples to be A. tamarense while only A. minutum and A. ostenfeldii were identified in samples from the Fal Estuary.

Dinophysis cells were found to allele with those from other global studies and a degree of heterogeneity was observed between individuals from the same population.

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